Conformational transmission in condensed lipid model compounds. Chain packing modification by head-group-induced conformational changes in the glyceryl backbone

On changing the phosphorus coordination from four to five via a trigonal bipyramidal geometry in phospholipid model systems, a conformational change is initiated around the C2-C3 linkage caused by enhanced electrostatic repulsion between the atoms O3 and O2, situated in the axis of the trigonal bipyramid. This is supported by the absence of conformational change upon substitution of O2 by methylene. With solid-state cross-polarization and magic-angle spinning 13C-NMR it is shown that such a conformational transmission leads to an increased packing density of the lipid chains. It seems to us that changes in lipid packing caused by conformational transmission may also occur when pentacoordination is present transiently, as under biological conditions, thereby inducing an activation of the transport proteins that are embedded in the membrane.

The different influences of ether and ester phospholipids on the conformation of gramicidin A. A molecular modelling study

With AMBER (assisted model building with energy refinement) molecular modelling techniques, the interactions between lipids which differ in the type of chain linkage (e.g., ether or ester) and gramicidin were approximated. It was found, theoretically, that replacement of the ester function in dipalmitoylphosphatidylcholine (DPPC) by an ether moiety induces a shift in the rotameric distribution of the Trp-15 side-chain in gramicidin A. Concomitantly, the channel entrance is contracted by approx. 0.4 A. The perturbation can be related to the strong hydrogen-bond formed between the lipid carbonyl group and the indole proton of the Trp-15 residue of gramicidin. In the ether lipid-gramicidin assembly a weaker H-bond is formed between Trp-15 and the phosphate moiety. To obtain a first indication of the influence of the strength of this H-bond on gramicidin A, a preliminary experimental study was set up. The transport properties of gramicidin A were studied using efflux measurements through vesicle walls containing ether and ester lipids, respectively. A change in the permeability of gramicidin A was found when ether lipids were added to a bilayer composed of the ester lipid dioleoylphosphatidylcholine (DOPC).

Conformational transmission in the glyceryl backbone of phospholipid model compounds, induced by a P(4-coordinated) into trigonal bipyramidal P(5-coord) transition

Triesterified phospholipid model compounds have been synthesized and extensively studied with 300-MHz 1H NMR in the monomer phase in order to get additional support for the effect of conformational transmission induced by a P(4-coord) into a trigonal bipyramidal P(5-coord) transition, as was suggested by Merkelbach and Buck. To elucidate any conformational preferences around the C2-C3 bond, the stereospecifically deuterated precursor 1,2-dihexanoyl-(3R)-sn-[3-2H]glycerol was synthesized. The results reveal that a coordinational change of phosphorus from four to five is transmitted in a significant increase in population of the conformer, in which the vicinally substituted oxygens O-2 and O-3 are trans located. The impact of this transmission seems not to be restricted to conformational changes in the adjacent C2-C3 bond, but is also present in specific rotations around the C1-C2 bond, thereby shifting the C1-C2 conformational equilibrium towards a decreased contribution of the trans arrangement of the acyl chains. As a consequence the interchain distance will be reduced and thus van der Waals interactions will be maximized. The results are interpreted in terms of increased electron density on O-3 when axially located in a P(5-coord) trigonal bipyramidal compound, thereby introducing enhanced electrostatic repulsions within the oxygen pairs O-3, O-2 and O-3, O-1. Relaxation of this energetically unfavourable geometry leads to the observed conformational shifts. Absence of conformational transmission, as found in P(5-coord) trigonal bipyramidal compounds with the 2-ester group substituted for an alkyl moiety, can be considered as additional support for the introduced concept. In the alkyl part of the model phospholipids, however, no conformational changes were observed by means of 13C NMR. Extrapolating this outcome to more condensed phases, a proposition could be made about the mechanism by which conformational changes in the head-group and/or glyceryl backbone will be compensated.