Fourier transform infrared spectroscopic investigation of rhodopsin structure and its comparison with bacteriorhodopsin.

FT-IR spectroscopy has been used to investigate the conformation of rhodopsin in bovine rod outer segment membranes, dispersed in aqueous suspension in both 2H2O and H2O. Detailed analysis of the amide I band was made, using second-derivative and deconvolution procedures. The frequency of the major amide I component is consistent with the presence of predominantly alpha-helices within the rhodopsin structure. A spectroscopic change occurs at acidic pH with the membranes in both 2H2O and H2O. The results for the membranes dispersed in H2O at pH 7 were used to estimate a value of 0.67 for w (amide II/amide I intensity ratio in H2O). This value of w gives an estimate of the unexchanged amide protons, in rhodopsin, of 51%. The extent of amide proton exchange at acidic p2H (p2H 5 and 2), in 2H2O was also determined. The conformation of rhodopsin in its unbleached and bleached states was investigated but no significant difference in the secondary structure was observed. A comparison, after second-derivative and deconvolution analysis, of the spectra of rhodopsin with that of bacteriorhodopsin shows that both proteins exhibit a similar number of amide I components. However, with bacteriorhodopsin the amide I band occurs at a higher frequency. Bacteriorhodopsin under similar conditions, in 2H2O, has 20% more unexchanged amide protons than does rhodopsin.