Protonation–deprotonation equilibria in tetrapyrroles. Part 1. Protonation titrations of 132-(demethoxycarbonyl)pheophytin a in methanolic hydrochloric acid by electronic absorption spectroscopy

Abstract

Spectrophotometric protonation titrations have been performed for 13²-(demethoxycarbonyl)-pheophytin a(1) using HCl as the acid and methanol as the solvent. The reversible formation of four spectroscopically different protonated species was observed. This is interpreted as indicating the formation of four different N-protonated cations: the mono-, di-, tri-and tetra-cations. The UV–VIS spectrometric parameters are given for each protonated species. The following pK values were obtained: pK₃=+4.14, pK₄=+2.06, pK₅=+0.30 ± 0.15 and pK₆=–0.34 ± 0.04. As four pyrrolic N-protonations have not been reported previously for any tetrapyrrole, the results are rationalized in terms of the very unsymmetrical structure of pheophytin 1 and the flexibility of its macrocycle to conformational distortions. On the basis of comparative protonation titrations with HCl in acetic acid, it could be deduced that solvent plays an important role in the protonation behaviour of tetrapyrroles. Alternative reaction possibilities in the HCl–methanol system, including C-protonation to the vinyl group or a methine bridge, O-protonation to the 13¹-oxo function followed by totalization, electrophilic substitution at a methine bridge, and aggregation, have been considered but found less likely than the four N-protonations. The reasons for this interpretation are discussed. meso-Chlorination was observed to occur under conditions where there was free access of ³O₂ into the reaction mixture. The most likely mechanism for the meso-chlorination is considered to be electrophilic substitution, where the most reactive molecular species is identified as the N-22 monocation. A new explanation for the regioselectivity of the electrophilic substitution in chlorins and phorbins is presented.