Mixed products of thymine and cysteine produced by direct and acetone-sensitized photoreactions.

Abstract

Abstract. The nature of the major mixed products of the photoreaction of thymine with L‐cysteine has been restudied. Our results, from application of recent advances in separation and spectroscopic technology to this problem, allow extension and require major revision of the published descriptions of the products produced in this system. We have found that the previously described major mixed photoproduct in the system, 5‐S‐L‐cysteinyl‐5, 6‐dihydrothymine, can be readily separated into diasteromers (Ia and Ib) that have quite different chromatographic and NMR spectral properties. Confirming previously published work, 5‐S‐L‐cysteinylmethyluracil (III) was also produced. Major modifications in the published descriptions of other mixed photoproducts produced in the photoreaction of thymine with cysteine are the following. While the product 5‐S‐L‐cysteinylmethyl‐5, 6‐dihydrouracil (II) is indeed produced, it does not correspond in its properties to the compound previously reported as this substance. Our experimental findings also indicate that observations, previously interpreted to imply formation of the disulfide 5‐S, S‐L‐cysteinyl‐5, 6‐dihydrothymine (IV), can instead be explained in terms of the properties of the β‐carbamoylsulfide functionality contained in Ia and Ib. Indeed, both diastereomers of the addition product of L‐cysteine with N‐ethylmaleimide, S‐(l‐ethyl‐2, 5‐dioxopyrrol‐idin‐3‐yl)‐L‐cysteine (VI), as well as 5‐carbamoyl‐4‐thiapentanoic acid (VII), exhibit properties assumed in the previous work to be specific to disulfide linkages; each of these compounds contains a β‐carbamoylsulfide functionality. Acetone photosensitization of reaction in the thymine‐cysteine system also yielded Ia, Ib, II and III as products. Again, no evidence was found for formation of IV.