Only water‐exposed carboxyl groups are protonated during the transition to the cation‐free blue bacteriorhodopsin

FTIR (Fourier‐transform‐infrared) difference spectra between cation‐free blue and purple bacteriorhodopsin were recorded. The results indicate that during the blue to purple transition no isomerization of the chromophore takes place. It is further observed that approx. 14 water‐exposed carboxyl groups ofamino acids are protonated in blue bacteriorhodopsin. The groups were deprotonated during the blue to purple transition. Protonation of carboxyl groups in the interior of the protein, which are not accessible to water, can be excluded.