| Abstract | The difference in the affinity for water of peptide groups embedded in different molecular environments was investigated. The chemical shift of an amide proton is sensitive to conformational variations, as well as to changes in the molecular environment [D.S. Wishat, B.D. Sykes and F.M. Richards, J. Mol. Biol., 222 (1991) 311–333]. Therefore, if the conformational motions are minimized or excluded, the observed changes in the chemical shift can simply be related to the environmental effects. The conformation(s) of the cyclic β-turn models studied in this work has been previously reported using X-ray, NMR, circular dichroism, and (FT-IR) spectroscopic methods, as well as MD calculations. [M. Hollósi, K.E. Köver, S. Holly, L. Radics and G.D. Fasman, Biopolymers, 26 (1987) 1527–1572; A. Perczel, M. Hollósi, B.M. Foxman and G.D. Fasman, J. Am. Chem. Soc., 113 (1991) 9772–9784; and H.H. Mantsch, A. Perczel, M. Hollósi and G.D. Fasman, Biopolymers, 33 (1993) 201–207]. The backbone of the cyclo[(δ)Ava—Gly—Pro—Aaa—Gly] (where Aaa = Ser(OtBu), Ser or Thr(OtBu), and δ(Ava) is δ-aminovaleric acid) compounds was found to be rigidly incorporated in the structure and to contain two intramolecular hydrogen bonds. These β-turn models also include one (or two) “free” amide group(s) that are not involved in any type of interaction. The “water titration” of these amide groups in acetonitrile, where they are involved in various degrees of hydrogen bonding, revealed their molecular environment. Owing to the rigidity of these structures, the observed changes in the amide proton chemical shifts, during titration were attributed to their involvement in hydrogen bonding. This was confirmed by monitoring the water titration simultaneously with FT-IR spectroscopy. The phenomenon described here, with the proposed characterization of the investigated peptide/water system, comprise an improvement in the NMR method for analyzing the hydrogen bonding of small rigid peptides. |
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