DSpace at VNU: Orientation determination of interfacial bent alpha-helical structures using Sum Frequency Generation vibrational spectroscopy

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DSpace at VNU: Orientation determination of interfacial bent alpha-helical structures using Sum Frequency Generation vibrational spectroscopy

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Chemical Physics 447 (2015) 15–21 Contents lists available at ScienceDirect Chemical Physics journal homepage: www.elsevier.com/locate/chemphys Orientation determination of interfacial bent a-helical structures using Sum Frequency Generation vibrational spectroscopy Khoi Tan Nguyen ⇑ Department of Applied Chemistry, School of Biotechnology, International University, Vietnam National University of HCMC, Ho Chi Minh City 70000, Viet Nam School of Chemical Engineering, University of Queensland, St Lucia, QLD 4072, Australia a r t i c l e i n f o Article history: Received 29 July 2014 In final form 26 November 2014 Available online December 2014 Keywords: Sum Frequency Generation Melittin Bent helix Supported lipid bilayers Antimicrobial peptides a b s t r a c t Sum Frequency Generation (SFG) has been shown to be a powerful and versatile technique in studies of proteins/peptides at surfaces and interfaces Recently SFG was successfully applied in studies of interfacial macro-molecules with increasing size and complexity In this report we continued to employ bond additivity model and group theory to demonstrate the importance of both the inter-helical tilt angle and the lengths of the helical segments assembling the structures being studies Specifically, a newly improved SFG data analysis of multiple a-helical structures on melittin was used to interpret the SFG experimental observation and also verified the findings with the recent insights brought by other spectroscopic techniques Ó 2014 Elsevier B.V All rights reserved Introduction Sum Frequency Generation (SFG) signal in the amide I band of the a-helical secondary structure was first observed in 2005 by Chen and coworkers [1] Since then SFG data analysis methods have been continuingly developed and applied to a variety of biological molecules of different levels of complexity including melittin, magainin 2, cecropin P1, fibrinogen, alamethicin, MSI-78, tachyplesin I, cytochrome, human islet amyloid polypeptide, and heterotrimeric G-protein [2–7] The orientation analysis of simple biological molecules consisting of a single a-helical segment has been successfully carried out on small peptides such as magainin 2, alamethicin and MSI-78 [3–5] Due to their simple structural properties and interaction schemes, these analyses were carried out using just the SFG signals in ssp and ppp polarization combinations For more complex structures/interaction schemes which inherently consist of more orientational parameters, SFG ssp and ppp signals are not sufficient to provide target orientational information because these two pieces of information can only be used to solve for a single orientation parameter if the absolute peptide coverage is unknown In such cases, complementary spectroscopic techniques (typically ATRFTIR), as well as additional computational ab-initio simulation and mathematical approaches/information theories, have been required to provide extra orientational information about the interfacial species [8–11] Even though these complementary approaches have ⇑ Address: School of Chemical Engineering, University of Queensland, St Lucia, QLD 4072, Australia Fax: +61 3365 4199 E-mail address: k.nguyen9@uq.edu.au http://dx.doi.org/10.1016/j.chemphys.2014.11.022 0301-0104/Ó 2014 Elsevier B.V All rights reserved certainly been useful in the interpretation of various biological secondary structures at the interfaces, they all have intrinsic limitations in the computational algorithms or methods of approximation For instance, the transition Raman polarizability of an oscillator can only be approximated due to its indefinite number of virtual excited states in the Raman process A systematic study employing group theory and the bond additivity model in the orientation determination of single ahelical structures of different lengths using ssp and ppp SFG amide I signals has been previously reported [12] However, to my knowledge, there have