PRESSURE-INDUCED TRANSMEMBRANE ALPHA(II)-HELICAL TO ALPHA(I)-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 alpha(II)- to alpha(I)-helical conversion f or the transmembrane helixes of bacteriorhodopsin. This structural tra nsition reaches completion near 3.7 kbar. Only negligible frequency sh ifts 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 292 7 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.