Fig. 2. Light-induced difference absorption spectra of the WT-KR2 (black curves) and 19 P219X mutants (colored curves) of KR2 in the presence of 500 mM hydroxylamine.

Positive and negative signals show the spectra before and after illumination corresponding to those of the rhodopsin and retinal oxime, respectively. Red and blue curves represent red- and blue-shifted mutants, respectively. Mutant and WT-KR2 spectra were normalized by use of a negative peak at 361 nm.