been no published reports of direct and formal determination of the interfacial orientation of a general multi-helical structure There have been a few recent SFG studies on bent helical structures [13–15]; however, the computational details the successive Euler’s transformation implementations in the calculation of the net dipole moment and the polarizability tensor were not explicitly discussed In addition, these studies assume strong vibrational coupling among all the backbone C@O in the protein molecules; which may not be true if there are substantial symmetry point group interruptions among helical units or when they are distant from each other Although the orientations of cytochrome b5 in model lipid bilayers have been reported recently, the validity of the analysis relied primarily on the assumed internal cancelation of the SFG hyperpolarizability which arises from helical segments pointing in opposite directions [11] Unfortunately, a general multi-helical structure may not possess such conformation, notably pardaxin, melittin, and cytochrome P450 16 K.T Nguyen / Chemical Physics 447 (2015) 15–21 There have also been a number of excellent SFG studies on peptides/proteins using both the amide I band and the C-H (or C-D) vibrational modes in the 2800–3100 cmÀ1 (or 2000–2200 cmÀ1) regime These studies tackle the orientation problem from different aspects such as investigating certain particular amino acid residues [7] or by studying the hyperpolarizability of the backbone C@O bonds The latter approach can be performed either qualitatively or quantitatively depending on the information being sought [6,16,17] For quantitative studies, the analyses may become unexpectedly complicated and the level of accuracy depends significantly on the models being implemented in the calculation Theoretically, the a-helical amide I band was shown to consist of two orthogonal vibrational modes A and E [18] The double mode added to the complexity of calculating the multi-helical molecular hyperpolarizability This study successfully investigates the significant impact of the molecular twist angle on the SFG amide I band signal obtained while this twist angle has been omitted in a few over-simplified SFG analyses of bent-helical structures [10,19] In addition, the inter-helical twist angle was confirmed to be not a crucial factor contributing to the SFG Ippp/Issp value, which is commonly used in the orientation analysis of helical structures Finally, we took into account the length of the helical motifs in the data analysis to characterize the molecular orientation of melittin in lipid bilayers and verify the findings with current literature The molecular orientation of melittin is of great interest since it gives direct hints to the peptide mode of action, which is still being hotly debated Traditionally, two main peptide-lipid interaction models have been proposed: barrel stave [20] and carpet/toroidal pore models [21,22] More recent studies observed a more sophisticated interaction picture in which melittin adopts a dual orientation distribution [10,23] or the pore formation occurs as a transient process [24,25] In this present study, an orientation distribution that aligns with recent studies on the mode of action of melittin was proposed, on the basis of the kink in the molecular structure of melittin being taken into account and the experimental evidence that the peptide adopts a single d orientation distribution reported recently using dual-fluorescence spectroscopy [26] It is worth emphasizing that the analysis in this present study may not be able to describe the peptide/lipid interaction in more complex cases in which the single d orientation distribution condition is not met; in such cases, further parameters can be sought using other optical spectroscopic techniques such as ATR-FTIR or FWM SFG data analysis – normal vibrational modes of multiple ahelical structures In fact, this paper is an advancement of a previously developed theoretical background [12,27]; interested readers may find these materials particularly helpful in providing fundamental details for the present study Therefore, full details on the SFG orientation analysis of single Pauling’s a-helical structures of different chain lengths will not all be reiterated here However, certain concepts which are