TRIPLET RADICAL ION-PAIR STATE OF THE ZN-PORPHYRIN VIOLOGEN DYAD AS A MAGNETIC-FIELD SENSITIVE PROBE OF PHASE-TRANSITIONS IN SMALL UNILAMELLAR VESICLES

The magnetic field effect on the recombination kinetics of the triplet radical ion pair state (RIPS) of the Zn-porphyrin-viologen dyad (P-Ph-Vi2+) in the small unilamellar vesicles (SUV) of D,L-dipalmitoyl-alpha-phosphatidylcholine has been studied by the nanosecond laser flash photolysis technique at 5-60-degrees-C. The increase in temperature from 25 to 40-degrees-C enhances the rate constant (k(r)) of the RIPS recombination in zero magnetic field from 0.9 x 10(6) to 1.6 x 10(6) s-1, while k(r) is temperature insensitive at 5-25 and 40-60-degrees-C. The typical break in the k(r) temperature dependence is observed in the temperature range of the phase transition of the SUV bilayers from the solid to the fluid state. The k(r) value in a strong magnetic field (B = 0.24 T) is equal to 2.7 x 10(5) s-1 and it is independent of temperature at 5-60-degrees-C The shape of the magnetic field dependence of k(r) is unaffected by the phase transition of the SUV bilayers and is characterized by the existence of an initial plateau of k(r) at B = 0 to 0.5 mT.