DYNAMICS OF ELECTRON-TRANSFER BETWEEN INTERCALATED POLYCYCLIC MOLECULES - EFFECT OF INTERSPERSED BASES

Both ethidium bromide (EB+) and N,N'-dimethyl-2,7-diazapyrenium dichloride (DAP2+) intercalate between base pairs in calf-thymus DNA. Upon illumination of intercalated EB+, an electron is transferred to an adjacent intercalated DAP2+ molecule, as evidenced by laser flash photolysis techniques. The rate of forward electron transfer decreases with increasing number of interspersed nucleic acid bases and, for the closest approach (almost-equal-to 10 angstrom), k(fet) = (1.3 +/- 0.2) x 10(9) s-1, whereas reverse electron transfer occurs on a much longer time scale (k(ret) = (5.0 +/- 0.8) x 10(7) s-1). Similar behavior was observed with acridine orange (AO) in place of EB+. Treating the kinetic data according to k(fet) = A exp(-beta-R), where R is the separation distance, gave a beta-value of (0.88 +/- 0.08) angstrom-1 for both EB+ (A = 1.1 x 10(13) s-1; DELTA-G-degrees = -25 kJ mol-1) and AO (A = 2.9 x 10(13) s-1; DELTA-G-degrees = -65 kJ mol-1). This attenuation factor is consistent with electron tunneling through the interspatial base pairs rather than via the phosphate or ribose functions.