title: Ring-opening photoreactions of 5-methylcytosine with 3-mercaptopropionic acid and other thiols.
abstract: The photoproducts from reaction of thymine with cysteine, an amino acid containing a sulfhydryl group, have been studied in detail, whereas results of less extensive studies have been reported for the uracil-cysteine system. However, products arising from corresponding reactions of cytosine and related compounds with compounds containing a sulfhydryl group have not been similarly studied. We report here the results of our study of the photoreaction of 5-methylcytosine (5MeCyt), a minor base occurring in mammalian DNA, with 3-mercaptopropionic acid (3MP), a model compound for cysteine. We found that this reaction proceeds at pH 7 to yield N-(N'-(2'-carboxyethyl)thiocarbamoyl)-3-amino-2-methylacrylamidine (Ia) as a primary photoproduct. A secondary thermal product, identified as 3-(2'-carboxyethylthio)-2-methylacrylamidine (IIa), appears if photoreacted solution is allowed to stand for appreciable times prior to workup; this latter compound is formed via an intermediate product. Heating of purified Ia at 100°C or standing at lower temperatures produces 3-amino-2-methylacrylamidine (IId); similarly, irradiation of Ia with UVB light in aqueous solution converts it into IId. Results from exploratory studies suggest that 5MeCyt similarly reacts with other thiols (2-mercaptoethanol, 2-mercaptoacetic acid) to form analogs of Ia and IIa. Other preliminary results suggest that 5-methyl-2'-deoxycytidine and 1,5-dimethylcytosine photoreact with 3MP to form compounds similar to Ia.
authors: Department of Pharmaceutical Chemistry, School of Pharmacy, University of California, San Francisco, CA, USA. shetlar@cgl.ucsf.edu
authors: Martin D
authors: MD
authors: Shetlar
authors: Janet
authors: J
authors: Chung
mesh_terms: D015097:3-Mercaptopropionic Acid / Q000737:chemistry*; D044503:5-Methylcytosine / Q000737:chemistry*; D015394:Molecular Structure; D055668:Photochemical Processes
chemical_list: D044503:5-Methylcytosine; D015097:3-Mercaptopropionic Acid