believed to be crucial for the development of this paper will be selectively discussed A sum frequency process can be considered as a hyper-Raman process of which hyperpolarizability is described by a third rank tensor This hyperpolarizability can be calculated using the transition dipole moment and the transition polarizability tensors Hence, the normal vibrational modes of the infrared absorption and Raman scattering processes should be thoroughly studied The hyperpolarizability tensor of the SFG process can be expressed as a combination of the IR absorption (x2) and the Raman scattering (x1): bijk ¼ X v ~ ij ðxSF Þjv ihv jl ~ k ðx2 Þjgi hgja ð0Þ g ð0Þ q À qv x2 xv ỵ icv 1ị where xSF ẳ x1 ỵ x2 ; xv is the frequency at which vibrational resonant transition occurs from jg i to jv i The quantity qg0ị q0ị v x2 xv ỵicv appears in the expression as a line-shape function The q values are the fractional populations at the vibrational states; while c dictates the line width of the spectral peak corresponding to the indicated vibrational transition The SFG macroscopic susceptibility tensor element vijk ði; j; k ẳ x; y; zị is related to the SFG molecular hyperpolarizability tensor element blmn ðl; m; n ¼ a; b; cị by an Euler angle projection [18,2729]: vijk;q ẳ Ns X ^Án b k ^ Þiblmn;q hð^i Á ^lÞð^j Á mÞð ð2Þ l;m;n Eq (1) reveals the dependence of the hyper-Raman tensor on the IR absorption and Raman scattering Each hyperpolarizability tensor corresponds to a vibrational mode that is both active among IR and Raman normal modes Therefore, the number of normal modes of the SFG process is reduced from three to two: A (symmetric, along the z axis) and E (asymmetric, in the xy plane) modes To illustrate this, the A and E modes are presented graphically in Fig It can be seen from Fig that the overall molecular symmetric A mode is directly dictated by the relative tilt angle between the two adjacent helical segments; whilst it is unlikely that the molecular E mode will be influenced by the corresponding twist angle The process of calculating the molecular hyperpolarizability tensors could be simplified if these two vibrational modes are well separated spectrally; unfortunately, the vibrational energy of the A and E modes are inherently only a few wavenumbers apart in the SFG spectra For this reason, the SFG signals of the A and E modes cannot be resolved due to the limited resolution of a typical SFG system Thus both modes are likely to be contributing to the amide I signal as shown below [10]: vzzz ẳ vE;zzz ỵ vA;zzz 3ị vyyz ẳ vxxz ẳ vE;yyz ỵ vA;yyz 4ị vyzy ẳ vxzx ẳ vzxx ẳ vzyy ẳ vE;yzy ỵ vA;yzy 5ị The calculation of the hyperpolarizability tensors using the bond additivity model and group theory has previously been systematically reported [12] In this model, the hyperpolarizability tensors are first calculated in the molecular fixed frame, which takes into account the relative positions of the amino acid units Hence, this analysis is highly dependent on the structural properties of the molecule, as will be discussed later The microscopic hyperpolarizability tensors are then calculated and transformed into measurable macroscopic quantities in the laboratory fixed frame using the appropriate Euler’s transformations [28] This A mode z z E mode y y x x A mode Fig A (left) and E (right) vibrational mode illustrations of an a-helix 17 K.T Nguyen / Chemical Physics 447 (2015) 15–21 transformation basically relates the intrinsic molecular hyperpolarizability tensors to the varying behaviors of the molecule in the specific coordinate system In this report, the hyperpolarizability tensors of the individual helical segments will be calculated and subsequently summed together with their corresponding modes The validity of this treatment depends heavily on the presumed rigidity in the molecular conformation of the peptide/protein being studied Not being a high resolution technique, SFG analyses typically require the molecular conformation acquired by higher resolution means For that reason, SFG typically works with other established high resolution techniques, such as X-ray crystallography and nuclear magnetic resonance, in boosting the power of spectroscopy and thereby providing insights into their biological functions In this study, the hyperpolarizability tensors of the two helical