PubMed · 13117764
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A V WOLF, M E McDOWELL. 1953. Sodium concentration deficit; nomogram for replacement.. https://doi.org/10.1152/jappl.1953.6.6.355
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The variation of atomic charges upon proton transfer in hydrogen bonding complexes of 4-methylimidazole, in both neutral and protonated cationic forms, and acetate anion, is investigated. These complexes model the histidine (neutral and protonated)-aspartate pair present in active sites of proteases where strong N--H...O hydrogen bonds are formed. Three procedures (Merz-Kollman scheme, Natural Population Analysis, and Atoms in Molecules Method) are used to compute atomic charges and explore their variation upon H-transfer in the gas phase and in the presence of two continuum media with dielectric constants 5 (protein interiors) and 78.39 (water). The effect of electron correlation was also studied by comparing Hartree-Fock and MP2 results for both complexes in the gas phase. Greater net charge interchanged upon H-transfer is observed in the anionic complex with respect to the neutral complex. Raising the polarity of the medium increases the amount of net charge transfer in both complexes, although the neutral system exhibits a larger sensitivity to the presence of solvent. Charge transfer associated to N--H...O and N...H--O bonds reveal the ionic contribution to the interaction depending on the number of charged subunits but the presence of solvent affects little this quantity. The lack of electron correlation overestimates all the charges as well as their variations and so uncorrelated calculations should be avoided.
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Inorganic tennis balls (ITBs), [[{Pt(betmp)(dach)}(2)Cu](2)(X)][X](3) (in which X=ClO(4) (-) (3), NO(3) (-) (4), Cl(-) (5) and Br(-) (6); dach=trans-1,2-diaminocyclohexane and betmp=bisethylthiomethylidenepropanedioate) and [[{Pt(dteym)(dach)}(2)Cu](2)(PF(6))][PF(6)](3) (7; dteym=1,3-dithiepane-2-ylidenemalonate), were prepared as crystals. Investigation of their X-ray crystal structures revealed that shapes of the cavities in ITBs show significant distortions that depend on the properties of the encapsulated anions. The CuCu* distance was observed to be longest in 7 and shortest in 5, the difference between them being 2.05 A. The flexibility of cavity structures of ITBs makes it possible to encapsulate various anions inside the cavity, while their distortions may be a reason for the difference in the encapsulating ability for anions, that is, anion selectivity. Especially, the distortions observed in 7 are so severe that the encapsulating ability of the cavity for PF(6) (-) is very low compared to other anions. The shapes of ITBs with ClO(4) (-) and BF(4) (-) ions inside their cavities are very similar; however, ClO(4) (-) is encapsulated by the cavity better than BF(4) (-), which is explicable by the difference of metal-anion interactions. This structural study on ITBs gives a clue to the origin of the anion selectivity of the cavity in ITBs previously investigated by (19)F NMR spectroscopy of the ITBs in methanol.