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Being largely driven by electrostatic interactions, hydration compensates hydrophobic repulsion and, thus, contributes decisively in structural expressions of molecules in a phospholipid membrane environment. Here, we explore the nature of the aqueous state associated with carbonyl moieties of a phospholipid bilayer. The task is of an obvious difficulty, since water clustering at a membrane interface implies the presence of various aqueous states giving rise to spectral inhomogeneity. The resultant frequency overlap of the optical response from states of differing nature obscures the structural analysis. We extract the information on water next to phospholipid carbonyls by monitoring the O-D stretch perturbation upon direct infrared excitation of lipid carbonyl groups. Modelling the signal with the help of quantum computations and molecular dynamics simulation, we extract the geometry for the optimal placement of water next to carbonyl moieties, and anticipate the time scale of the water molecule displacement, leading to the disruption of the hydrogen bond, upon direct excitation of the C=O group. The picture we provide here is of general and applied value. The practical importance stems from the necessity of experimentally characterizing the hydration of polar moieties in polypeptides (of pharmacological significance) when in a phospholipid environment, a task not yet achieved.

We explore the two-dimensional infrared response of D(2)O and of OD impurity at the interface of phospholipid membrane fragments. The spectra of the two aqueous states are inhomogeneously broadened due to the absorption of water molecule associated with the membrane interface in different structural sites. The nonlinear spectra of the two species allow disentangling of the spectral contributions of the aqueous states of two types: where the stretching modes are under effective mixing and where the stretching modes are uncoupled. By reviewing the results of the experimental studies with the support of molecular dynamic simulation we identify the spectral signatures of the main structural motives responsible for the inhomogeneous distribution of resonances in the infrared OD stretching region. Our analysis provides a quantitative estimate of the statistical population of the different aqueous species at the polar interface of the bilayer.

Molecular dynamics simulations of two monounsaturated phosphatidylcholine (PC) bilayers made of 1-palmitoyl-2-oleoyl-PC (POPC; cis-unsaturated) and 1-palmitoyl-2-elaidoyl-PC (PEPC; trans-unsaturated) were carried out to investigate the effect of a double bond in the PC beta-chain and its conformation on the bilayer core. Four nanosecond trajectories were used for analyses. A fully saturated 1,2-dimyristoyl-PC (DMPC) bilayer was used as a reference system. In agreement with experimental data, this study shows that properties of the PEPC bilayer are more similar to those of the DMPC than to the POPC bilayer. The differences between POPC and PEPC bilayers may be attributed to the different ranges of angles covered by the torsion angles beta10 and beta12 of the single bonds next to the double bond in the oleoyl (O) and elaidoyl (E) chains. Broader distributions of beta10 and beta12 in the E chain than in the O chain make the E chain more flexible. In effect, the packing of chains in the PEPC bilayer is similar to that in the DMPC bilayer, whereas that in the POPC bilayer is looser than that in the DMPC bilayer. The effect of the cis-double bond on torsions at the beginning of the O chain (beta4 and beta5) is similar to that of cholesterol on these torsions in a myristoyl chain.