segments of melittin were calculated separately then subsequently transformed into the laboratory fixed frame using the z–x–z convention as described previously [12] The tilt and twist angles of the first helical segment were simple chosen to be the same as those of the peptide molecule The set of tilt and twist angles (h2, W2) of the second helical segment requires careful treatment due to the shift in the molecular frame of the second segment caused by the rotations of the first helical segment The angles (h2, W2) were then calculated to be: À1 h2 ¼ h þ h12 þ sin  p for W 0; cosh12 4sinh12 ð1 À cos WÞ q5 2 sin h12 cos W ỵ cos h12 6ị h2 ẳ h h12 þ sin for W p ;p  The asymmetric E mode of melittin can then be calculated as: vE;zzz ¼ X X b ^z Á n ^ ịibE1;lmn ỵ h^z ^lị^z mị c0 ị^z nb0 ÞibE2;l0 m0 n0 hð^z Á b l Þð^z Á m l0 ;m0 ;n0 l;m;n 12ị vE;xxz ẳ vE;yyz ẳ X X b ^z n ^ ịibE1;lmn ỵ h^x Á^lÞð^x Á mÞð l;m;n l 0 c0 Þð^z Á nb0 ÞibE2;l0 m0 n0 hð^x Á b l Þð^x Á m ;m0 ;n0 13ị vE;xzx ẳ vE;zxx ẳ X b ^x Á n ^ÞibE1;lmn hð^x Á ^lÞð^z Á mÞð l;m;n X ỵ l 0 c0 ị^x nb0 ịibE2;l0 m0 n0 hð^x Á b l Þð^z Á m ð14Þ ;m0 ;n0 As discussed previously in Eqs (2)–(4), the zzz, xzx and xxz hyperpolarizability tensors are then calculated as: vzzz ¼ X b ^z Á n ^ ÞibA1;lmn hð^z Á ^lị^z mị l;m;n X ỵ c0 ị^z nb0 ÞibA2;l0 m0 n0 l Þð^z Á m hð^z Á b l0 ;m0 ;n0 ỵ X b ^z n ^ ịibE1;lmn h^z ^lị^z mị l;m;n X ỵ l vxxz ẳ vyyz ẳ X ỵ cosh12 4sinh12 ð1 À cos WÞ Â qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi5 sin h12 cos2 W ỵ cos2 h12 ỵ 7ị ỵ c0 Þhð^z Á nb0 ÞibE2;l0 m0 n0 l Þhð^z Á m hð^z Á b ð15Þ ;m0 ;n0 X b ^z Á n ^ ÞibA1;lmn hð^x Á ^lÞð^x Á mÞð l;m;n c0 Þð^z Á nb0 ÞibA2;l0 m0 n0 hð^x Á b l Þð^x Á m l ;m0 ;n0 X b ^z Á n ^ ÞibE1;lmn hð^x Á ^lÞð^x Á mÞð l;m;n X c0 Þð^z Á nb0 ÞibE2;l0 m0 n0 l Þð^x Á m hð^x Á b ð16Þ l0 ;m0 ;n0 W2 ẳ W12 ỵ W 8ị where h12 is the inter-helical tilt angle between the two segments; h and W are the molecular tilt and twist angles, respectively The transformed SFG susceptibilities of the segments were then combined together, yielding the molecular SFG susceptibility Due to the deviation of the molecular symmetry property, the molecular twist angle is no longer azimuthally symmetric Consequently, the molecular SFG susceptibility quantity is a function of both the molecular tilt and twist angles The non-zero hyperpolarizability tensors of the symmetric A mode of melittin can then be calculated from the segmental tensors: vA;zzz ẳ X b ^z n ^ ịibA1;lmn ỵ hð^z Á^lÞð^z Á mÞð l;m;n X c0 Þð^z Á nb0 ÞibA2;l0 m0 n0 l Þhð^z Á m ð^z Á b l0 ;m0 ;n0 9ị vA;xxz ẳ vA;yyz ẳ X þ l b ^z Á n ^ ÞibA1;lmn hð^x Á ^lÞð^x Á mÞð l;m;n c0 Þð^z Á nb0 ÞibA2;l0 m0 n0 ð^x Á bl Þhð^x Á m ð10Þ ;m0 ;n0 vA;xzx ẳ vA;zxx ẳ ỵ X X l0 ;m0 ;n0 X b ^x Á n ^ ÞibA1;lmn hð^x Á ^lÞð^z Á mÞð l;m;n c0 Þð^x Á nb0 ÞibA2;l0 m0 n0 ð^x Á bl Þð^z Á m ð11Þ vxzx ẳ vzxx ẳ X ỵ l ỵ X b ^x Á n ^ ÞibA1;lmn hð^x Á ^lÞð^z Á mÞð l;m;n c0 Þð^x Á nb0 ÞibA2;l0 m0 n0 l Þð^z Á m hð^x Á b ;m0 ;n0 X b ^x Á n ^ ÞibE1;lmn hð^x Á ^lÞð^z Á mị l;m;n ỵ X c0 ị^x nb0 ịibE2;l0 m0 n0 l Þð^z Á m hð^x Á b ð17Þ l ;m0 ;n0 Results and discussions 3.1 Melittin – a revisited problem Melittin is an amphipathic peptide consisting of 26 amino acid residues (GIGAVLKVLTTGLPALISWIKRKRQQ) divided into three parts, two of which adopt helical secondary structures (amino acid residues positioned from to11 and 12 to 21) and a non-helical segment of five amino acid residue long (22–26) (Fig 2) Even though the interaction between melittin and lipid bilayers has been studied extensively by various techniques such as ATRFTIR, Raman, fluorescence, circular and linear dichroism, dielectric relaxation, NMR and SFG, results are inconclusive and contradictory This apparent variation in results may be due to the molecules bent structure arising from the Pro residue at position 14 in the amino acid sequence Some studies suggested a horizontal orientation of melittin in the lipid bilayers whilst 18 K.T Nguyen / Chemical Physics 447 (2015) 15–21 Fig The structure of melittin Tertiary structure of melittin [30] some proposed that melittin forms channels by orienting vertically in the membrane [23,31–38] In fact, research about the mode of action of melittin was fairly fruitful in the years of 2012 and 2013 A series of very recent papers employing X-ray diffraction, oriented circular