Spin label EPR studies of the effect of gramicidin S on lipid chain dynamics

The effect of gramicidin S (GS) on acyl chain dynamics of 14-doxylstearic a cid spin label (14-SASL) monitoring the centre of dimyristoylphosphatidylch oline (DMPC) bilayers was studied with electron paramagnetic resonance (EPR ) spectroscopy. The chain-melting transition of DMPC was shifted down and b roadened monotonously by several degrees with increasing peptide concentrat ion in the range of lipid/peptide ratios of 1/p = 5/1-1511. The effective r otational correlation time tau (eff) of 14-SASL became larger with increasi ng amounts of peptide bound to fluid membranes of DMPC. From the dependence of tau (eff) on the lipid/peptide molar ratio it was estimated that about 10 DMPC molecules per monomer GS are involved in non-covalent peptide-lipid interaction. Optimized spectral subtractions showed that, well above the c hain-melting transition, 90% of the EPR spectral intensity of the spin-labe led peptide-lipid complex (1/p = 5/1) originated from spectra recorded in b ilayers (without peptide) at lower temperature. This was not the case 6 deg reesC above the phase transition where two components with comparable inten sities were detected. The two components were identified as different motio nal modes of the label molecules, rather than distinct mobile/immobile lipi d populations. (C) 2001 Elsevier Science B.V. All rights reserved.