Lanyi (1990Lanyi ( , 1991a postulated an irreversible transition the FTIR spectra occur between M 2 and M G . This between M 1 and M 2 . This could not, however, hitherto suggests, that the tertiary structural changes between be unambiguously proven experimentally, although the M 1 and M 2 are responsible for the switch opening the presence of two M intermediates (M 1 and M 2 ) had been cytoplasmic half-channel of BR for reprotonation to deduced much earlier from spectroscopic measurements complete the catalytic cycle. These tertiary structural (Korenstein et al., 1978). changes seem to be triggered by a charge redistribution UV-VIS (Varo and Lanyi, 1991b;Varo et al., 1992; which might be a common feature of retinal proteins Zimanyi et al., 1992) and Fourier transform infrared also in signal transduction.(FTIR) spectroscopy (Ormos, 1991;Perkins et al., 1992) Keywords: conformational changes/hydration/ revealed a transition from M 1 to M 2 by a slight shift in M intermediates/photocycle/proton pumping the absorption maximum of the M intermediate and changes in the amide-I region (1650-1670 cm -1 ), respectively. Later, the FTIR results were, however, re-interpreted as giving no evidence for a M 1 to M 2 transition, due to