Carbon-13 chemical shifts accompanying helix formation

Conformational changes of alamethicin induced by solvent and temperature. A 13C-NMR and circular-dichroism study

13C nuclear magnetic resonance (NMR) and circular dichroism (CD) have been used for studies on the conformation of alamethicin. The 13C NMR spectrum is assigned with the aid of signals of synthetic partial sequences and selective proton decoupling. The solvent and temperature-dependence of the 13C NMR spectra, T1 measurements and the use of lanthanide-shift reagents allow the differentiation between the amino acids belonging to a rigid alpha-helical portion of the alamethicin sequence and those belonging to a more flexible part. The 13C NMR results are in agreement with results obtained from extended solvent and temperature-dependent CD studies which indicate a highly stabilized nonpolar and intrachenar alpha-helical part. The concentration-dependence of the CD spectrum of alamethicin in a nematic phase revealed aggregation phenomena which might simulate those observed in natural and synthetic membranes. After dissolving alamethicin in aqueous alcohol there is a time-dependence of the ellipticity of the Cotton effects showing a sort of memory effect on the mode of dissolution. Four different conformations can be characterized by CD spectra depending on the solvent and concentration. A model illustrating the dynamic conformations and aggregation phenomena within a membrane is proposed.

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.

Long-range, pH-dependent effects on the carbon-13 nuclear magnetic resonance spectra of oxytocin

The effects of hydrogen ion concentrations on the carbon-13 nuclear magnetic resonance spectra of oxytocin were investigated. The starting pD of 3.0 was increased stepwise to 8.4. A change of the state of protonation of the N-terminal amino group of oxytocin is accompanied by changes in chemical shifts of carbon-13 nuclei of amino-acid residues located in the 20-membered ring of the hormone. The resonance positions of the acyclic peptide portion, Pro-Leu-Gly-NH(2), remain constant. The pD-induced chemical-shift changes of carbons up to five bonds removed from the site of protonation are interpreted in terms of "through-bond" and "through-space" mechanisms. Chemical-shift changes of carbons more than five bonds removed are proposed to have a conformational origin. It is suggested that a change in the charge density of the amino group perturbs the dihedral angle of the -CH(2)-S-S-CH(2)- moiety of oxytocin, which in turn significantly affects the overall conformation of the 20-membered ring of the hormone.