Two-point connected complex of zinc aminoporphyrin with histidine protected at -amino and imidazolyl- nitrogen atoms

Zinc porphyrins possessing an (N,N-dialkylamino)methyl group at the peripheral position were prepared from beta-octaethylporphyrin. The synthetic zinc aminoporphyrins were complexed with an N alpha,Nim(tau)-protected histidine through the coordination of the histidyl imidazolyl-tau nitrogen atom with the central zinc atom and the concomitant hydrogen bonding of the histidyl carboxy group with the peripheral amino group. The 1:1 complexation in the chloroform was dependent on the size of the alkyl groups on the peripheral aminomethyl nitrogen atom. The diethyl substitute complexed more strongly than the dimethyl and dipropyl counterparts. The complexation of zinc (diethylamino)methyl-porphyrin with an N alpha,Nim(pi)-protected histidine was 260-fold stronger than that with the above N alpha,Nim(tau)-protected histidine to exhibit the association constant of 2.6 x 106 M-1. The significant enhancement was ascribable to the less sterically demanding and more coordinating Nim(tau)-atom compared with the Nim(pi)-atom. The achiral zinc aminoporphyrin was complexed with the chiral N alpha,Nim(pi)-protected histidine to give a circular dichroism active species in a visible Soret region. Graphical Abstract A zinc porphyrin possessing a peripheral (N,N-diethylamino)methyl group prepared from beta-octaethylporphyrin was complexed with an N alpha,Nim(pi)-protected histidine through coordination of the less sterically demanding histidyl imidazolyl-tau nitrogen atom with the central zinc atom and concomitant hydrogen bonding of the histidyl carboxy group with the amino periphery. The 1:1 CD-active complexation in the chloroform was significantly large with K = 2,600,000 M-1.