Spectral studies of protonated and anionic forms of porphyrins with an asymmetric substitution system

The directed synthesis of asymmetrically substituted porphyrins—tetraphenylporphyrine derivatives containing amino acid residues as a functional group, which can be used as “anchor” groups, for incorporation into the structure of a protein molecule, was carried out. The obtained compounds were characterized by a number of spectral methods confirming their structure and purity. Their base and acid ionization constants in the AN–HClO₄ and DMSO–potassium cryptate (KOH[222])—systems were measured by spectrophotometric titration. It was found that the asymmetric substitution architecture contributed to the stabilization of the protonated forms of porphyrins. This allowed, for the first time, to extract and spectrally characterize the mono- and doubly-protonated forms (H₃P⁺ and H₄P²⁺) of each ligands in the AN–HClO₄ system. The analysis of the spectral changes of the monoamino derivative led to identify three stages of the protonation, which first involves the nitrogen atom of the periphery substituent (pK_b = 13.26) and then the central nitrogen atoms of the macrocycle (pK_b1 = 11.50; pK_b2 = 9.65; pK_b1,2 = 21.15). The relative stability of the first intermediate is assumed to be caused by charge delocalization. The electron-donor nature of the solvent in the DMSO–KOH [222] system led to the leveling of the ionization constants for the first and second stages, which allowed us to determine only the total values for the porphyrins studied. The successive stages of acid–base interactions were analyzed in the article.

Graphical abstract