A group of synthetic peptides including Boc-Lys-Phe-X-Y, X = Ala (I, III) or Thr (II), Y = Pro (I, II) or Ala (III) was studied by means of 1H NMR spectroscopy and theoretical conformational analysis. Compound I in DMSO shows two conformers with the trans- and cis-configuration of the peptide bond Ala-Pro. The salt bridge between the Lys ε-amino group and the C-terminal carboxyl is featured by magnetic nonequivalence of the Lys CεH2 protons. The space structure of I and II was found to possess a salt bridge fixed by an unusual turn in the chain formed by the Lys side chain and the C-terminal dipeptide with the trans-peptide bond X-Pro. Since a stable ionic bond in III and in the cis-conformer of I has not been observed, its contribution to stabilization of the space structure of the peptides in DMSO appears rather small.
The number of structural parameters were determined for spin-labelled angiotensin in aqueous solution with the use of fluorescence spectroscopy and rH NMR relaxation induced by the spin label. At the same time all measured parameters were estimated theoretically by means of energy calculations and Monte-Carlo techniques. The matching procedure for experimental and computational data allows one to suggest a dynamic equilibrium between conformers of the molecule in aqueous solution and to estimate the values of their weights.
The main features of the assumed biologically active conformation of the tuftsin molecule have been described using the energy calculations as well as the n.m.r. and CD‐spectroscopy measurements performed for the conformationally rigid active cycloanalogue, Thr‐L‐ys‐Pro‐Arg. The biologically active conformation has been shown to be of BBRB or BLRB backbone type with the trans‐conformation of proline residue.
Despite the rapid expansion of Two-dimensional (2D) NMR methods in organic chemistry, the potential of this approach for the interpretation of 29Si NMR spectra has not yet been fully realized. Therefore, we applied 2D heteronuclear correlation spectroscopic techniques to the interpretation of the 2ySi and 'H spectra of some trimethylsilyl-substituted sugars.
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iFirst, a complete analysis of the 'H NMR spectra of the ring protons of these compounds was undertaken. The correlation of 'H and 29Si chemical shifts through the 2ySi-O-C-1H coupling provides a sound assignment of the 29Si signals in the 29Si NMR spectra, and allows the verification of the analysis of the 'H spectra (Fig. 1). The correlation of 29Si and 'H chemical shifts The shift correlation with polarization transfer is performed via the long-range coupling 3J(29SiOC1H) =3.5 Hz. Digital resolution, 1.95 Hz per point for 'H and 0.58Hz per point for *%i; recycle delay, 2s; At1,,=0.5ms; T, = 147 ms; T~= 147 ms; phase cycling for quadrature detection in f,; acquisition, 3 h.
A combined 'H-NMR and molecular mechanics study of [Cpp', Sar7]AVP was performed in order to select the most probable conformations in DMSO solutions. The NMR constraints obtained were employed in the selection of starting conformations of the cyclic moiety of the analog. In particular, the diminished accessibility of the Asn' NH proton to solvent and the close contact between Cpp' and Cys6 C'H protons suggests a 8-turn conformation at the Phe'-Gln4 residues. Energy minimization was carried out both in the ECEPP/2 (rigid-valence geometry) and in the AMBER (flexible-valence geometry) force fields. Comparison of the experimental and calculated values of NMR characteristics has revealed that conformations containing type I, 11, and 111 P-turns at the Phe3-Gln4 residues are in reasonable agreement with the experimental data, with a dynamic equilibrium between the 81 (PIII) and p11 type structures of the cyclic part being the most probable. All of these conformations prefer the negative chirality of the disulfide bridge (xz 2 -90").Five representative conformations were chosen for the acyclic tail: one with a PI, one with a bII'-turn at the Sar7-Arg' residues, two extended-type conformations, and a conformation with a ;-turn at Sar7. Because only high-energy extended conformations were in agreement with NMR data, it was concluded that the acyclic tail has considerable conformational flexibility in solution. The conformations obtained are discussed in terms of the structure-function relationship of the neurohypophyseal hormone analogs.
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