Transversal Distribution of Acyl-Linked Pyrene Moieties in Liquid-Crystalline Phosphatidylcholine Bilayers. A Fluorescence Quenching Study

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Abstract
Quenching of the fluorescence of pyrene-labeled phospholipids by dibromolipids was used to determine the chain length dependence of the bilayer depths of the pyrenyl moieties. Six 1-palmitoyl-2-(pyrenyl-n-acyl)-phosphatidylcholines (PyrnPC) were examined, with end-labeled pyrenyl chains varying in length, n, from 4 to 14 carbons. These lipids were incorporated, at a concentration of 0.3 mol%, into bilayers composed of various mixtures of 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) and of one of three 1-palmitoyl-2-(x,y-dibromostearoyl)phosphatidylcholine quencher lipids (Brx,yPC; x,y = 6,7; 9,10; or 11,12). Parallel experiments were carried out with bilayers containing 50 mol % cholesterol. Quenching in these systems is dynamic, as demonstrated by the identical dependence of steady-state fluorescence intensities and excited state lifetimes of Pyr8PC on the mole fraction of Br6,7PC. Stern--Volmer analysis of the Brx,PC mole fraction dependence of PyrnPC fluorescence yielded apparent quenching constants, KSV, which show a systematic relation with both the length of the pyrenyl acyl chain and the position of the bromine atoms. The quenching data were further analyzed by plotting KSV as a function of n (defined above), or b (the average of the two bromine positions for each PyrnPC), or n--b (the separation between pyrenes and bromines). In all cases, the data were fit by Gaussian functions yielding estimates of the centers and the apparent 1/e half-widths of the transversal distributions of the pyrenyl moieties in methylene units (mu). Both in the absence and in the presence of cholesterol, the position of each PyrnPC Gaussian center is equal to the sum of n plus a constant d approximately 2.5 mu, corresponding to the distance from the effective center of the pyrenyl moiety to its point of attachment to the acyl chain.(ABSTRACT TRUNCATED AT 250 WORDS)

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