Thermal behaviour of cubic phases rich in 1-monooleoyl- rac -glycerol in the ternary system 1-monooleoyl- rac -glycerol/ n -octyl-b- D -glucoside/water Gerd Persson 1 ,Ha ˚ kan Edlund 1 and Go¨ ran Lindblom 2 1 Department of Natural and Environmental Sciences, Mid Sweden University, Sundsvall, Sweden, 2 Department of Biophysical Chemistry, Umea ˚ University, Umea ˚ , Sweden Using synchrotron X-ray diffraction the thermal behaviour was studied of the cubic phases in the 1-monooleoyl-rac- glycerol (MO)/n-octyl-b- D -glucopyranoside (OG)/ 2 H 2 O system with 58 or 45 wt % MO concentration and varying OG/ 2 H 2 O contents. These MO contents correspond to a Pn3m cubic single-phase or a Pn3m cubic phase in excess water on the binary MO/water axis of the ternary phase diagram. The cubic liquid crystalline phases are stable with small fractions of OG, while higher OG concentrations trigger a cubic-to-lamellar phase transition. Moreover, with increasing OG concentration the initial Pn3m structure is completely converted to an Ia3d structure prior to the L a phase being formed. Upon heating this effect is reversed, resulting in an Ia3d-to-Pn3m phase transition. For some samples additional peaks were observed in the diffracto- grams upon heating, resulting from the metastability notoriously shown by bicontinuous cubic phases. This judgement is supported by the fact that upon cooling these peaks were absent. Remarkably, both the Ia3d and the Pn3m cubic structures could be in equilibrium with excess water in this ternary system. A comparison is made with previous results on n-dodecyl-b- D -maltoside (DM), showing that cubic phases with OG have higher thermal and composi- tional stability than with DM. Keywords: 1-monooleoyl-rac-glycerol; n-octyl-b- D -gluco- side; monoolein-rich cubic phases; thermal behaviour. Access to the complete structure of membrane proteins is one of the cornerstones in obtaining a better understanding of their function in the biological cell. For larger proteins the most important method for achieving this information is X-ray diffraction, which require high quality crystals of the protein. It is frequently possible to crystallise water-soluble proteins, which can be inferred from the large number of structures determined so far [1]. However, it is considerably more difficult to get suitably good crystals of membrane proteins, mainly due to the need to remove them from their native membrane environment, and solubilize them in mild detergent micelles. The solubilization process may lead to denaturation of the proteins, thus destroying them. Therefore, one of the foremost current issues is concerned with the problem of obtaining such crystals. A new approach to solve this problem was introduced by Landau and Rosenbusch in 1996 [2]. Their method includes the use of a bicontinuous cubic liquid crystalline phase as the crystallization medium. The general idea behind this approach was to introduce the proteins into an envi- ronment that mimics the native milieu [2], and the bicon- tinuous cubic phases formed by 1-monooleoyl-rac-glycerol (MO) [3,4] was utilized to meet these basic requirements. The exact mechanisms involved in the crystallization process are yet to be elucidated, although an attempt to describe the process has been published [5]. It should be noted that since the introduction of this method, only two to three membrane proteins have been successfully crystallised [2,6–8]. Therefore, for the method to be generally functional it is necessary to have a detailed understanding, at the molecular level, of what is driving the protein crystallization. To utilize this method fully, several crucial issues need to be solved. From a colloid or surfactant chemistry point of view knowledge about the microstructure of the liquid crystalline phases involved, and information about the possible effect(s) different additives may have on the liquid crystalline phases present are very important. A recent paper presented the compati- bility of a number of substances with the MO cubic phases [9], but among these additives only one was a surfactant (cetyltrimethylammonium bromide). Moreover, the most frequently used surfactant for the solubilization of the membrane proteins is n-octyl-b- D -glucopyranoside (OG). However, to our knowledge, only a few attempts to partially investigate the effect of OG on the stability of the MO cubic phases have been published [10,11], and it seems appropriate Correspondence to G. Persson, Department of Natural and Environmental Sciences, Mid Sweden University, Holmgatan 10, SE-851 70 Sundsvall, Sweden. Fax: + 46 60 148802, Tel.: + 46 60 148932, E-mail: Gerd.Persson@kep.mh.se Abbreviations:DM,n-dodecyl-b- D -maltoside; DSC, differential scanning calorimetry; MO, 1-monooleoyl-rac-glycerol; OG, n-octyl-b- D -glucopyranoside; PMOS, phosphomolybdic acid in sulphuric acid solution; SAXD, small-angle X-ray diffraction; TLC, thin-layer chromatography. (Received 8 July 2002, revised 27 October 2002, accepted 11 November 2002) Eur. J. Biochem. 