Intermolecular potentials--past, present, future.

Membrane fusion is a ubiquitous process in eukaryotic cells. When two membranes fuse, lipid must undergo molecular rearrangements at the point of merging. To understand how lipid structure transitions occur, scientists studied the phase transition of lipid between the lamellar (L(alpha)) phase and the inverted hexagonal (H(II)) phase, based on the idea that lipid must undergo a similar rearrangement as in fusion. However, previous investigations on the system of dioleoylphosphatidylcholine (DOPC) and dioleoylphosphatidylethanolamine (DOPE) did not reveal intermediate phases between the L(alpha) and H(II) phases. Recently, we found a rhombohedral phase of diphytanoylphosphatidylcholine between its L(alpha) and H(II) phases using substrate-supported samples. Here we report the observation of two new phases in the DOPC-DOPE system: a rhombohedral phase and a distorted hexagonal phase. The rhombohedral phase confirms the stalk hypothesis for the L(alpha)-H(II) transition, but the phase of stable stalks exists only for a certain range of spontaneous curvature. The distorted hexagonal phase exists only in a lipid mixture. It implies that lipids may demix to adjust its local spontaneous curvature in order to achieve energy minimum under stress.

Chemistry, Physical↗

PubMed2003

Variable character of O-O and M-O bonding in side-on (eta(2)) 1:1 metal complexes of O2.

The structures and the O-O and M-O bonding characters of a series of reported side-on (eta(2)) 1:1 metal complexes of O(2) are analyzed by using density functional theory calculations. Comparison of the calculated and experimental systems with respect to O-O bond distance, O-O stretching frequency, and O-O and M-O bond orders provides new insights into subtle influences relevant to O(2) activation processes in biology and catalysis. The degree of charge transfer from the generally electron-rich metals to the dioxygen fragment is found to be variable, such that there are species well described as superoxides, others well described as peroxides, and several cases having intermediate character. Increased charge transfer to dioxygen takes place via overlap of the metal d(xy) orbital with the in-plane pi* orbital of O(2) and results in increased M-O bond orders and decreased O-O bond orders. Comparison of theory and experiment over the full range of compounds studied suggests that reevaluation of the O-O bond lengths determined from certain x-ray crystal structures is warranted; in one instance, an x-ray crystal structure redetermination was performed at low temperature, confirming the theoretical prediction. Librational motion of the coordinated O(2) is identified as a basis for significant underestimation of the O-O distance at high temperature.

Chemistry, Physical↗