The effect of gramicidin A on phospholipid bilayers.

The helical polypeptide, gramicidin A has been widely studied as a model for the interactions of hydrophobic proteins with lipid bilayer membranes. Many reports are now available of the physical effects of mixing gramicidin A with phospholipid membranes, however, the interpretation of these data remains unclear. The purpose of this communication is to examine the controversial claim that high concentrations of gramicidin A' cause disorder within the L alpha phase of phosphatidylcholine-water dispersions. Solid-state nuclear magnetic resonance (NMR), density gradient and X-ray diffraction techniques are used to confirm the existence of such an effect and mechanisms are discussed which account for the known effects of gramicidin A on lipid bilayers.

Location and activity of ubiquinone 10 and ubiquinone analogues in model and biological membranes.

Deuteriated analogues of ubiquinone 10 (Q10) have been dispersed with plasma membranes of Escherichia coli and with the inner membranes of beetroot mitochondria. Orientational order at various deuteriated sites was measured by solid-state deuterium nuclear magnetic resonance (2H NMR). Similar measurements were made, using the compounds dispersed in dimyristoylphosphatidylcholine (DMPC) and egg yolk lecithin and dispersions prepared from the lipid extracts of beetroot mitochondria. In all cases only a single unresolved 2H NMR spectrum (typically 1000-Hz full width at half-height) was observed at concentrations down to 0.02 mol % Q10 per membrane lipid. This result shows that most Q10 is in a mobile environment which is physically separate from the orientational constraints of the bilayer lipid chains. In contrast, a short-chain analogue of Q10, in which the 10 isoprene groups have been replaced by a perdeuteriated tridecyl chain, showed 2H NMR spectra with quadrupolar splittings typical of an ordered lipid that is intercalated into the bilayer. The NADH oxidase activity and O2 uptake in Escherichia coli and in mitochondria were independent of which analogue was incorporated into the membrane. Thus, despite the major difference in their physical association with membranes, or their lipid extracts, the electron transport function of the long- and short-chain ubiquinones is similar, suggesting that the bulk of the long-chain ubiquinone does not have a direct function in electron transporting activity. The physiologically active Q10 may only be a small fraction of the total ubiquinone, a fraction that is below the level of detection of the present NMR equipment.(ABSTRACT TRUNCATED AT 250 WORDS)