270, 56–65 (2003) Ó FEBS 2003 doi:10.1046/j.1432-1033.2003.03361.x to extend these studies further. In a previous paper we presented the entire phase diagram for the ternary system MO/OG/ 2 H 2 O [12], from which it can be concluded that only a small fraction of OG is sufficient to convert the MO-rich cubic phases to a lamellar liquid crystalline (L a ) structure, and that these cubic phases are also found in equilibrium with excess water (cf. the binary MO/water- system). In this report, the thermal behaviour of such cubic phases rich in MO has been investigated by X-ray diffrac- tion. The study comprises both one- and two-phase regions, including cubic phases in equilibrium with excess aqueous solution. Materials and methods Materials 1-Monooleoyl-rac-glycerol (MO) (> 99% purity) and n-octyl-b- D -glucopyranoside (OG) (> 98% purity) were purchased from Sigma Aldrich Chemie GmbH, Germany and the substances were used without further purification. 2 H 2 O (99.9% in 2 H) was obtained from Cambridge Isotope Laboratories, USA. Sample preparation Samples were prepared by weighing the appropriate amounts of each substance into 8-mm glass tubes, which were sealed with removable caps. The samples were homogenized by rotation and stored in the dark at 25 °C. All samples were inspected both visually and between crossed polarizers to check homogeneity and optical anisotropy. The samples were then placed in 1.5-mm capillaries. All samples were stored in 5 °C for 12 h and then at 25 °C for 4 days prior to measuring. Methods Small-angle X-ray diffraction (SAXD). The small angle X-ray diffraction, SAXD, experiments were performed at the Austrian Academy SAXS station at the ELETTRA synchrotron (Trieste, Italy) using the 8 keV beam, corres- ponding to a wavelength of 0.15 nm. The temperature was controlled by an in-line differential scanning calorimeter (DSC), and the temperature-scanning rate used was 1 °CÆmin )1 during heating, while the cooling rate was approximately 5 °CÆmin )1 . To ensure that a powder pattern was obtained, the capillary was rotated with an angular velocity of 1.26 radÆs )1 using a purpose-built device. To minimize the exposure time we used a sampling time of 10 sÆmin )1 and the shutter was closed during the residual 50 s. The space group of the structure of the cubic liquid crystalline phase was achieved from the locations of the peaks in the SAXD diffraction patterns [13]. For the two space groups identified in this work (Pn3m and Ia3d)the first four Bragg reflections appear at spacing ratios of Ö2:Ö3:Ö4:Ö6andÖ6:Ö8:Ö14 : Ö16, corresponding to the Miller indices (110), (111), (200), (211) and (211), (220), (321), (400), respectively. The cubic cell lattice parameter, a, is obtained as the slope of a plot of the reciprocal spacings (1/d hkl ) of the Bragg reflections vs. m ¼ (h 2 + k 2 + l 2 ) 1/2 , where h, k and l are the Miller indices. Thin layer chromatography (TLC). The SAXD samples were checked for purity and radiation damage by thin layer chromatography (TLC) in a mixture of hexane, diethyl ether and acetic acid in the ratio 80 : 20 : 1 (v/v/v), using a 20 cm long and 0.2 mm thick silica plate (Fluka). Spots were developed by spraying the plate with 2.5% (v/v) phospho- molybdic acid in sulphuric acid solution (PMOS), followed by heating. The appearance of two unidentified spots and another corresponding to oleic acid showed that there was some degradation of MO. However, comparison with unexposed samples show that this very small degradation is not a result of the exposure to X-rays, but rather a result of the presence of water which leads to hydrolysis of MO [14]. Results Cubic phases Cubic liquid crystalline phases with a number of different structures have been reported [13,15, and references therein]. These are usually grouped into two main types, namely the discrete and bicontinuous structures. The discrete structure is also known as the micellar type, since it is constructed from micellar entities, normal