The basicities of the nitrogen atoms in the porphyrin nucleus; their dependence on some substituents of the tetrapyrrolic ring

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Abstract
Porphyrins have been shown to be stronger bases than previously supposed, two protons being added in two stages to the nitrogen atoms in the conjugated tetrapyrrolic ring as the pH of an aqueous solution is decreased from pH 12 to 0. The existence of mono-cations of certain porphyrins in aqueous solution with characteristic visible and ultra-violet absorption spectra has been established. pK$_{a}^{\prime}$ values for the two more basic ring nitrogen atoms of coproporphyrin I, deuteroporphyrin IX dimethyl ester disulphonic acid, N-methyl coproporphyrin I free acid and its tetramethyl ester have been determined by spectrophotometric titration in various buffer systems. Information on the basicity of uroporphyrin I and mesoporphyrin IX is also included. N-Methyl coproporphyrin has been prepared. The relationship of these pK$_{a}^{\prime}$ values to the thermodynamic dissociation constants is discussed. Comparison of the pK$^{\prime}$ values obtained has led to certain conclusions regarding the effect on its basicity of some substituents of the tetrapyrrolic ring, and possible structures for the free bases and cations are considered in the light of these findings.