dichroism, electrochemical impedance spectroscopy as well as various fluorescence spectroscopic and computational simulation techniques to investigate the mechanism of melittin membrane permeability converged to a general idea that melittin does not form a typical toroidal pores when disrupting the lipid bilayers as previously believed, but rather via a transient process in which the peptide does not adopt the transmembrane orientation at equilibrium [23–26,39,40] Being an intrinsic surface sensitive technique, SFG was also used by the Chen group in studying the interaction between melittin and DPPG/dDPPG lipid bilayer They proposed that melittin adopts both vertical and horizontal orientations in DPPG/dDPPG lipid bilayer [10] However, by introducing some small modifications into the data analysis, a description of this peptide-membrane interaction can be obtained which better aligns with findings reported in the current literature It has been shown by molecular simulation (using CHARMM force field) that the melittin molecular conformation remains rigid during the interaction with the lipid bilayer [41] Using this result, the molecular SFG hyperpolarizability tensors of melittin can be calculated The structure of melittin under different conditions has been studied using a number of techniques In particular, the inter-helical tilt angle h12 was determined to be around 2.48 ± 0.05 (rads) in DTPC lipid [42], 2.25 rads in crystal form [43], or 2.2 ± 0.3 (rads) for DPC bound melittin [44] and 2.3 ± 0.16 (rads) for melittin lyophilized from methanol [45] This inter-helical tilt value was found to be significantly lower to the value of 1.5 ± 0.6 (rads) for PC bound melittin, based on transferred NOE [46] Although being heavily influenced by the surrounding conditions, the inter-helical angel of melittin was found not to differ greatly among peptide crystals prepared from aqueous solution, melittin lyophilized from methanol and DPC bound melittin This inter-helical angle, however, changes significantly when melittin binds to DTPC in hydrated gel state and dry form [42] Therefore, in studies of interactions between fully hydrated melittin and model cell membranes, the inter-helical angle has been widely assumed to be close to its value in the crystal form, which is 2.25 rads The detailed expressions of the above modes are relatively lengthy and therefore are not reported here Instead, their relationships with the molecular tilt and twist angles are presented visually in Fig (plotted using Eqs (15) and (16)) The calculation resulted in a possible range of the ratio vzzz/vyyz of [1.4, 2.8], which is broader than the range calculated previously for melittin [10] This may be due to the omission of the slight kink at Pro14 that Chen and colleagues assumed This new broader range suggests the possibility that melittin can actually adopt a single orientation distribution in a 1,2-dipalmitoyl-sn-glycero-3-phosphoglycerol (DPPG) lipid bilayer In Chen et al.’s study, a ratio vzzz/vyyz of 1.4 was experimentally obtained and it was concluded that melittin adopts two distinct orientations upon its interaction with the DPPG bilayer at a concentration of 780 nM [10] They employed a fairly sophisticated and elegant analysis involving the maximum entropy approach as well as the assumption of double delta orientation distributions However, in their analysis, melittin structure was assumed to be linear so that its symmetry properties guaranteed the angles W and u to be azimuthal This assumption will possibly be valid if the second helical segment is substantially shorter than the latter or the inter-helical tilt angle is negligible (close to 0o) since the molecular twist angle W, directly affects the tilt angle of the second helical segment which in turn alters the projections of the symmetric and asymmetric vibrational modes in the laboratory fixed frame Based on the analysis presented here, an experimental ratio vzzz/vyyz of around 1.4 implies a possible range of [0, 0.40 (±0.02) (rads)] and [1.40 (±0.07), 2.00 (±0.10) (rads)] for the molecular tilt and twist angles respectively (Fig 3(c)) It is worth mentioning that the region when the ratio vzzz/vyyz falls in [1.4,1.5] at h e [1,1.4] is due to a mathematical artifact in Fig 3(c) and hence will not be considered To assess the effects of broader