or reversed, arranged in a cubic lattice. In cubic phases of the bicontinuous category a surfactant bilayer is associated with an infinite minimal periodical surface resulting in a structure that is continuous with respect to both water and amphiphile. These bicontin- uous phases can also be of the normal or reverse kind [16]. Within the two main groups of cubic structures several spatial arrangements are possible, leading to a number of different space groups [13]. In the binary system of MO/water at 25 °C two reversed bicontinuous cubic phases, belonging to the space groups Ia3d and Pn3m, are present [17,18]. Upon addition of small amounts of OG the Pn3m structure is transformed via Ia3d to an L a structure. From Fig. 1 it can be inferred that, at 25 °C, the molar ratios between OG and MO (calculated on the total lipid concentration) needed for inducing the Ia3d and lamellar phases are in the order of 1 : 90 and 1 : 16, respectively. Figure 2 shows partial phase diagrams, where the OG concentration is plotted against temperature at a constant MO concentration of 57.8 wt %. The compositions of the samples studied are found in Fig. 1 (located on an almost Fig. 1. Section of the ternary MO/OG/ 2 H 2 O phase diagram at 25 °C, showing the location of the samples studied. Pn3m, MO-rich cubic phase of space group Pn3m; Ia3d, MO-rich cubic phase of space group Ia3d; L a + Ia3d, lamellar + cubic two-phase area; L a , tentative boundary of the lamellar phase [12]. Ó FEBS 2003 Thermal behaviour of MO-rich MO/OG cubic phases (Eur. J. Biochem. 270)57 straight line in the figure). We assume that a heating rate of 1 °CÆmin )1 is slow enough to allow for the phase-transitions to occur, while the cooling rate is set by the equipment. The cooling was performed to determine whether the initial structure would reappear on a reasonable time scale, since cubic structures are prone to be meta-stable. Table 1 summarizes heating scans of SAXD measurements at different compositions (A–E) with the corresponding lattice parameters. It can be concluded that the general behaviour upon heating, in this area of the phase diagram, is to shift the aggregate structure towards a more curved surface in the liquid crystalline phase (Fig. 2A). Moreover, the lattice parameters of both cubic phases tend to decrease with increasing temperature. For the Ia3d phase a increases with the OG content, while for the Pn3m structure it seems to decrease compared to the structure in the binary system [4]. An additional, very strong, peak is observed for sample A, located between (110) and (111) peaks in the Pn3m pattern in the temperature range 25–35 °C (Figs 3 and 4B). Extra peaks are also observed for samples B (weak) and C (medium) that are located between (220) and (321) peaks in the Ia3d pattern at temperatures 25–30 °C and 20–35 °C, respectively. We have not observed any optical anisotropy in this temperature range for these samples. Upon cooling, Fig. 2. Temperature vs. composition for the samples A to E as deter- minedbySAXD.(A) Heating scans at a rate of 1 °Cmin )1 ; (B) cooling scans at a rate of approx. 5 °CÆmin )1 . The MO concentration is approx.58wt%.(d) Pn3m, (black circle containing white cross) Pn3m + additional peak, (s) Ia3d, (white circle containing black dot), Ia3d + additional peak, (shaded circle) Ia3d + Pn3m,(shaded triangle) Ia3d +L a . Table 1. Sample composition and the lattice parameter, a, obtained upon heating scans. Heating rate 1 °CÆmin )1 . To ensure that a powder pattern was obtained, the capillary was rotated with an angular velo- city of 1.26 radÆs )1 . Sample wt % MO/OG/ 2 H 2 O T (°C) Pn3m (nm) Ia3d (nm) La (nm) A 57.81/0.34/41.85 20 9.66 30 9.64 40 9.50 45 9.20 B 57.81/1.31/40.88 21 14.29 30 14.31 40 13.93 50 13.91 55 8.68 13.81 60 8.46 65 8.48 C 57.44/2.54/40.02 22 15.55 31 15.55 41 14.99 45 15.15 D 57.7/3.37/38.93 21 16.16 4.86 25 16.23 4.81 35 16.08 4.79 40 15.46 4.71 45 14.31 4.62 50 14.90 E 57.95/4.20/37.85 21 18.25 4.86 25 17.92 4.88 35 17.86 4.84 45 15.55 4.69 50 15.55 4.64 55 14.31 F 44.77/0.30/54.93 21 9.78 30 9.84 40 9.68 45 9.38 G 44.67/1.14/54.19 21 10.89 30 10.76 40 10.52 45 10.03 H 44.54/2.29/53.17 21 11.12 17.15 30 17.30 40 17.06 45 16.95 58 G. Persson et al.(Eur. J. Biochem. 270) Ó FEBS 2003 these metastable structures do not occur in these three samples, and no unexpected peak occurs either in the heating scans of samples D and E. The origin of the extra peaks observed is unclear and we can only speculate at their origin. One plausible explanation is that they result from another cubic Fig. 3. Diffractograms of the samples (A–E) shown in Fig. 1. (a) Heating rate of 1 °CÆmin )1 , (b) cooling rate of approx. 