Luminescence kinetics following pulse irradiation—IV. A purine pyrimidine crystal complex

Abstract

The behaviour of the excited states formed after ionizing irradiation in a base-paired crystalline complex between 9-methyladenine and 1-methylthymine (AT complex) has been studied by observing the associated luminescence. The luminescence spectra and decay kinetics of the AT complex and its separate components have been observed over a temperature range of 93 K to 300 K, and from 2μs to 1 s after the start of a 1·6 μs duration electron pulse. At low temperatures the early luminescence decay kinetics are complex and probably arise from trapping within the molecular or crystal structure. The major luminescence yields, however, arise from exponential portions of the decay curves. In this region the spectra and kinetics of the AT complex and the 1-methylthymine are closely similar although the luminescence yield of the complex is much greater than that of the 1-methylthymine and is closer to the yield from 9-methyladenine. These results and results obtained at other temperatures are interpreted in terms of the transfer of excitation energy between the bases with emission occurring from the thymine base as is believed to occur in DNA. Possible mechanisms of energy transfer are discussed.