Mechanism of exciplex formation between Cu-porphyrin and calf-thymus DNA as revealed by saturation resonance Raman spectroscopy

The excited-state complex (exciplex) formation that results from the photoinduced interaction of water-soluble cationic copper(II) 5,10,15,20-tetrakis[4-(N-methylpyridyl)]porphyrin [Cu(TMpy-P4)] with calf-thymus DNA has been studied in detail by resonance Raman (RR) spectroscopy using both ~10 ns and ~50 ps laser pulses. The obtained Raman saturation dependences were simulated using the rate equations approach on the basis of two different models. The first model was taken from the work of Strahan et al. and is based on the assumption of the preferential intercalation of Cu(TMpy-P4) at GC sites and includes the process of fast translocation of metalloporphyrin from GC sites into outside-bound AT sites under the action of a laser pulse. Another model was based on the assumption of binding of Cu(TMpy-P4) to both GC and AT sites of DNA and exciplex formation involving copper porphyrin initially located at AT sites. Our results show that: (i) the exciplex is formed during the action of a laser pulse, with both ~10 ns and ~50 ps duration; (ii) photoinduced accumulation of free Cu(TMpy-P4) molecules in a buffer solution and changes in the stationary distribution over metalloporphyrin binding modes do not take place under high-power irradiation; (iii) the mechanism of Cu(TMpy-P4) translocation is not sufficient for the explanation of the process of exciplex formation; (iv) further time-resolved studies are required to suggest a more elaborate mechanism for this process