orientation distributions of the tilt and twist angles on the ratio vzzz/vyyz, one can assume that these angles adopt a more realistic normal distribution with a fixed standard deviation Ideally, both the tilt and twist angles can be assumed as normally distributed and the vzzz/vyyz ratio can be calculated and plotted accordingly Unfortunately, 19 K.T Nguyen / Chemical Physics 447 (2015) 15–21 ð2Þ ð2Þ ð2Þ ð2Þ (a) (b) (c) (d) ð2Þ Fig (a)–(c): vzzz ; vyyz and vzzz vzzz =vyyz as a function of the molecular titl (h,rad) and twist (W,rad)) angles In (c), the plot was truncated to the [1.4,1.5] (d): Definitions of the molecular tilt (h) and twist (W) angles the attempt to calculate the expected value of the hyperpolarizability tensor elements in the whole continuous range of both the tilt and twist angles failed because it was rather computational intensive for a personal computer to handle To simplify this process, either the tilt or twist angle was be held fixed while the other was assumed to adopt a normal distribution In particular, the twist was first fixed at 1.70 rads while the tilt angle was assumed a normal distribution of mean 0.20 rads and standard deviation of 0.26 rads; a vzzz/vyyz ratio of 1.45 was obtained Then, the tilt angle was set to 0.20 rads while the twist angle was assumed a normal distribution with mean and standard deviation of respectively 1.70 and 0.26 rads; a vzzz/vyyz ratio of 1.43 was accordingly obtained As a reference, the ratio vzzz/vyyz was also calculated to be 1.41 as both the tilt and twist angles were assumed to follow the d-distribution at 0.20 and 1.70 rads, respectively These three values of the vzzz/vyyz are sufficiently close, which validates the simplification of using the d-distribution for both the tilt and twist angles in the analysis As discussed extensively in earlier studies [27], SFG ssp and ppp signals are only able to provide one piece of information of structural significance For this bent helical structure, the orientation description includes both the tilt and twist angles Therefore, a second technique which possesses comparable experimental setup is, in principle, required However, given the narrow orientational range obtained above, one is able to conclude that one of the two helical segments of melittin adopts a vertical orientation in relative to the membrane surface This tilt and twist ð2Þ ð2Þ vð2Þ zzz vzzz =vyyz ratio range of angle combination supports the peptide penetration model in which one helical segment orients almost vertically and the other orients almost horizontally that leads to the coexisting vertical and horizontal peptide orientation predicted by Chen et al [10] The interpretation of this study is consistent with a recent model of melittin in lipid bilayers proposed by Postupalenko et al using dual-fluorescence spectroscopy In their study, L9 residue was found to protrude into the bilayer while the W19 residue was reported to localize at the interface [26] Moreover, the interaction picture presented here is surprising in line with the classical model Vogel proposed using Raman spectroscopy by the fact that one of the two helical segments of melittin adopts a vertical and the latter adopts a horizontal orientation in the lipid membranes [30] Interestingly, in a recent study using X-ray diffraction and oriented circular dichroism, Huang et al recently suggested that when the pore induction occurs, the ratio between vertical/horizontal helical content is 0.56 which falls in the range of [0.55,0.68] achieved by projection calculations [23] Although the proposed molecular orientation proposed here seems to align well with Huang’s and Vogel’s, there is a problem relating these pieces of orientation information into the toroidal pore disruption mode of melittin because the length of its transmembrane oriented segment is definitely not sufficient to span the whole lipid bilayer More specifically, the thickness of the hydrophobic core of a typical POPC lipid bilayer is around nm [39], while the transmembrane segment is only able to penetrate at most 1.5 nm into the bilayer 20 K.T Nguyen / Chemical Physics 447 (2015) 15–21 posing lipids [22,34] The analysis of this study on DPPG bound melittin explains the dual orientation distribution as well as the transient pore formation proposed by a number of