5 °CÆmin )1 . Ó FEBS 2003 Thermal behaviour of MO-rich MO/OG cubic phases (Eur. J. Biochem. 270)59 structure. The energy needed for the Pn3m–Ia3d phase transition is very small [3]. A peculiarity was observed upon cooling the samples C and D where Pn3m and Ia3d patterns coexist instead of the Ia3d and lamellar patterns observed in the heating scans (Fig. 3 and Table 2). We have not investigated whether a lamellar structure will appear at sufficiently long waiting time. All these observations are most probably caused by the notorious problem of reaching a true thermodynamic equilibrium for bicontinuous cubic phases [13]. Fig. 3. (Continued). 60 G. Persson et al.(Eur. J. Biochem. 270) Ó FEBS 2003 In samples D and E a rather drastic broadening in the line shape of the peaks indexed to the Ia3d structure occurs just below 40 °C, indicating the occurrence of a more disordered structure. Also, the lattice parameter jumps to a smaller value in this region (cf. Figure 3 and Table 1). Finally, visual as well as microscope observations, show differences in the optical appearance of the two cubic phases, one being slightly turbid (Pn3m), while the other one is completely clear (Ia3d). As a final point it could also be mentioned that the transition enthalpies between the different cubic structures are so small that they are very difficult to observe by conventional DSC [3]. Cubic phases in excess water In the binary system of MO/water only the Pn3m cubic phase is found in equilibrium with excess water. At 25 °C this structure is retained upon addition of small amounts OG (samples F–H). The composition of the excess aqueous solution has not been determined, but from the ternary phase diagram it is obvious that the aqueous phase contains very little OG, referred to hereafter as excess water [12]. As in the previous paragraph, Fig. 1 shows the sample locations in the ternary system, while the compositions and corres- ponding lattice parameters for the heating and cooling scans are summarized in Tables 1 and 2. Upon heating, the Pn3m structure is retained in sample F and G, while in sample H the structure is changed to an Ia3d space group (Figs 5 and 6). However, upon cooling the structure returned to the Pn3m structure at 35–30 °C, which is a higher transition temperature than obtained from the heating scan, again pointing to the difficulty of obtaining a true thermodynamic equilibrium for bicontinuous cubic phases. Fig. 4. Diffractogram of sample A showing (A) without and (B) with the extra peak. The dots above the peaks indicate the calculated positions obtained for the Pn3m structurebasedonthefirstpeakinthedif- fractogram. Table 2. Sample composition and the lattice parameter, a, obtained upon cooling scans. Cooling rate approx. 5 °CÆmin )1 . To ensure that a powder pattern was obtained, the capillary was rotated with an angular velocity of 1.26 radÆs )1 . Sample wt % MO/OG/ 2 H 2 O T (°C) Pn3m (nm) Ia3d (nm) La (nm) A 57.81/0.34/41.85 45 9.15 40 9.09 30 9.12 25 9.11 B 57.81/1.31/40.88 64 8.40 54 8.44 47 8.55 33 8.64 30 8.74 13.19 26 9.00 13.91 20 14.16 C 57.44/2.54/40.02 45 15.22 39 15.27 34 9.71 15.08 20 9.71 15.13 D 57.7/3.37/38.93 45 15.04 40 9.67 14.66 34 9.55 14.93 28 9.62 14.84 23 10.30 15.31 18 15.50 E 57.95/4.20/37.85 52 14.43 43 14.43 31 14.56 25 14.49 18 14.95 4.76 F 44.77/0.30/54.93 44 9.23 40 9.19 30 9.32 21 9.58 G 44.67/1.14/54.19 45 9.83 41 9.89 31 9.75 19 9.90 H 44.54/2.29/53.17 43 16.75 40 16.69 34 10.66 16.69 29 10.68 15.31 18 10.48 Ó FEBS 2003 Thermal behaviour of MO-rich MO/OG cubic phases (Eur. J. Biochem. 270)61 Similarly to the results obtained for the Ia3d cubic phase the lattice parameters for the Pn3m cubic phase in equilibrium with excess water increase with the OG content. Furthermore, as for the cubic phases without excess water the lattice parameters tend to decrease with increasing temperature. Also, for the samples with excess water, additional peaks appear at specific temperature intervals upon heating, while they are absent when cooling. The temperature range of these extra peaks lies between 30 and 40 °C for sample G and H, while sample F shows a wider temperature range. The additional peak is quite strong in samples F and G positioned between (110) and (111) in the Pn3m pattern, while in sample H it is very weak occurring between (220) and (321) in the Ia3d pattern. Discussion The aim of this work is to investigate how the structure and thermodynamic stability of the cubic phases formed by MO in water is affected by the presence of OG, which is commonly utilized to solubilize or extract membrane proteins from native membranes. The OG micellar solution containing the membrane protein may hold quite a high concentration of the surfactant. In this study we have shown that the cubic phases in fact are very sensitive to relatively small amounts of OG. An increase in the OG concentration results in a decrease in the absolute value of the curvature of the lipid bilayer, building up the cubic phase structure. This conclusion is drawn from the observation that upon addition of OG an increase in the size of the unit cell within a one- phase area arises, and from the order of occurrence of the phases formed. The reason for this change in the curvature can be understood in terms of a simple geometric considera- tion of the shape of the molecules involved [19–21]. MO, forming reversed nonlamellar phases, is considered to be wedge shaped having quite a small glycerol head group, while the C18 hydrocarbon chain is rather bulky with its double bond between C9 and C10. On the other hand, the highly water soluble OG has a more conical shape with the base located at the glucose head group and a relatively short hydrocarbon chain with only eight carbons, yielding a packing parameter of less than one at high water contents [21]. The hydrocarbon chain of OG is of similar length as the distance from the head group to the double bond in MO. Thus, for an OG molecule present in the MO bilayer its head group will be located at the bilayer interface together with the MO head groups, while the hydrocarbon tail of OG penetrates deeper into the bilayer. It will reach approximately down to the MO double bond. The combined MO-OG Ômolecular entityÕ will attain a shape of a cylinder, resulting in a packing parameter that is closer to one. Moreover, the flexibility possessed by a bilayer containing MO only will be reduced, resulting in a preference for an arrangement of a ÔflatÕ bilayer, eventually forming an L a phase. Similarly, the effect of an increase in temperature on the phase behaviour can be understood with this simple model based on the molecular shape. Within a region of a cubic phase in the diagram an increase in the temperature generally results in a decrease in the size of the unit cell. This is interpreted to be an effect of an increase in the curvature of the lipid bilayer affected by the increased mobility of the hydrocarbon chains together with a possible decrease in the hydration of the glucoside head groups (cf. the nonionic alkylethylenoxide surfactants [22]), i.e. the lipid molecules will attain a more wedge-like shape. To put it another way, with increasing temperature the bilayer gets thinner, with the minimal and the parallel surfaces of the polar/nonpolar interface approaching each other, again resulting in an increased wedge shape of the lipids. Previously, Ai and Caffrey investigated the effect of a different sugar lipid, n-dodecyl-b- D -maltoside (DM), on MO cubic phases [23], and it seems useful to compare our results with their study. They showed that the addition of DM converts the Pn3m cubic phase to an L a phase via an Ia3d structure. For both OG and DM the order in the phases formed upon addition of the surfactants is similar. However, on a molar basis, the stability of the liquid crystalline phases is higher for additions of OG than of DM, as can be seen when comparing the fraction of each surfactant necessary to induce phase transitions. At 25 °C, the ratios between the sugar surfactant and MO, where an Ia3d structure was formed, is in the order of OG/MO ¼ 1 : 90 and DM/MO ¼ 1 : 170, while for the lamellar structure the appropriate fractions between the surfactant and MO are OG/MO ¼ 1:16or DM/MO ¼ 1 : 21. Note however, that these ratios refer to total concentrations and not to the actual fraction of OG or DM in the bilayer, since there is a large difference in critical micellar concentration for these surfactants (25 m M for OG [24]; 0.15 m M for DM [25]). Therefore, it is fair to assume that there is somewhat less OG incorporated in the MO bilayer than these ratios indicate. However, this is probably of minor importance in a comparison of the effect between the two sugar surfactants. Furthermore, the cubic Ia3d and L a phases containing OG have a higher thermal stability than if