Abstract
Isotopically labeled tyrosines have been selectively incorporated into bacteriorhodopsin (bR). A comparison of the low-temperature bR570 to K Fourier transform infrared-difference spectra of these samples and normal bR provides information about the role of tyrosine in the primary phototransition. Several tyrosine contributions to the difference spectrum are found. These results and comparison with the spectra of model compounds suggest that a tyrosinate group protonates during the bR570 to K transition. This conclusion is strongly supported by the results of UV difference spectroscopy.