Molecular mobility of polypeptides containing proline as determined by 13C magnetic resonance

Molecular mobility and structure of elastin deduced from the solvent and temperature dependence of 13C magnetic resonance relaxation data

13C relaxation parameters, T1, line width, and NOE, have been determined for backbone carbons of ligamentum nuchae elastin swollen by 0.15 M NaCl, 0.15 M NaCl-formamide, 0.15 M NaCl-ethanol, dimethyl sulfoxide, and formamide. The data have been analyzed in terms of (a) a single correlation time model and (b) a model employing a log-chi2 distribution of correlation times used by Schaefer (1973) to analyze solid cis-polyisoprene 13C relaxation data. Employing the latter mode, one obtains an approximately self-consistent quantitative analysis of all the elastin data. An average backbone correlation time, tau, of ca. 2 nsec is calculated for elastin swollen in the presence of polar organic solvents at 37 degrees, in approximate agreement with tau of 0.4 nsec obtained for bulk cis-polyisoprene at 35 degrees. The influence of solvent and temperature on elastin spectra indicate that the larger tau value (approximately 80 nsec) obtained for elastin swollen by 0.15 M NaCl at 37 degrees is a consequence of weak interchain polar and hydrophobic interactions, a result which is in accord with the reported viscoelastic behavior exhibited by water-swollen elastin at 37 degrees. The results obtained further suggest that Gly, Pro, and Val residues are significantly more mobile than Ala residues, which are located in the cross-link regions. Hence, the NMR data support the view that water-swollen elastin is composed of a network of mobile chains, except possibly in the cross-link regions.

Conformational flexibility of luteinizing hormone-releasing hormone in aqueous solution. A carbon-13 spin-lattice relaxation time study

The carbon-13 nuclear magnetic resonance (13C NMR) spectra of luteinizing hormone-releasing hormone (LH-RH) and lower homologous peptides have been assigned in aqueous solutions at various pH values. 13C spin-lattice relaxation times (T1) have been measured for all proton-bearing carbons at 25.2 and 67.9 MHz. From the T1 data the rates of overall molecular motion and intramolecular motion of side chains have been estimated. LH-RH is a flexible molecule in solution, having segmental motion along the backbone as well as in the nonaromatic side chains.

Molecular dynamics and structure of the random coil and helical states of the collagen peptide, alpha 1-CB2, as determined by 13C magnetic resonance

Carbon-13 chemical shifts, spin-lattice (T1) and spin-spin (T2) relaxation times, and 13C-[1H] nuclear Overhauser enhancements (NOE) have been determined for the random coil and triple helical states of the alpha 1-CB2 fragment of rat skin collagen. Assignment of all aliphatic resonances of this 36 residue peptide in the random coil state (30 degrees) has been achieved with the aid of model polypeptides containing pyrrolidine residues. The chemical shifts and intensities of the Pro and Hyp C-gamma resonances show that (see article) 90% of the X-Pro and X-Hyp bonds are trans in both helix and coil conformations. From T1 measurements rotational correlation times (tau-eff) of ca. 0.45 nsec are calculated for interior C-alpha carbons in the coil, while taueff values of the side chain and near terminal carbons are found to be 2-9 times smaller. These results along with the narrow natural line widths (3-5 Hz) and maximal NOE values (2.8 plus or minus 0.3) demonstrate the high degree of backbone mobility, due to segmental motion, in the unordered state of the peptide. By contrast, the broad lines (50-90 Hz) and small NOE values (1.3 plus or minus 0.3) for the alpha carbons in the helical state (2 degrees) suggest much slower motion. The line widths and NOE values together with the C-alpha T1 values (0.025-0.040 sec) correspond to correlation times which are in reasonable agreement with those calculated for an axially symmetric rigid ellipsoid, undergoing rotational diffusion, having dimensions approximating those of a collagen-type triple helical aggregate of three alpha 1-CB2 chains. A satisfactory computer simulation of the experimental 2 degrees spectrum is obtained by assigning the narrow aliphatic resonances in the spectrum (line widths 5-40 Hz) to (a) carbons in the small amounts of alpha 1-CB2 (3 mol %) and alpha 1-CB1 (2.5 mol %) random coil conformations, (b) carbons in the flexible terminal triplets of the helix, and (c) Ala, Leu, and Phe methyl and phenyl carbons. The side chain carbon line widths obtained from the simulation--when compared with side chain line widths calculated for a rotating rigid ellipsoid with internal motion--indicate rapid axial reorientation of methyl and phenyl groups. With the exception of the Hyp residue the line widths suggest local motion for at least some carbons in most other side chain moieties. The Hyp C-beta and C-gamma line widths indicate the presence of little if any rapid Hyp ring motion.