Cobalt(III) porphyrins of general formula Co(P)(L)2, where P = tetrakis[4-(N-methylpyridiniumyl)]porphine (TMPyP) or tetrakis(4-carboxylatophenyl)porphine (TCPP) and L = amino acid, have been studied in water solution by H-1 NMR spectroscopy. Complexes with the stoichiometry Co(P)(L)2 are the predominant species above pH 7, whereas at lower pH values Co(P)(L)(H2O) species are also present. In the general case, the two amino acid ligands were bound to the cobalt atom through the NH2 group. In the case of histidine, three different complexes with CoTMPyP, two symmetrical and one unsymmetrical, were detected in proportions that varied with the pH of the solution: at pH 7, histidine was bound to cobalt exclusively through imidazole N-3, and at higher pH (pH > 10), it was bound only through the NH2 group. Similar behavior was found for two other amino acids containing two potential ligands: methionine and lysine. The predominant species at pH 10 were the NH2-bound complexes, in both cases. The conformational analysis of these complexes has been performed by using two sets of NMR data, the vicinal interproton coupling constants, J(NH-H-alpha), J(H-alpha-H-beta), and J(H-beta-H-alpha), and the induced shifts, DELTA-delta. All amino acids studied adopt a largely predominant geometry characterized by a nearly eclipsed conformation around the N-C-alpha bond and conformation I around C-alpha-C-beta. This geometry allows the side chain of the amino acid to interact with the porphyrin macrocycle by either stacking (aromatic amino acids), hydrophobic (e.g. leucine), or electrostatic (e.g. aspartic acid) interactions. Existence of the last interactions were confirmed by the conformational analysis of the complexes of the same amino acids with the anionic porphyrin CoTCPP, which revealed that only the polar amino acids aspartic acid, serine, and asparagine had significantly different geometries in the two types of complexes. Comparison of the constant K for the "open form" arrow-pointing-both-left and right "closed form" equilibrium in the free and complexed amino acids has shown that the gain of stability -DELTA-DELTA-G-degrees of the closed form upon complexation increases in the order His < TyrO- < Phe < TyrOH < Trp. Temperature dependence of DELTA-DELTA-G-degrees values indicates that the enthalpy change contributes to the stabilization due to the stacking interaction in the case of the aromatic amino acids.
