and 1.6% in the 3–5 µm bandpass Alternatively, an object at 1000 K emits 8.9% of its total flux in the 8–12 µm bandpass and 36% in the 3–5 µm bandpass This discussion of infrared emitting objects and which is the better emitter is important to keep in mind as we discuss biological infrared detectors Bacterial Thermoreception Cellular processes are influenced by temperature, and therefore, cells must possess temperature-sensing devices that allow for the cell’s survival in response to thermal changes Virtually all organisms show some kind of response to an increase or decrease in temperature, but sensing mechanisms are not well understood When bacterial cells are shifted to higher temperatures, a set of proteins known as “heat-shock” proteins are induced These proteins include molecular chaperones that assist in refolding proteins that aggregate at higher temperatures as well as proteases that degrade grossly misfolded proteins (12,13) Changes in temperatures can also be sensed by a set of coiled-coil proteins called methyl-accepting proteins (MCPs), that regulate the swimming behavior of the bacterium Escherichia coli (14) Coiled-coil proteins are formed when a bundle of two or more alpha-helices are wound into a superhelix (Fig 6) (15) The MCPs can be reversibly methylated at four or five glutamate residues (16) Methylation and demethylation, it is presumed, is the trigger that dictates the response during temperature that the denaturation and renaturation process i lows cells to adapt quickly to changes in temper shown in Fig 7, the change in the structure of measured by circular dichroic spectroscopy as a of temperature We observed that the thermal u folding is reversible and the protein displayed 10 ery To date, of all the proteins tested by us, TlpA the highest degree of reversibility with respect to mal unfolding transition It is likely that TlpA, MCPs, represent an adaptation of the coiled-coil temperature sensor by coupling its folding and un temperature cues In addition, the ability of short coiled-coil peptides to undergo rapid thermal den and renaturation (Naik and Stone, unpublished tions), suggests that the coiled-coil motif would b for designing new peptide-based thermosensing Snake Infrared Reception The longest and best studied system of biologica sensing is the snake system Snakes from two Crotalidae (pit vipers) and Boidae (boas and pyt sense infrared radiation by using specialized orga crotalines, two infrared pit organs are positioned side of the head between the eyes and upper jaw an array of infrared pit organs line the upper jaw, and the number of pit organs is species spe ability of these organs to detect thermal energy Molar ellipticity at 222 nm (change in protein structure) 5 10 −5 55°C −5 10 −5 25°C −1 10 −4 −1.5 10 −4 −2 10 −4 Figure 6 A cartoon showing the coiled-coil structure of MCP-II from Escherichia coli 75°C 0 100 10°C 0 1 10 10°C 2 3 4 Time (min) 5 Figure 7 Reversibility of the thermal unfolding o Shed pit 0 400 450 500 550 600 Wavelength (nm) 65 Figure 9 Fiber-optic spectrophotometry, visible wav Figure 8 SEM micrograph of IR pit organ surface described by Noble and Schmidt in the 1930s (18) Bullock and co-workers at UCLA further defined this area by their electrophysiological studies in the 1950s His publications from this period continue as the referenced sources for the stated sensitivity of 0.003◦ C for crotaline infrared pit organs (19,20) Hartline continued to further the study of thermoreception in snakes throughout the 1970s, and he wrote a wonderful review article for the layperson in 1982 (21) For more than three decades, the center of snake infrared research has been in Japan based on the work of Terashima and Goris Recently, this group published a book that compiles their research papers from this past decade (22) Much of this previous body of work has been electrophysiological and descriptive using electron microscopy techniques We recently published a detailed examination of the morphology of Boidae infrared pits using both atomic force microscopy (AFM) and scanning electron microscopy (SEM) (23) Our results were consistent with the earlier results of Amemiya et al (24) In both publications, the function of the unique surface morphology that covers the infrared pit organs was speculated about (see Fig 8) This speculation centered on the hypothesis that unwanted wavelengths of light, that is, visible, were being scattered and desired wavelengths of light, that is, infrared, were being preferentially transmitted To prove the speculation about visible light, we conducted a series of spectroscopy experiments to test the spectral properties of infrared pit scales compared to other parts of the snake (Fig 9) This data suggested that the IR pit organ surface microstructure indirectly aids infrared detection by scattering unwanted visible wavelengths of light Using various samples and repeated measurements, there was consistently more than a fourfold reduction in the amount of transmitted visible light This loss of transmission was attributed to scatter due to ments using a helium–neon laser at 632 nm and detector Shed IR pit skin transmission dropped f function of detector distance compared to eye sca mission; this indicated an increased scattering a limited sample absorption The increased visible l ter can be accounted for by using a simple Raylei of scatter and incorporating the micropit dimensi ferent snakes (23) This difference in skin surface morphology a tion of location on the snake is a wonderful ex evolved tissue engineering These unique dimen confined to a few square millimeters within the gan From the standpoint of chemical compositi is no difference, as indicated by FT-IR analysis The FT-IR spectra from shed IR pit skin and shed (eye) skin are identical to the amide bands of ker dominate the absorbance profile Interestingly, regions of high skin transmission correspond to r high atmospheric transmission (3–5 and 8–12 m As mentioned previously, the sensitivity of crot viper) infrared detection, widely stated as 0.003 to the seminal work by Bullock and co-workers ( ever, this value was never measured directly but r trapolated from calculated assumptions Further measured values were determined as water was over the pits—a conductive mode rather than mechanism of heat transfer The function of prey has been studied extensively for these sensors (2 ing this function in mind, we attempted to exa phenomenon of snake infrared reception in the c the thermal radiative transfer among the sensor, background The actual molecular mechanism for infrared function is an active area of research in our g others Several models were proposed by de Cock and based on his work, we sought to construct tive transfer model that would measure the rad of a biological object as a function of distance (2 Cock Buning (27) presented thresholds and corre 0.2 0 0 4 6 8 10 Wavelength (microns) 12 14 Figure 10 FT-IR analysis of shed crotaline skin detection ranges, but this analysis did not take into account the form factor relationships between emitter and detector and ignored the effect of the thermal background from the soil and atmosphere The output from our model is the change in radiant flux ( Q) at the infrared pit organ as a 37◦ C object is moved When this value becomes negative, the object (prey) no longer has a thermal signature greater than the background—essentially, it becomes invisible from an infrared, or thermal perspective What was surprising in this analysis was how quickly the Q value became negative, indicating extremely short detection distances of the order of 50 nm wide) and mesopores (2–50 nm) to micropores (