PHOTO-DECARBOXYLATION OF IRON(III) PORPHYRIN AMINO-ACID COMPLEXES IN AQUEOUS-SOLUTION

Irradiation (lambda > 390 nm) of acidic anaerobic aqueous solutions of iron(III) tetrakis(2-N-methylpyridyl)-porphyrin, in the presence of m ono- or di-basic amino acids or their N-acylated derivatives, generate s the iron(II) porphyrin and the corresponding acyloxyl radical; subse quent decarboxylation of the latter gives the corresponding ammonioalk yl or amidoalkyl radical, The rate and course of each reaction have be en monitored by UV-VIS spectroscopy and EPR spin-trapping techniques. The large differences in the observed rates of Iron(II) porphyrin form ation are controlled by two factors: the binding affinity of the carbo xy group for the iron(III) porphyrin to form the photoactive complex a nd the competitive reactions of the acyloxyl radical [decarboxylation and regeneration of the iron(III) porphyrin complex], following photol ysis. With cationic carboxylate ligands, such as glycine or L-alanine, charge repulsion with the cationic porphyrin results in the former ef fect predominating, whereas with neutral ligands, for example N-acetyl glycine, it is the latter that determines the overall rate of reaction . In aqueous base, the amino acids ligate to the iron(III) porphyrin v ia the amino rather than the carboxy group. Subsequent irradiation bri ngs about an electron transfer from the ligand to give the iron(II) po rphyrin and the amino acid cation-radical which reacts further to give an alpha-amino radical either by decarboxylation or proton loss.