The efficiency of porphyrins in sensitizing the photooxidation of amino acids was found to be dependent upon the presence and upon the nature of a complexed metal ion and upon the nature of the side chains which protrude from the tetrapyrrolic core of the molecule. In particular, chlorophyll a, which contains the diamagnetic ion Mg2+, causes a very fast photooxidation of tryptophan, methionine, histidine, and tyrosine in aqueous solution, pH 6.1; on the contrary, under the same condit'ons hemin, which contains the paramagnetic ion Fe3+, is devoid of photodynamic activity. Moreover, hematoporphyrin, which contains no metal ion, acts selectively on the amino acids tryptophan and methionine both in aqueous solution, in the pH range 2.5-6.5, and in 5-100% acetic acid solution. Since tryptophan is no longer photoreactive when the α-amino group is protected, the hematoporphyrin-sensitized photooxidation can be utilized for the specific modification of the methionyl residues in a polypeptide molecule. The feasibility of the method in the case of proteins was tested with hen's egg-white lysozyme. Irradiation of the enzyme in the presence of hematoporphyrin and in neutral aqueous solution results in a reduction of the enzymic activity to about 54%; amino acid analysis showed that this partially active product contains one methionine sulfoxide residue per protein molecule: this uniquely photoreactive residue was identified as methionine-12. The other methionyl residue which is present in lysozyme, i.e., methionine-105, is not susceptible of photooxidation, unless the conformation of the protein molecule is modified by running the irradiation in concentrated acetic acid solution. The different photoreactivity of the two methionyl residues has been interpretated on the basis that methionine-105 is less accessible to the solvent than methionine-12 in the native lysozyme. © 1969, American Chemical Society. All rights reserved.
