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 photoi nduced 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 similar to 10 ns and similar to 50 ps laser pulses. The obtained Raman saturation d ependences were simulated using the rate equations approach on the basis of two different models, The first model was taken from the work of Strahan e t al, and is based on the assumption of the preferential intercalation of C u(TMpy-P4) at GC sites and includes the process of fast translocation of me talloporphyrin from GC sites into outside-bound AT sites under the action o f a laser pulse. Another model was based on the assumption of binding of Cu (TMpy-P4) to both GC and QT sites of DNA and exciplex formation involving c opper porphyrin initially located at AT sites. Our results show that: (i) t he exciplex is formed during the action of a laser pulse, with both similar to 10 ns and similar to 50 ps duration; (ii) photoinduced accumulation of free Cu(TMpy-P4) molecules in a buffer solution and changes in the stationa ry distribution over metalloporphyrin binding modes do not take place under high-power irradiation; (iii) the mechanism of Cu(TMpy-PL) translocation i s 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. Copyright (C) 1999 John Wiley & Sons, Ltd.