Fluorescence lifetimes of carbocyanine lipid analogs in phospholipid bilayers

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Abstract
The fluorescence lifetimes for the 1,1''-dialkyl-3,3,3''3''-tetramethylindocarbocyanine (CNdiI) dyes (N = 12, 18, and 22) in a variety of lipid bilayer membranes were measured. Effects of bilayer physical state, probe chain length, probe concentration, charge, lipid head group, and cholesterol concentration were examined. Even in single-phase membranes these probes did not exhibit single-exponential decays. Rather, the data were fit by biexponential decays with lifetimes of .apprx. 0.3-0.4 and .apprx. 0.9-1.3 ns with no significant improvement in .chi.2 convergence with the addition of a third component. Average lifetimes were dependent upon lipid phase and to a lesser degree surface charge and the phospholipid head group. In dipalmitoylphosphatidylcholine (DPPC)-cholesterol membranes, the C18diI lifetime was sensitive to membrane reorganizations at both 20 and .apprx. 33 mol% cholesterol. In egg phosphatidylcholine (EPC) bilayers, the C18diI lifetime was essentially independent of its concentration below 1:103.