Kinetics of the electron transfer reaction between 3ZnTPP* and methyl viologen in lecithin vesicles studied by global analysis

✍ Scribed by Paulo J.G. Coutinho; Sílvia M.B. Costa

• Publisher: Elsevier Science
• Year: 1994
• Tongue: English
• Weight: 988 KB
• Volume: 82
• Category: Article
• ISSN: 1010-6030
• DOI: 10.1016/1010-6030(94)87010-1

✦ Synopsis

Suspensions of lecithin vesicles incorporating zinc tetraphenylporphyrin in high and low local concentrations (lipid-to-porphyrin

ratio of 50~1 and 1OOO:l) and methyl viologen dichloride in the external aqueous phase were studied by laser flash phatolysis. Electron transfer from the porphyrin triplet to methyl viologen was shown to occur with low efficiencies ranging from 0.12 to 0.21 depending on the acceptor concentration. The data obtained followed non-homogeneous kinetics which depended on the quencher gradient concentration, on the local porphyrin content in the vesicle interface, and on its asymmetric distribution. Kinetic data were treated with a global analysis method, showing that only 30% of the triplet porphyrin sites were active as regards electron transfer to a quencher solubilized in the aqueous external phase, whereas the remaining 70% of the porphyrin sites were quenched but did not participate in the electron transfer process. In high concentrated vesicles, triplet-triplet annihilation and triplet-ground state interactions compete with electron transfer and were taken into account in the kinetic evaluation. The pre-exponential factor determined for this reaction in the outer membrane of lecithin vesicles is k&R,) =5 x 10' s-' and electron transfer occurs at short edge-to-edge distances, ranging from 1.1 8, to 3 A.