Photodynamic cross-linking of proteins -: V.: Nature of the tyrosine-tyrosine bonds formed in the FMN-sensitized intermolecular cross-linking of N-acetyl-L-tyrosine

Some types of proteins are intermolecularly cross-linked via the formation of Tyr-Tyr (dityrosine) bonds on illumination in the presence of certain kinds of photosensitizers. The study of reactions of this type is of importance since they appear to be involved in several processes of biomedical importance such as the photosensitized (photodynamic) treatment of tumors, pathological effects of sunlight (skin photoaging, induction of cataracts), etc. The mechanisms involved in the photosensitized formation of dityrosine cross-links are not clear, and the chemical nature of the cross-link(s) is not known. In the present work, these problems have been studied using N-acetyltyrosine as a model substrate. This compound was illuminated under aerobic conditions at pH 6.0 with FMN as the photosensitizer. The Tyr-Tyr cross-linked products were isolated and purified by a preparative silica gel 60 column and by preparative reverse phase HPLC. Three main cross-linked products, designated as 1 (C6, C6'-linked di-(N-acetyltyrosine)), 2 (C6, 07'-linked di-(N-acetyrtyrosine)), and 3 (C6, C4'-linked di-(N-acetyltyrosine)), were identified, and their chemical structures were determined by MS, two-dimensional NMR spectral methods, and other standard techniques. Detailed reaction pathways for the formation of the carbon-carbon (1 and 3) and carbon-oxygen (2) coupled products were proposed. (C) 2000 Elsevier Science S.A. All rights reserved.