Molecular dynamics of lipid association at the hydrophobic interface of gramicidin S

Mihailescu, D.; Horváth, L. I.
March 1999
European Biophysics Journal;1999, Vol. 28 Issue 3, p216
Academic Journal
Abstract Gramicidin S was incorporated into dimyristoylphosphatidylcholine dispersions and the observed two-component EPR spectra of spin-labelled lipids at 30 Celsius were analysed by a two-stage algorithm, including spectral subtractions and two-site exchange simulations. A limited range of temperatures around 30 Celsius was found suitable for such measurements. It has been found that negatively charged labelled lipids display a selectivity towards the intramembranous part of the peptide. The relative association constants for spin-labelled stearic acid (14-SASL) and phosphatidylserine (14-PSSL) were K[sub r] = 2.08+/-0.10 and 1.18+/-0.08, respectively, when compared with the zwitterionic phosphatidylcholine label (14-PCSL, K[sub r] equivalent to 1). The lateral diffusion of spin-labelled lipids in peptide-free regions causes exchange between those labels in the bulk fluid lipid phase and motionally restricted boundary labelled lipids at the apolar interlace of gramicidin S. Owing to exchange, the spectral anisotropy of labelled lipids giving rise to the slow-motion spectral component was gradually decreased, and there was an augmentation of spectral intensity in between the motionally restricted (slow motion) and the last tumbling (motionally averaged) labelled lipid components. Two-component exchange simulations allowed the determination of off-rates of labelled phospholipids, showing an inverse proportionality with lipid-protein selectivity. Spin-labelled procaine exhibited limited selectivity and fast exchange similar to the on-coming non-specifically associated lipids.


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