Recognition of histidines with a synthetic zinc amino-oxochlorin regioisomer via synergetic coordination and hydrogen bonding

Zinc 13-(N,N-diethylamino)methyl-3,3,7,8,12,17,18-heptaethyl-2-oxochlorin was prepared from 2,3,7,8,12,13,17,18-octaethylporphyrin via the regioselectively acid-catalyzed double dehydration of cis-12,13-diethyl-cis-12,13-dihydroxy-2-oxobacteriochlorin to the corresponding 12-ethyl-13-vinyl-2-oxochlorin. The synthetic zinc amino-oxochlorin was axially ligated with the imidazolyl group of N^α-protected histidines in chloroform, and concomitantly the amino group of the former was hydrogen-bonded with the carboxy group of the latter. The resulting two-point bonding supramolecules served as models for the complexes of chlorophylls with peptides in photosynthetic apparatuses including light-harvesting antennas and charge-separating reaction centers. A similar 1:1 complex was produced by the interaction of the zinc amino-chlorin with the related methionine possessing thioether and carboxylic acid moieties.