Porphyrin–tyrosine interactions: Photo‐CIDNP and NMR studies

Abstract

Strong 'H and I3C nuclear polarizations are observed when aqueous solutions of synthetic water-soluble porphyrins are irradiated in the presence of tyrosine and some of its derivatives. These polarizations are strongly pH dependent. Evidence is also shown for the formation, in the dark, of a complex between the reactants. The association constants are evaluated from the NMR chemical shifts on the aromatic tyrosine protons induced by the presence of the porphyrins. The nature of the intermediate radical pair generating the CIDNP effects is discussed. An electron-transfer reaction from tyrosine to the excited triplet state of the porphyrin, or within the porphyrin-tyrosine excited complex, is expected to be the primary step in the reaction. It is followed by subsequent proton transfer within the initial ion radical pair. The spin polarizations arise principally from the back-transfer step, as the reactants are the only products which are polarized.