PK(A) OF THE PROTONATED SCHIFF-BASE OF BOVINE RHODOPSIN - A STUDY WITH ARTIFICIAL PIGMENTS

Artificial bovine rhodopsin pigments derived from synthetic retinal an alogues carrying electron-withdrawing substituents (fluorine and chlor ine) were prepared. The effects of the electron withdrawing substitute nts on the pK(a) values of the pigments and on the corresponding Schif f bases in solution were analyzed. The data suggest that the apparent pK(a) of the protonated Schiff base is above 16. However, the alternat ive possibility that the retinal Schiff base linkage in bovine rhodops in is not accessible for titration from the aqueous bulk medium cannot be definitely ruled out.