EVIDENCE OF LOCAL CONFORMATIONAL FLUCTUATIONS AND CHANGES IN BACTERIORHODOPSIN, DEPENDENT ON LIPIDS, DETERGENTS AND TRIMERIC STRUCTURE, AS STUDIED BY C-13 NMR

We examined how the local conformation and dynamics of [3-C-13]Ala-lab eled bacteriorhodopsin (bR) are altered as viewed from C-13 NMR spectr a when the natural membrane lipids are partly or completely replaced w ith detergents. It turned out that the major conformational features o f bR, the alpha(II)-helices, are generally unchanged in the delipidate d or solubilized preparations. Upon partial delipidation or detergent solubilization, however, a significant conformational change occurs, a scribed to local conversion of alpha(II)-->alpha(I)-helix tone Ala res idue involved), evident from the upfield displacement of the transmemb rane helical peak from 16.4 ppm to 14.5 ppm, conformational change ton e or two Ala residues) within alpha(II)-helices from 16.3 to 16.0 ppm, and acquired flexibility in the loop region (especially at the F-G lo op) as manifested from suppressed peak-intensities in cross-polarizati on magic angle spinning (CP-MAS) NMR spectra, On the other hand, forma tion of monomers as solubilized by Triton X-100, Triton N-101 and n-do decylmaltoside is characterized by the presence of a peak at 15.5 ppm and a shifted absorption maximum (550 nm). The size of micelles under the first two conditions was small enough to yield C-13 NMR signals ob servable by a solution NMR spectrometer, although C-13 CP-MAS NMR sign als were also visible from a fraction of large-sized micelles. We foun d that the 16.9 ppm peak (three Ala residues involved), visible by CP- MAS NMR, was displaced upfield when Schiff base was removed by solubil ization with sodium dodecyl sulfate, consistent with our previous find ing of bleaching to yield bacterioopsin. (C) 1998 Elsevier Science B.V . All rights reserved.