recent studies [10,23–26,30,39] On a broader scale, this study does not only provides new orientational information on DPPG lipid bilayer bound melittin, but also opens up opportunities for the implementation of this high throughput technique in structural biology This report serves as a foundation for SFG studies on biological systems under physiological conditions Fig Representative average molecular orientation of melittin in lipid bilayers Conflict of interest Therefore, the pore formation is unlikely to occur; the orientation of melittin in DPPG lipid bilayers appears as presented in Fig A study on the effect of melittin on DPPG/dDPPG lipid bilayer previously carried out by Chen et al supports the interpretation of this study on melittin orientation in DPPG/dDPPG bilayers [47] Using isotope labeling, Chen et al were able to differentiate the proximal and distal leaflets and thus observe the behaviors of each leaflet when interacting with melittin at the peptide concentration of 780 nM They found that the integrity of the proximal leaflet was maintained during the course of the interaction while the integrity of the distal leaflet was disturbed significantly, which agrees well with the presented findings, that melittin cannot span the whole lipid bilayer Chen et al observed that the original spectral features of the proximal leaflet in the CD region 2000– 2300 cmÀ1 remained unchanged upon peptide interaction, while that of the distal leaflet in the CH region 2800–3100 cmÀ1 completely diminished Although a decrease of the overall SFG signal in the CD region was observed, this could only be due to the increase in the symmetry property of the bilayer caused by the flip-flopping of the lipid molecules This observation fully supports interpretation of the present study (Fig 4) in which the horizontal helical segment destroys the integrity of the only one phospholipid layer yet leaves the other intact It is worth noting that using vesicle leakage experiment, Wiedman et al observed a two phase interaction between melittin and POPC lipid bilayers There was observed a rapid burst of leakage occurred in the first phase followed by a second phase during which leakage slowed down and then stopped before completion (at relevant peptide:lipid ratios) [25] Wiedman et al.’s observation is an excellent indication of the self-healing ability of POPC lipid bilayer which again does not support the toroidal pore mode of action of melittin At this stage, one may wonder if it is not the toroidal pore mode of action melittin follows, then how melittin lyses the cells There have been both experimental and computational evidence suggesting that melittin creates transient pores in lipid bilayers [24,25] Even though the proposed orientation information does not directly suggest a transient behavior, it actually supports this model by providing the peptide orientation information at equilibrium which implies that the translocation of melittin is required for the cell lysis to occur Conclusions Carrying information on both chemical and orientational properties of interfacial species, SFG vibrational spectroscopy possesses a rather complicated data analysis Despite rather lengthy transformations of the hyperpolarizability tensors, SFG vibrational spectroscopy was employed to determine a multi a-helical structure without any of the qualitative simplifications that have previously been assumed [10,11] As pointed out in earlier studies, the complexity of the interaction scheme that melittin adopts, might vary with factors such as membrane composition, hydration levels, temperature, and the phase states of the membrane com- There is no conflict of interest Acknowledgements This research is funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under Grant number 106.16-2012.67 I sincerely thank Dr Gay Marsden for her generous assistance with the manuscript preparation and the proofreading References [1] X.Y Chen, J Wang, J.J Sniadecki, M.A Even, Z Chen, Langmuir 21 (2005) 2662 [2] A.P Boughton, P Yang, V.M Tesmer, B Ding, J.J.G Tesmer, Z Chen, Proc Natl Acad Sci USA 108 (2011) E667 [3] K.T Nguyen, S.V Le Clair, S.J Ye, Z Chen, J Phys Chem B 113 (2009) 12358 [4] P Yang, A Ramamoorthy, Z Chen, Langmuir 27 (2011) 7760 [5] S.J Ye, K.T Nguyen, Z Chen, J Phys Chem B 114 (2010) 3334 [6] L Fu, G