Pressure-Induced Transmembrane αII- to αI-Helical Conversion in Bacteriorhodopsin:  An Infrared Spectroscopic Study

Decreases in the infrared spectroscopic amide A frequencies from 3308 to 3291 cm -1 of native purple membrane as a function of pressure are consistent with a gradual α II - to α I -helical conversion for the transmembrane helixes of bacteriorhodopsin. This structural transition reaches completion near 3.7 kbar. Only negligible frequency shifts are observed, however, as a function of pressure in the 1660 cm -1 amide I region, suggesting that additional factors such as hydrogen bonding and helix−helix interactions are important in modulating the frequency of this mode. The pressure dependence of the feature at 2927 cm -1, arising from a Fermi resonance couplet originating from both protein and lipid methyl groups, indicates that global environmental changes occur near 4 kbar.