Hydration of Cis and Trans N-Methylformamide as Revealed by the Use of O-17-Nmr, Molecular Mechanics, and Ab-Initio Calculations

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Hydration of Cis and Trans N-Methylformamide as Revealed by the Use of O-17-Nmr, Molecular Mechanics, and Ab-Initio Calculations (EN)

Gerothanassis, I. P. (EN)

Πανεπιστήμιο Ιωαννίνων. Σχολή Θετικών Επιστημών. Τμήμα Χημείας (EL)
Gerothanassis, I. P. (EN)

The solvation of cis and trans N-methylformamide (NMF) by water was investigated using a combination of O-17-nmr spectroscopy, classical molecular mechanics [MM2(77) and Mm2(87)] force field, and ab initio 4-31G* gradient optimization calculations. In dilute aqueous solution, the O-17-nmr spectra of NMF indicate strong shielding by 66.9 and 66.1 ppm for the cis and trans amide oxygens, respectively, compared to those values obtained in dilute toluene solution. This demonstrates that both isomers are equally solvated by molecules of water, which are further hydrogen bonded to molecules of water of the bulk solvent. Molecular mechanics simulations were carried out for cis and trans NMF in a cluster of water molecules. Radial distribution functions show structural contacts by several water molecules at the amide CO and NH group, which are significantly more pronounced with MM2(87) calculations. Ab initio 4-31G* gradient optimization calculations on the supermolecule trans NMF-(H2O)(3) indicates the presence of more than two hydrogen-bond contacts at the carbonyl oxygen. This is in agreement with MM2 calculations and provides further evidence for multiple acceptor properties of the amide oxygen and an out of the amide plane arrangement of the bound molecules of water. Comparison of the integration data to the first radial distribution function (rdf) minima shows that the local solvation of CO and NH groups is very similar for both cis and trans isomers. The intermolecular geometric parameters of the supermolecule trans NMA-(H2O)(3) and the first rdf maxima resulting from Mm2(87) and MM2(77) calculations are compared with distribution of water molecules around the CO and NH groups of peptides and proteins resulting from x-ray and neutron diffraction experiments. The rdfs involving the methyl group of NMF demonstrate the nonrandom distribution of solvent sites with first maxima in reasonable agreement with distribution water molecules around the apolar side chain of amino acid residues in proteins. (C) 1995 John Wiley & Sons, Inc. (EN)

intermolecular potential functions (EN)

Biopolymers (EN)



Wiley-Blackwell (EN)

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