William A. Gibbons

Reference lineshape adjusted difference NMR spectroscopy. II. experimental verification

Abstract The accuracy and reliability of difference NMR spectroscopy can be substantially improved by a simple correction based on changes in the lineshape of an internal reference line. This reference lineshape adjusted (RLSA) method is experimentally demonstrated to behave according to theoretical predictions under variations in drift of the main magnetic field, in rf power, in homogeneity of the main field, in spinning side bands, in detector phase, and in sweep rate. The RLSA method is experimentally demonstrated to automatically compensate for changes in all these instrumental parameters provided H, is sufficiently small that saturation is avoided. In addition, the internal reference must be constant in composition, concentration, and resonance position, and must be totally resolved from the rest of the spectrum. Under these conditions, the RLSA method is shown to give difference spectra that are visibly superior to simple difference spectra.

A 1H nuclear magnetic resonance study of the opioid peptide dynorphin-(1–13) in aqueous solution

A 1H n.m.r. study of the conformation of the opioid peptide dynorphin-(1-13) was performed in aqueous solution using one-dimensional (1D) and two-dimensional (2D) n.m.r. techniques. Chemical shifts, scalar 3Jφ values, amide proton–deuteron exchange rates, temperature coefficients of chemical shifts, and spin-label perturbation of amide proton relaxation rates formed a self-consistent picture of a polypeptide chain possessing no major preferred secondary conformations. 600 MHz 2D J-resolved spectroscopy was used to analyse the coupling patterns for the consituent amino acid residues. Side-chain rotamer analysis based upon 3Jαβ coupling constants for five of the residues indicated no significantly large populations of any χ1 rotamers.

Confirmation of the solution structure of tyrocidine a using perturbation of proton relaxation rates by nitroxide spin labels

The spin–lattice relaxation rate enhancements of the protons of tyrocidine A upon addition of 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) were analysed in solution and were shown to be consistent with the peptide conformation. The effects on tyrocidine A proton spin–lattice relaxation rates for three TEMPO derivatives in two different solvents have also been studied in order to obtain information about the influence of TEMPO substituents on nitroxide-biomolecule interactions. The neutral free radical TEMPO exhibits less specificity in its interaction with tyrocidine A than its derivatives and, hence, it is the most suitable probe for generally investigating conformational moieties of biomolecules in solution. By corollary, charged nitroxides should be probes of the microenvironment surrounding the specific site of interaction.

The derivation of carbon–proton internuclear distances in organic natural products from 13C relaxation rates and nuclear overhauser effects

The precise definition of the hydrogen bonding patterns of natural product organic molecules and biopolymers in solution has hitherto been very difficult. It is demonstrated here that proton–carbon n.O.e. difference spectroscopy and carbon relaxation rates readily yield carbon assignments, the donor and acceptor groups of hydrogen bonds, and the cis- and trans- stereochemistry around C–O and C–N single bonds.N.O.e. ratio methods and cross-relaxation rates gave proton–carbon distances that are in good agreement with the corresponding crystallographically derived distances.

The solution structure of [Ala4]-desdimethylchlamydocin: a 1H n.m.r. relaxation study

Many fungal peptides exhibit plant toxicity in a host-specific manner. Here we report a proton relaxation and two dimensional (2D) n.m.r. study of the [Ala4]-desdimethyl analogue of the fungal tetrapeptide chlamydocin. Interproton distances calculated from n.m.r. parameters agreed substantially with the corresponding distances in the crystalline form of dihydrochlamydocin. A solution conformational analysis was performed based on n.m.r. distance measurements and φ, ψ, and ω angles of the four residues plus χi rotamer populations. These data support the use of proton relaxation parameters as a basis for accurate solution conformational analysis. As a corollary, the data can also indicate within experimental error that the crystal and solution conformations of chlamydocin and the [Ala4]-desdimethyl analogue, respectively, are identical.

Synthesis and solution structure of [Val3]-HC toxin by 1H and 13C nuclear magnetic resonance relaxation parameters

The [Val3] analogue of the fungal tetrapeptide HC toxin has been synthesised. Conformational parameters of the peptide in solution have been measured by a variety of n.m.r. techniques. The 1H and 13C spectral features were assigned by using two-dimensional n.m.r. methods. Accurate backbone distances (proton–proton and proton–carbon) and those relative to the acceptors and donors of the hydrogen bonds were measured by using homo- and hetero-nuclear Overhauser effects and 1H and 13C relaxation parameters. The peptide conformation thus derived was identical with that of the parental toxin.

13C relaxation studies of the structure and flexibility of the carboxylic ionophore lasalocid A

Carbon-13 spin lattice relaxation times (T1) were determined for all the protonated carbon atoms of the carboxylic ionophore lasalocid A in non-hydroxylic (CDCl3) and hydroxylic (CD3OD) media. In CDCl3, the motions of the protonated backbone carbon atoms C-4 to C-24 (inclusive) are all equally restricted. The correlation time for overall molecular reorientation, τeff, calculated from an average NT1 value of 565 ms, is 8.6 × 10–11 s, where N is the number of attached protons. The carbon atoms in the side chains are more mobile than those in the backbone. The carbon backbone in CD3OD is characterized by segmental motion at the aromatic end, as evidenced by increased NT1, values for C-4, C-5, C-7, and C-8. The reminder of the backbone appears to be rigid and to have a τeff virtually identical with that observed for the entire backbone in CDCl3. These results are discussed in terms of the mechanism that has been proposed for ion complexation and transport by carboxylic ionophores in biomembranes.