THE ALKALINE TRANSITION OF BIS(N-ACETYLATED) HEME UNDECAPEPTIDE

Alkaline forms of the ferric bis(N-acetylated) heme undecapeptide of c ytochrome c (N-ac-HUP) and some of its derivatives have been studied b y electronic absorption and electron paramagnetic resonance spectrosco pies. Surprisingly, even at pH > 12, no evidence could be found for th e formation of a hydroxyl ion adduct, in direct contrast to a previous report concerning ferric heme peptides encapsulated in detergent mice lles (Mazumdar et al. Inorg. Chem. 1991, 30, 700-705). A spectroscopic ally determined pK(a), of similar to 9 is assigned to the deprotonatio n of the constituent histidine ligand of heme iron in N-ac-HUP. The pr esent findings are not entirely in keeping with those of an earlier st udy concerning the properties of N-acetylated heme octapeptide (Wang e t al. J. Biol. Chem. 1992, 35, 15310-15318), the differences observed being attributed to the buffering media employed in the two investigat ions. The implications of the current results in relation to a better understanding of the alkaline transitions observed in hemoglobins and myoglobins is considered.