Quantitative characterization of the structure of rhodopsin in disc membrane by means of Fourier transform infrared spectroscopy.

Fourier transform infrared (FTIR) spectroscopy has been used for the detailed characterization and quantification of the secondary structure of bovine rhodopsin in native disc membranes. FTIR spectra were obtained in aqueous media, both in 1H2O and in 2H2O. Analysis of spectra by means of Fourier self-deconvolution, complemented with maximum likelihood restoration and Fourier derivative, has allowed the characterization of major amide I secondary structure-sensitive component bands of structural relevance which had not been detected before. In consequence, we show a richer secondary structure for rhodopsin than previously described. Our results indicate a total regular helix content around 51%, which would include not only the main alpha 1-type helix but also 3(10)-like helix. The presence of distorted helicoid sequences might furthermore increase to a certain extent the total helix amount. It is also indicated that a significant proportion of the amino acid residues are involved in extended/beta-structures and in reverse turns, as well as in "random" segments, which had not been directly demonstrated before. 61 +/- 4% of rhodopsin is determined to be solvent-accessible, which is a substantially higher value than previously reported. Helices account for most of the inaccessible moiety.