Effect of a Polar Environment on the Conformation of Phospholipid Head Groups Analyzed with the Onsager Continuum Solvation Model

Conformational study of methylphosphocholine: a prototype for phospholipid headgroups in membranes

Phospholipid bilayers constitute the largest structural component of cell membranes, in which choline phospholipids are abundant. In this study, through a theoretical sampling on a methylphosphocholine (MePC) potential energy surface, a set of conformers was selected as a prototype for the membrane phospholipid head. We performed a detailed conformational study of such a prototype, both as an isolated moiety and in a solvated system. We used the polarizable continuum model (PCM) to account for solvation effects. We used a quantum-mechanical methodology based on density functional theory (DFT) and the 6-31G(d,p) basis set for the calculations. Through this methodology we were able to obtain a set of conformations that presented a mirror-image pattern, in good agreement with the experimental geometric values for the different phosphocholine derivatives. Potential curves for the main parameters of the dihedral space of MePC were obtained and are provided to guide future force-field parameterizations.

A comparison of the gas phase acidities of phospholipid headgroups: experimental and computational studies

Proton-bound dimers consisting of two glycerophospholipids with different headgroups were prepared using negative ion electrospray ionization and dissociated in a triple quadrupole mass spectrometer. Analysis of the tandem mass spectra of the dimers using the kinetic method provides, for the first time, an order of acidity for the phospholipid classes in the gas phase of PE < PA << PG < PS < PI. Hybrid density functional calculations on model phospholipids were used to predict the absolute deprotonation enthalpies of the phospholipid classes from isodesmic proton transfer reactions with phosphoric acid. The computational data largely support the experimental acidity trend, with the exception of the relative acidity ranking of the two most acidic phospholipid species. Possible causes of the discrepancy between experiment and theory are discussed and the experimental trend is recommended. The sequence of gas phase acidities for the phospholipid headgroups is found to (1) have little correlation with the relative ionization efficiencies of the phospholipid classes observed in the negative ion electrospray process, and (2) correlate well with fragmentation trends observed upon collisional activation of phospholipid [M - H](-) anions.

Conformational studies of sphingolipids by NMR spectroscopy. II. Sphingomyelin

Sphingomyelin (SM) is the most prevalent sphingolipid in the majority of mammalian membranes. Proton and 31P nuclear magnetic resonance spectral data were acquired to establish the nature of intra- and intermolecular H-bonds in the monomeric and aggregated forms of SM and to assess possible differences between this lipid and dihydrosphingomyelin (DHSM), which lacks the double bond between carbons 4 and 5 of the sphingoid base. The spectral trends suggest the formation of an intramolecular H-bond between the OH group of the sphingosine moiety and the phosphate ester oxygen of the head group. The narrower linewidth and the downfield shift of the resonance corresponding to OH proton in SM suggest that this H-bond is stronger in SM than in DHSM. The NH group appears to be involved predominantly in intramolecular H-bonding in the monomer. As the concentration of SM increases and the molecules come in closer proximity, these intramolecular bonds are partially disrupted and the NH group becomes involved in lipid-water interactions. The difference between the SM and DHSM appears to be not in the nature of these interactions but rather in the degree to which these intermolecular interactions prevail. As SM molecules cannot come as close together as DHSM molecules can, both the NH and OH moieties remain, on average, more intramolecularly bonded as compared to DHSM.