Ma, E.C.Y Yan, J Am Chem Soc 132 (2010) 5405 [7] T Weidner, N.F Breen, K Li, G.P Drobny, D.G Castner, Proc Natl Acad Sci USA 107 (2010) 13288 [8] L Fu, D.Q Xiao, Z.G Wang, V.S Batista, E.C.Y Yan, J Am Chem Soc 135 (2013) 3592 [9] D.Q Xiao, L Fu, J Liu, V.S Batista, E.C.Y Yan, J Mol Biol 421 (2012) 537 [10] X.Y Chen, J Wang, A.P Boughton, C.B Kristalyn, Z Chen, J Am Chem Soc 129 (2007) 1420 [11] K.T Nguyen, R Soong, S.C Im, L Waskell, A Ramamoorthy, Z Chen, J Am Chem Soc 132 (2010) 15112 [12] K.T Nguyen, S.V Le Clair, S.J Ye, Z Chen, J Phys Chem B 113 (2009) 12169 [13] Y.W Liu, T.L Ogorzalek, P Yang, M.M Schroeder, E.N.G Marsh, Z Chen, J Am Chem Soc 135 (2013) 12660 [14] P Yang, A Boughton, K.T Homan, J.J.G Tesmer, Z Chen, J Am Chem Soc 135 (2013) 5044 [15] B Ding, J.E Laser, Y.W Liu, P.R Wang, M.T Zanni, Z Chen, J Phys Chem B 117 (2013) 14625 [16] L Fu, J Liu, E.C.Y Yan, J Am Chem Soc 133 (2011) 8094 [17] K.T Nguyen, J.T King, Z Chen, J Phys Chem B 114 (2010) 8291 [18] C Hirose, N Akamatsu, K Domen, J Chem Phys 96 (1992) 997 [19] B Ding, L Soblosky, K Nguyen, J.Q Geng, X.L Yu, A Ramamoorthy, Z Chen, Sci Rep.-Uk (2013) [20] G Schwarz, R.T Zong, T Popescu, Biochim Biophys Acta 1110 (1992) 97 [21] K Matsuzaki, K Sugishita, N Ishibe, M Ueha, S Nakata, K Miyajima, R.M Epand, Biochemistry-Us 37 (1998) 11856 [22] H Vogel, Biochemistry-Us 26 (1987) 4562 [23] M.T Lee, T.L Sun, W.C Hung, H.W Huang, Proc Natl Acad Sci USA 110 (2013) 14243 [24] K.P Santo, S.J Irudayam, M.L Berkowitz, J Phys Chem B 117 (2013) 5031 [25] G Wiedman, K Herman, P Searson, W.C Wimley, K Hristova, BbaBiomembranes 1828 (2013) 1357 [26] V.Y Postupalenko, O.M Zamotaiev, V.V Shvadchak, A.V Strizhak, V.G Pivovarenko, A.S Klymchenko, Y Mely, Bioconjugate Chem 24 (2013) 1998 [27] J Wang, S.H Lee, Z Chen, J Phys Chem B 112 (2008) 2281 [28] A.J Moad, G.J Simpson, J Phys Chem B 108 (2004) 3548 [29] G.J Simpson, J.M Perry, C.L Ashmore-Good, Phys Rev B 66 (2002) [30] H Vogel, F Jahnig, Biophys J 50 (1986) 573 [31] K Hristova, C.E Dempsey, S.H White, Biophys J 80 (2001) 801 [32] D Allende, S.A Simon, T.J McIntosh, Biophys J 88 (2005) 1828 [33] I Constantinescu, M Lafleur, Bba-Biomembranes 1667 (2004) 26 [34] S Frey, L.K Tamm, Biophys J 60 (1991) 922 [35] A.S Ladokhin, S.H White, Bba-Biomembranes 1514 (2001) 253 [36] C Steinem, H.J Galla, A Janshoff, PCCP (2000) 4580 [37] S Toraya, K Nishimura, A Naito, Biophys J 87 (2004) 3323 [38] L Yang, T.A Harroun, T.M Weiss, L Ding, H.W Huang, Biophys J 81 (2001) 1475 [39] S.J Irudayam, M.L Berkowitz, Bba-Biomembranes 1818 (2012) 2975 [40] S.J Irudayam, T Pobandt, M.L Berkowitz, J Phys Chem B 117 (2013) 13457 K.T Nguyen / Chemical Physics 447 (2015) 15–21 [41] M Andersson, J.P Ulmschneider, M.B Ulmschneider, S.H White, Biophys J 104 (2013) L12 [42] Y.H Lam, S.R Wassall, C.J Morton, R Smith, F Separovic, Biophys J 81 (2001) 2752 [43] T.C Terwilliger, D Eisenberg, J Biol Chem 257 (1982) 6016 [44] F Inagaki, I Shimada, K Kawaguchi, M Hirano, I Terasawa, T Ikura, N Go, Biochemistry-Us 28 (1989) 5985 21 [45] R Bazzo, M.J Tappin, A Pastore, T.S Harvey, J.A Carver, I.D Campbell, Eur J Biochem 173 (1988) 139 [46] A Okada, K Wakamatsu, T Miyazawa, T Higashijima, Biochemistry-Us 33 (1994) 9438 [47] X.Y Chen, J Wang, C.B Kristalyn, Z Chen, Biophys J 93 (2007) 866 ... properties of interfacial species, SFG vibrational spectroscopy possesses a rather complicated data analysis Despite rather lengthy transformations of the hyperpolarizability tensors, SFG vibrational spectroscopy. .. peptide orientation information at equilibrium which implies that the translocation of melittin is required for the cell lysis to occur Conclusions Carrying information on both chemical and orientational... The q values are the fractional populations at the vibrational states; while c dictates the line width of the spectral peak corresponding to the indicated vibrational transition The SFG macroscopic

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Mục lục

  • Orientation determination of interfacial bent α-

    • 1 Introduction

    • 2 SFG data analysis – normal vibrational modes o

    • 3 Results and discussions

      • 3.1 Melittin – a revisited problem

    • 4 Conclusions

    • Conflict of interest

    • Acknowledgements

    • References

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