Peptide conformation. 17. cyclo-(L-Pro-L-Pro-D-Pro). Conformational analysis by 270- and 500-MHZ one- and two-dimensional proton NMR spectroscopy

Abstract

6297Ni' dehydrogenates propane and isobutane by an apparently exothermic 1,2 process,25 which occurs with moderate cross section at low energy (Table I). It is not clear why the 1,2 process "turns off" with straight-chain hydrocarbons with more than three carbons. If the initial insertion into a C-H bond is reversible, then the metal ion may sample several insertion sites in a single encounter.26 To explain the present results requires that further reaction of the C-C insertion intermediates occurs with greater facility (higher frequency factors, lower activation energies) than that of the C-H insertion intermediates. As noted in the Ex-(26) Multiple encounter reactions are a well-established feature of ionmolecule reactions owing to the strong ion-neutral attractive interaction. See, for example: Dugan, J. V., Jr.; Rice, J. H.; Magee, J. L. Chem. Phys. Left. 1969,3,323. Sullivan, S. A.: Beauchamp, J. L. J. Am. Chem. SOC. 1977,99, 5017.perimental Section, the 1,2 process is observed with n-butane at relative kinetic energies above -1 eV. Acknowledgment.Abstract: The 270-and 500-MHz IH NMR spectra of cyclo-(~-Pro,-~-Pro) in various solvents have been analyzed with the aid of one-and two-dimensional (2-D) NMR techniques; 500-MHz 2-D-J-resolved spectra were used to determine starting parameters for chemical shift values and to assign overlapping spin multiplets of the individual nuclei in the 21-spin system. 2-D spin-echo-correlated spectra yield connectivities between coupled spins. In particular, many small long-range couplings have been detected by this technique. Stereochemical assignment of geminal proline protons has been performed by 2-D nuclear Overhauser enhancement (2-D NOE) spectroscopy and by interpretation of aromatic solvent-induced shift (ASIS) effects, coupling constants, and carbonyl anisotropy. A twisted-boat backbone conformation similar to the structure determined by X-ray analysis was derived from the data.

Conformational Analysis of Two Cyclic Analogs of Angiotensin: Implications for the Biologically Active Conformation

“…In the case of cyc/ofSar1, Cys3, Mpt5]-AT, it might be suggested that the side chain rotamers of Tyr4 could play a role in this nonequivalence. However, such a large chemical shift difference was also found in rigid peptides, such as cyclo-(L-Pro-L-Pro-D-Pro) (Kessler et al, 1982), in which the anisotropy effect of the neighboring carbonyl group contributed to a 0.6 ppm shift change between the protons of the proline. This is consistent with the rotamer analysis of the Tyr4 side chains (see below), in which the most populated conformation of xi (g-) places the aromatic ring at the distance of more than 4.5 Á from the Mpt5 -protons, thus ruling out the ring current effect contributing significantly to the nonequivalence of the Mpt5 protons.…”

“…However, such a large chemical shift difference was also found in rigid peptides, such as cyclo-(L-Pro-L-Pro-D-Pro) (Kessler et al, 1982), in which the anisotropy effect of the neighboring carbonyl group contributed to a 0.6 ppm shift change between the 6 protons of the proline. This is consistent with the rotamer analysis of the Tyr4 side chains (see below), in which the most populated conformation of XI (g-) places the aromatic ring at the distance of more for the amide protons of the residues 3,6, and 7 indicate that these are solvent-exposed NH's, whereas the smaller A6/AT values, of 2.2-2.8 X l t 3 ppm/K, observed for the Arg2 and Tyr4 residues, indicate that these amide protons are somewhat shielded from solvent ( Table 2).…”

Chapter 3 Developments in the Karplus Equation as they Relate to the NMR Coupling Constants of Carbohydrates

“…50 An early observation of this effect was the non-equivalence of J endo-endo and J exo-exo in norbornanes, 51,52 which was also predicted to occur in cyclopentane, tetrahydrofuran, and in 7-hetero-substituted nor-bornanes, 50 and also explains 50 the observed non-equivalence of cisoid H–C β –C γ –H and H–C γ –C δ –H couplings in prolines. 53,54 This effect is not described by the typical basic or generalized Karplus equations, and the computations of de Leeuw et al . 50 suggested that it could be approximated by the function: $$\mathrm{\Delta }J=T{cos}^2(P-P_{\text{b}})$$ where Δ J is a decrease in the size of the coupling due to the transmission effect, P is the phase angle of pseudorotation in the five-membered ring, P b is the phase angle of the envelope form for which a maximum effect is reached, and T depends strongly on the nature of the atom through whose orbitals the effect is transmitted.…”

Geometry of the nitrogen-hydrogen...oxygen-carbon (N-H...O:C) hydrogen bond. 2. Three-center (bifurcated) and four-center (trifurcated) bonds

“…The acceptor atom involved in the longer contact (the "minor component") is referred to as X throughout the text. Note that ¿(H-O) > d{H-X) (2) The acceptor atoms involved in the minor components of a four-center bond are referred to as X and Y, where d(H-O) > d(H-X) > d(H-Y) (3) Three-Center Bonds: Preliminary Observations For the purposes of the present study, we define a three-center bond as one in which the proton forms two contacts to hydrogen-bond acceptor atoms, such that both are in the "forward" direction [i.e., ( --O), a(N-H-X) > 90°] and shorter than the sum of the van der Waals radii of the atoms involved [i.e., </(H-), ¿/( -X) >0]. By this definition, 304 of the 1509 N-H-0=C bonds in our sample are three centered.…”