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

Zinc porphyrins possessing an (N,N-dialkylamino)methyl group at the peripheral position were prepared from β-octaethylporphyrin. The synthetic zinc aminoporphyrins were complexed with an Nα,Nim(τ)-protected histidine through the coordination of the histidyl imidazolyl-τ 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 Nα,Nim(π)-protected histidine was 260-fold stronger than that with the above Nα,Nim(τ)-protected histidine to exhibit the association constant of 2.6 × 106 M−1. The significant enhancement was ascribable to the less sterically demanding and more coordinating Nim(τ)-atom compared with the Nim(π)-atom. The achiral zinc aminoporphyrin was complexed with the chiral Nα,Nim(π)-protected histidine to give a circular dichroism active species in a visible Soret region.