Thermoluminescence and emission spectra studies of 6-methylmercaptopurine and 6-methylmercaptopurine riboside x irradiated at 10 K

Abstract

In this work we report the first thermoluminescence (TL) and emission spectra studies of the sulphur-containing DNA base analog 6-methylmercaptopurine (6MeMP), and its riboside, 6-methylmercaptopurine riboside (6MeMPR),x irradiated at 10 K. The 6MeMP glow curve exhibits TL peaks at 28, 40, 68, 78, 112, and 140 K with typical emission from each peak consisting of a band possessing maxima at 500 and 530 nm. Only three prominent peaks were found in the 6MeMPR glow curve: 42, 52, and 140 K. Typical emission from each of these TL peaks consisted of a band with maxima at 455 and 485 nm. Thermal activation energies and frequency factors associated with each glow peak were extracted from the experimental data and used in formulating a model to explain the observed emission. Ultraviolet (UV)-induced glow curves, emission spectra, and photobleaching studies were also conducted in efforts to deduce the mechanisms of charge recombination in 6MeMP and 6MeMPR. The results indicate that metastable states exist in both 6MeMP and 6MeMPR which are filled by ionizing radiation. TL occurs when these trapped charges are thermally released and decay to the ground state (So) via the first excited singlet state (S1) or the first excited triplet state (T1). An energy level diagram depicting the TL emission process is presented which shows that the energy separation of S1 and T1 is approximately 0.14 eV. Further, we find that some of the TL peaks in both samples possess thermal activation energies less than 0.14 eV, suggesting that they lie between S1 and T1 in energy; these peaks are characterized by unusually small frequency factors (0.3 to 3000) s−1. Our results are consistent with a model originally proposed by Weissbluth et al. and modified by Tatake et al. to explain TL in nucleic acid bases. Finally, we suggest that the 140 K peak in 6MeMP results from the thermal destruction of an electron-adduct radical whose molecular structure and thermal properties have been previously determined by ESR studies.