Solvent effects on the sensitized photoxygenation of lidocaine

Detection of O-2((1) (g)) phosphorescence emission, lambda (max) = 1270 nm. following laser excitation and steady state methods were employed to deter mine both the total constant, k(T)(LID), and the chemical reaction rate con stants, k(R)(LID), fur reaction between the anaesthetic lidocaine and singl et oxygen in several solvents. Values of k(T)(LID) range from 0.20 +/- 0.09 x 10(6) M-1 s(-1) in trifluoroethanol to 45.8 +/- 2.40 x 10(6) M-1 s(-1) i n N,N-dimethylacetamide. Values of k(R)(LID) are at least one order of magn itude lower than k(T)(LID) values in a given solvent. Solvent effect on que nching rates shows that reaction mechanism involves formation of a charge t ransfer exciplex. Correlation of k(T)(LID) values with solvent parameters d oes not follow that observed for a typical tertiary amine such as triethyla mine. Although k(T)(LID) values are lower in hydrogen bond donor solvents, this solvent effect is significantly smaller than that for triethylamine, a nd no expected decrease in Lidocaine reactivity with change from aprotic to protic solvents was found. This result is ascribed to weaker hydrogen bond ing between the amino moiety in lidocaine and the solvent. Otherwise, hydro gen bond acceptor solvents increase k(T)(LID) to a greater extent than that triethylamine. This can be explained by intra-molecular hydrogen bonding o r electrostatic interactions that stabilize lidocaine and hydrogen bond acc eptor solvents disrupt these interactions. (C) 2001 Elsevier Science B.V Al l rights reserved.