bR, N-like and O-like intermediate states of [ 15 N]methionine-labelled wild type and D85N⁄T170C bacteriorhodopsin were accumulated in native membranes by controlling the pH of the preparations. 15 N cross polariza-tion and magic angle sample spinning (CPMAS) NMR spectroscopy allowed resolution of seven out of nine resonances in the bR-state. It was possible to assign some of the observed resonances by using 13 C⁄ 15 N rota