Electrostatic lessening

The repulsive, electrostatic forces of sialic acids contribute to the rigidity of the cell surface, as was shown hy an increase in the defor-mability of sarcoma cells after enzymic removal of sialic acid residues.424 Enzymic release of sialic acids from the zona pellucida of rabbit ovum lessens the rigidity of this cell, and spermatozoa can no longer penetrate it.425 Glycoproteins on the surface of sea-urchin eggs... 

When two HX molecules are paired together, it is clear that a H-bond can form between the proton of one molecule and the X atom of the other. But the situation is less clear cut if the H atom of the proton acceptor molecule is replaced by another halogen atom. In such a case, the dihalogen will not be polarized much at all, lessening the electrostatic part of the interaction. 

The balance of electrostatic and delocalization interactions in an isolated molecule may be perturbed by the influence of the solvent. In calculations based on Eq. 7, the analysis of solvation-energy terms suggested that the electrostatic contribution stabilizing the ap orientation of the acetal s ment is the conformationally dominant term. For example, in 2-methoxyoxane, the difference in energy of the (ap, ap) and (ap, sc) conformers in water, compared to that in the isolated molecule, caused by solute-solvent electrostatic interactions alone, amounts to 4 kJ.mor. Accordingly, the inter-and intra-molecular, electrostatic interactions operate in reverse directions in acetals. Whereas the intramolecular, electrostatic interactions are responsible, together with delocalization interactions, for the aiq)earance of the anomeric, reverse anomeric, and exo-anomeric effects, the solute-solvent electrostatic interactions lessen their im nitude, and may even cancel them. Of course, the solvent may also influence the electron distribution and energy of MO s in a molecule. In this way, the orbital interactions of lone-pairs and delocalization contributions to the anomeric effect may be scaled by the solvent, but this mechanism of the environmental effect is, in most cases, of only minor importance. 

It should be noted that electrostatic repulsion exists in the enol form. However, this electrostatic repulsion is less in the enol form than in the keto form because the single C-0 bond has a smaller polarization gradient than the carbonyl bond, hence lessening the repulsion of the partially positively charged carbons. 

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