Head-group overlap

Entropic confinement forces occur at ultrathin (<5 nm) surfactant films and between bilayers in solution. They are mainly responsible for the stability observed in so-called Newton black soap films. It arises from the steric repulsion occurring when adsorbed layers overlap. These forces operate by various modes, like undulation, peristaltic fluctuations, or by head-group overlap. ... 

At low concentrations, a hollow vesicle results with usually just one double layer and, as the concentration is increased, the number of double layers can increase in a transition from unilamellar vesicles to multilamellar structures. Since the hydro-plilic head groups are exposed on the inside as well as the outside of the vesicular structure this provides an opportunity to entrap hydrophilic guest drug molecules both inside the center of the vesicle and, if multilamellar, between the phospholipid bilayers as well. On the other hand, hydrophobic molecules can become incorporated in the hydrophobic regions of the bilayers where the hydrophobic tails overlap. 

Further, if the interface is h3rperbolic, one of the focal surfaces to the interface must lie at a distance equal to the chain length (1) from the interface, where the head-groups are located. (This condition ensures that no gaps between chains or chain overlap occur.) Consequently, the cross-sectional area for this parallel surface must vanish ... 

Thomas et al. were first to report a CoMFA QSAR pharmacophore model for anandamide (4) audits analogs (Thomas etal. 1996). These authors used molecular dynamics studies to explore conformations of 4 that present pharmacophoric similaritieswith2l -THC(l). A J-shaped or looped conformation of 4 was identified that had good molecular volume overlap with 1 when (1) the carboxyamide of 4 was overlaid with the pyran oxygen (0-5) in 1 (2) the head group hydroxyl of 4 was overlaid with the C-1 phenolic hydroxyl group of 1, (3) the five terminal carbons of the 4 fatty acid acyl chain were overlaid with the C-3 pentyl side chain of 1 and... 

All resonance structures for the same molecule must have the same sigma framework (w sigma bonds form from the head on overlap of hybridized orbitals). Furthermore, they must be correct w Lewis structures with the same number of electrons (and consequent charge) as well as the same number of unpaired electrons. Resonance structures with arbitrary separation of charge are unimportant, as are those with fewer covalent bonds. These unimportant resonance structures only contribute minimally (or not at all) to the overall bonding description however, they are important in some cases such as for a w carbonyl group. 

These reactions are regarded as electron tunnelling processes, and the results are interpreted in terms of the estimated distributions of occupied and unoccupied electronic levels in the redox systems aq-e, D-D+, and A-A- involved. If the overlap of the occupied levels of the donor with the unoccupied levels of the acceptor is not good, transfer is slow, irrespective of how thermodynamically favourable it may be. These levels are shifted, relative to one another, by the charge on the micellar head-group, which determines the direction of the electrical double layer at the interface. Reaction rates are therefore influenced both by the charge on the micelle and the concentration of electrolyte in the aqueous phase.47 48 The lifetime of normally unstable intermediates may be enormously enhanced by working in a micellar system.49... 

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