Side-chain vs. main-chain conformational flexibility in aromatic dipeptides

Abstract

4195The application of eq 15 to the 2Jcc, in P-[Dlglucose 16 and a- [D]Glucose 17 gives results in good conformity with the experimental data if we assume that the OH group at the C2 carbon contributes +3 Hz to the total (vide supra) and that the contributions from eq 15 for each of the three oxygen substituents at C1 and C3 are additive. The total of the four contributions are +2.5 Hz and -0.1 Hz, which are in good conformity with the experimental values of 3.5 Hz and C1.5 Hz, respectively. ConclusionsGeminal 13C-'3C coupling constants exhibit dependencies on substituent orientation, hybridization, and CI-C2-C3 bond angle effects. Some of the trends are qualitatively reproduced by means of the INDO-FPT MO method, but the calculated magnitudes are often 5-6 Hz too negative.By means of a valence-bond bond-order formulation, 2Jccr in a propanic fragment are related to geminal, vicinal, and long range 'H-IH or to geminal and vicinal I3C-'H coupling constants. The long range terms make relatively small contributions, but the small magnitudes of 2Jcc. in saturated hydrocarbons must be due to a cancellation between the geminal and vicinal terms. In freely rotating systems a good linear correlation is observed between the experimental data for zJcc, and 2JcH.Substituent effects at the C2 carbon follow the same trends observed for geminal 'H-'H and I3C-'H coupling constants wherein electronegative substituents lead to more positive values of ZJccJ. By means of VB formulation it is shown that substituents at C1 or C3 should lead to an angular dependence of the vicinal type. A single equation of this type is found to provide a good correlation of the experimental data.

Structural characterization of NRAS isoform 5

“…19,20 In addition, it has been observed that clusters of TFE may mimic the interior of a protein, induce turns, and fix aromatic rings. 19,[21][22][23][24] Therefore, our calculated structure of NRAS isoform 5 is consistent with the TFE literature.…”

Kinetically trapped structure in the renaturation of reduced oxindolealanine 62 lysozyme

“…The behavior in the Trp63 residue was shown to be reflected on that of 0X62 in an earlier NMR analysis of 0X62 lysozyme (Blake et al, 1981). Moreover, the Trp63 residue was supposed to be affected by 0X62 in the unfolded state because there were interactions between the amino acid side chains in the sequence of Trp-Trp or Trp-Tyr (Rizzo & Jackie, 1983). Therefore, Trp62 may be confirmed to be one of the key residues in the renaturation of reduced lysozyme.…”

Conformational transformation of ascidiacyclamide analogues induced by incorporating enantiomers of phenylalanine, 1-naphthylalanine or 2-naphthylalanine

“…Small spectral changes with increasing CH 3 CN concentrations were observed for [ l ‐1Nal] and [ l ‐2Nal]ASC and were also seen for [ l ‐Phe]ASC. Because the molar ellipticities in the range of 200–230 nm may be governed more by the side‐chain naphtyl chromophores than by the folded structure of the main‐chain amide chromophores , these spectra differed among [ l ‐Phe], [ l ‐1Nal], and [ l ‐2Nal]ASC in the range of 200–230 nm. All the spectra for the d ‐enantiomer‐incorporated analogues, [ d ‐Phe], [ d ‐1Nal], and [ d ‐2Nal]ASC in TFE were similar to those of the l ‐enantiomer‐incorporated ASCs (dashed lines).…”

“…Small spectral changes with increasing CH 3 CN concentrations were observed for [L-1Nal] and [L-2Nal]ASC and were also seen for [L-Phe]ASC. Because the molar ellipticities in the range of 200-230 nm may be governed more by the side-chain naphtyl chromophores than by the folded structure of the main-chain amide chromophores [25,26], these spectra differed among (Table S1). Such guested solvent molecules are often found in the open structures in crystal of ASC.…”

“…Two isomeric amino acids differ only on the position at which the naphthalene ring is linked to the Cβ atom. The hindrance caused by the bulky moiety near the peptide bond is much greater with the 1Nal residue than the 2Nal residue and has a significant impact on the peptide conformations and pharmacological properties of the peptide that contains the 1Nal or 2Nal residue . For this study, we synthesized five ASCs [ l ‐1Nal], [ l ‐2Nal], [ d ‐Phe], [ d ‐1Nal], and [ d ‐2Nal]ASC (Figure ), and here we describe their characterization using 1 H NMR, circular dichroism (CD) spectroscopy, and X‐ray diffraction.…”