the binding of histidine was relatively retarded in the presence of sulfate anion ( kd = 150 mm for 0.53 m sulfate present ) , and imidazole alone yielded no frequency shift , indicating the binding of the carboxyl group of histidine to the protein cationic site on one hand and a weak charge-transfer interaction between the imidazole group and the heme group on the other .
C0003075
anion
C0038720
sulfate