Hydration-Induced Changes of Structure and Vibrational Frequencies of Methylphosphocholine Studied as a Model of Biomembrane Lipids

Abstract

The chemical characteristics of the polar parts of phospholipids as the main components of biological membranes were investigated by using infrared (IR) spectroscopy and theoretical calculations with water as a probe molecule. The logical key molecule used in this study is methylphosphocholine (MePC) as it is not only a representative model for a polar lipid headgroup but itself has biological significance. Isolated MePC forms a compact (folded) structure which is essentially stabilized by two intramolecular C-H...O type hydrogen bonds. At lower hydration, considerable wavenumber shifts were revealed by IR spectroscopy: the frequencies of the (O-P-O)- stretches were strongly redshifted, whereas methyl and methylene C-H and O-P-O stretches shifted surprisingly to blue. The origin of both red- and blueshifts was rationalized, on the basis of molecular-dynamics and quantum-chemistry calculations. In more detail, the hydration-induced blueshifts of C-H stretches could be shown to arise from several origins: disruption of the intramolecular C-H...O hydrogen bonds, formation of intermolecular C-H...O(water) H-bonds. The stepwise disruption of the intramolecular hydrogen bonds appeared to be the main feature that causes partial unfolding of the compact structure. However, the transition from a folded to extended MePC structure was completed only at high hydration. One might hypothesize that the mechanism of hydration-driven conformational changes as described here for MePC could be transferred to other zwitterions with relevant internal C-H...O hydrogen bonds.

Spatial Distribution of Lipid Headgroups and Water Molecules at Membrane/Water Interfaces Visualized by Three-Dimensional Scanning Force Microscopy

“…So far, the membrane/water interface has been extensively studied by various techniques. [4][5][6][7][8][9][10][11] These previous works have shown that the water molecules adjacent to a membrane strongly interact with the headgroups and give significant influence on its mechanical strength and fluidity. [12][13][14] However, molecular-scale origin for such a grave influence has remained elusive.…”

Advanced Instrumentation of Frequency Modulation AFM for Subnanometer-Scale 2D/3D Measurements at Solid-Liquid Interfaces

“…So far, the membrane-water interface has been extensively studied by various techniques [17,19,[100][101][102][103][104][105]. These previous works have shown that the water molecules adjacent to a membrane strongly interact with the headgroups and give significant influence on its mechanical strength and fluidity [106][107][108].…”

Solvated potential energy surfaces for MePC

“…Although the experimentally observed changes due to solvation on the asymmetric stretching modes of the -[P(O 2 )] --and -CH 2 -groups are quantitatively well represented in Approaches 2 and 3, it could be interesting to analyze their 2-fold origin proposed in the literature [17]. One origin is the possibility of direct ''improper hydrogen bonds'' [24,25] and the other origin is the conformational changes induced by hydration [26]. (…”