DM is present in the phases, i.e. the temperature at which the phase boundaries shifts towards higher OG concentration (Fig. 2A). Thus, with OG the Ia3d phase is stable up to about 50–55 °C, while for DM the Ia3d-to-Pn3m phase transition occurs at about 40 °C. We have not determined how OG affects the two-phase area of water/Pn3m.The effect of OG and DM on the phase behaviour of the MO- system can be explained by a consideration of the effective packing parameter, resulting upon addition of the sugar surfactant. The maltoside head group is a disaccharide and therefore the DM head group is larger than that of OG with a monosaccharide head group, but because the hydrocar- bon tail is longer for DM the packing parameter is closer to one than for OG. This difference is also reflected in the binary phase diagrams of DM/water [26] and OG/water [27]. In the DM/water system only micelles and a lamellar phase is present, while in the OG system both hexagonal and bicontinuous cubic structures as well as micelles and a lamellar phase are formed at room temperature. When OG is present in the MO bilayer, the shorter tail will only affect the part of the MO molecule from the head group to the double bond, leaving the rest of the MO hydrocarbon chain free, while introducing DM into the bilayer will affect the packing more, as the longer tail will reach approximately to the centre of the bilayer. Thus, considering only the hydrocarbon tail, addition of DM should not be very different from adding another MO molecule, but the properties of the head groups are slightly different and must also be considered. A study of the effects of maltose 62 G. Persson et al.(Eur. J. Biochem. 270) Ó FEBS 2003 Fig. 5. Diffractograms of the samples with excess water (F–H) shown in Fig. 1. (a) Heating rate of 1 °CÆmin )1 ; (b) cooling rate of approx. 5 °CÆmin )1 . Ó FEBS 2003 Thermal behaviour of MO-rich MO/OG cubic phases (Eur. J. Biochem. 270)63 and glucose on the Pn3m phase of the MO system shows that both sugars are tolerated to rather high concentrations, but the lattice parameter decreases with increasing sugar concentration. The lattice parameter decreases more rapidly for maltose [23], indicating an effect on the hydration of the cubic phase to a larger extent than for glucose. It is thus obvious that the longer hydrocarbon chain and the larger head group of DM are the causes for the stronger effect on the phase behaviour for this surfactant than for OG. Therefore, DM exhibits a larger effect at low concentrations, and shows a stronger influence on the temperature depend- ence on the cubic Ia3d and the lamellar phases than OG. Conclusions and final remarks In this report, we have investigated the thermal behaviour of the MO-rich cubic phases found in the ternary phase diagram of MO/OG/ 2 H 2 O. It is shown that only small amounts of OG (OG : MO ¼ 1 : 16) are sufficient to transform the cubic structure to a lamellar one. Addition of OG to the Pn3m cubic phase converts it to an Ia3d structure in analogy with previous results on the MO system containing a different sugar surfactant [23]. The results obtained in this study on the stability of the cubic phases in the ternary lipid system may be of importance for getting a better understanding of the crystallization process of membrane proteins. In particular, it is of great importance to realize that for the OG-solubilized proteins added to the cubic phase, the OG content is limited to a few percent to keep a stable cubic phase. 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(1986) Aqueous solution properties of nonionic n-dodecyl b- D -maltoside micelles. J. Phys Chem. 90, 4581–4586. 27. Nilsson, F. & So ¨ derman, O. (1996) Physical-chemical properties of the n-octyl b- D -glucoside/water system. A phase diagram, self- diffusion NMR, SAXS study. Langmuir 12, 902–908. Supplementary material The following material is available from http://www. blackwellpublishing.com/products/journals/suppmat/EJB/ EJB3361/EJB3361sm.htm Table 1. Structures, indices and spacings obtained for some temperatures during heating. Ó FEBS 2003 Thermal behaviour of MO-rich MO/OG cubic phases (Eur. J. Biochem. 270)65 . n-octyl-b- D -gluco- side; monoolein -rich cubic phases; thermal behaviour. Access to the complete structure of membrane proteins is one of the cornerstones in obtaining a better. (cf. the binary MO/water- system) . In this report, the thermal behaviour of such cubic phases rich in MO has been investigated by X-ray diffrac- tion. The