Photoprocesses of Naphthalene Imide and Diimide Derivatives in Aqueous Solutions of DNA

Despite the growing number of redox-active chromophores utilized to photoinduce oligonucleotide cleavage, detailed correlations between the degree of ground-state complexation and product yields have not been developed. To elucidate the specific role of singlet and triplet excited states in nucleotide photooxidation, the photochemical reactivities of N-(2-(N-pyridinium)ethyl)-1,8-naphthalene imide (NI) and N,N‘-bis-[2-(N-pyridinium)ethyl]-1,4,5,8-naphthalene diimide (NDI) with calf-thymus DNA have been explored as a function of ground-state complexation with the DNA polymer. Upon addition of calf-thymus DNA to a phosphate buffered solution of the naphthalene imide derivatives, distinct changes in the UV absorption spectrum of the chromophores, along with single isosbestic points, are observed. Analysis of these changes using the noncooperative model of McGhee and von Hippel yield association constants of (2.46 ± 0.42) × 10⁴ M^-1 and (7.78 ± 0.11) × 10⁵ M^-1 for NI and NDI, respectively. Pulsed 355 nm excitation of either NI or NDI in the presence of calf-thymus DNA produced the reduced NI^•- and NDI^•- species that absorbed maximally at 400 and 480 nm, respectively, from the triplet excited states. For both compounds, the yield of radical anion from self-quenching processes was substantial (φ_I^•- = 0.11 ± 0.01 and 0.25 ± 0.01 for NI and NDI, respectively). However, pulsed excitation of NI in the presence of DNA resulted in the production of radical species that were not attributed to self-quenching processes. For both compounds, the fraction of associated imide was systematically varied between 0 and 1. The intersystem crossing yield was found to decrease linearly with the fraction bound to DNA from 0.71 to 0.08 for NI and 0.35 to 0.004 for NDI.