Fourier transform infrared spectroscopy at a resolution of 1 cm-1has been used to study the conformation of dark-adapted bacteriorhodopsin in the native purple membrane, in H2O and D2O suspensions. A detailed analysis of the amide I bands was made using derivative and deconvolution techniques. Curvefitting results of four independent experiments indicate, after estimation of the methodological errors, that native bacteriorhodopsin contains 52-73% α-helices, 13-19% reverse turns, 11-16% β-sheets, and 3-7% unordered segments. Our analysis has enabled the identification of several components corresponding to α-helices, β-sheets, and reverse turns. Besides the αIand αII-helices (peaking at 1658 and 1665 cm-1), we propose that two more infrared bands arise from α-helical structures: one at 1650 cm-1from αIand another one at 1642 cm-1 in H2O suspension, which could originate from type III β-turns (i.e., one turn of 310-helix). The relatively high content of reverse turns suggests the presence of one reverse turn per loop, plus another one in the C-terminal segment. On the other hand, several reasons argue that the calculated mean β-sheet content of around 14% should be decreased somewhat. These β-sheets could be located in the noncytoplasmatic links of the bacteriorhodopsin molecule. © 1992, American Chemical Society. All rights reserved.