O-17 NMR studies of the solvation state of cis/trans isomers of amides and model protected peptides
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Gerothanassis, I. P.
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Πανεπιστήμιο Ιωαννίνων. Σχολή Επιστημών και Τεχνολογιών. Τμήμα Βιολογικών Εφαρμογών και Τεχνολογιών
(EL)
Gerothanassis, I. P.
(EN)
O-17 shielding constants have been utilized to investigate solvation differences of the cis/trans isomers of N-methylformamide (NMF), N-ethylformamide (NEF), and tert-butylformamide (TBF) in a variety of solvents with particular emphasis on aqueous solution. Comparisons are also made with protected peptides of the formulas CH3CO-YOH, CH3CO-Y-NHR (Y = Pro, Sar), and CH3CO-Y-Z-NHR (Y = Pro; Z = D-Ala) selectively enriched in O-17 at the acetyl oxygen atom. Hydration at the amide oxygen induces large and specific modifications of the O-17 shielding constants, which are practically the same for the cis and trans isomers of NMF, NEF, and the protected peptides. For tert-butylformamide, the strong deshielding of the trans isomer compared to that of the cis isomer may be attributed to an out-of-plane (torsion-angle) deformation of the amide bond and/or a significant reduction of solvation of the trans isomer due to steric inhibition of the bulky tert-butyl group. Good linear correlation between delta(O-17) of amides and delta(O-17) of acetone was found for different solvents which have varying dielectric constants and solvation abilities. Sum-over-states calculations, within the solvaton model, underestimate effects of the dielectric constant of the medium on O-17 shielding, while finite-perturbation-theory calculations give good agreement with the experiment. (C) 1996 Academic Press, Inc.
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