We recently published procedures describing the isolation of absolute infrared
spectra for the intermediates of the bacteriorhodopsin (BR) photocycle and from
these, obtaining transitional difference spectra between consecutive intermediates.
In that work, we concentrated mainly on proton-binding centers and the route of
proton transport across the membrane. In the current study, we used isolated spectra
for the amide I, amide II, and amide III envelopes to obtain quantitative
information on the extent of conformational change accompanying each transition in
the photocycle. Our main finding was that most of the conformational changes occur
in the conversion of the M<sub>F</sub> intermediate to N. In our earlier
publication, a new proton acceptor, absorbing at 1650 cm<sup>-1</sup> was
identified, which appeared to accept a proton from Asp96COOH during the
transformation of BR† to L. Below, we present evidence that supports this
interpretation and propose a possible role for this new component.
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