No abstract

Power supplies used in field-cycling nuclear resonance experiments are described. These are used to switch an air coil from high field to low field as rapidly as possible, then back to high field after a short time. This sequence must be repeated every few seconds or minutes. An example of a simple regulator is described, and methods of output switching with relays are discussed. A circuit is described which delivers up to 10 kG (15 A) in a 2.6 cm i.d. superconducting coil with a switching speed of 5×105 G/sec and an accuracy probably as great as 1 G. Speed is achieved by automatic connection, during up or down switching, of energy storage capacitors across the magnet, operating at much higher voltage than the steady-state voltage level across the magnet and pass transistors.

Previous work by several groups has shown that the combination of spin--spin coupling constants and spectral density components (derived from spin--lattice relaxation and/or nuclear Overhauser measurements) may aid in the task of conformational determination of peptides in solution. Using the peptide formyl-L-methionyl-L-leucyl-L-phenylalanine, which is a potent specific chemotactic agent for leucocytes, we show the following: (a) that 3JNHCH coupling constants are consistent with a high degree of rigidity in the peptide backbone in solution, (b) that 3H isotopic substitution in combination with relaxation data taken at different Larmor frequencies enables spectral density, and thence conformational, information to be obtained, (c) that side-chain conformations for this molecule mirror, in some aspects, those found in the solid state for other peptides containing the same residues, and (d) that temperature dependence of amide chemical shifts does not have direct implication concerning the existence of intramolecular hydrogen bonds in peptides. We are able to propose a family of conformations which appear to interchange rapidly on the NMR time scale and are characterized by a distribution of side-chain rotamers. The basic backbone conformation is, or closely approximates, a small beta antiparallel pleated sheet and as such suggests a possible mode of receptor--chemotactic peptide interaction.

SynopsisThe application of rotating frame spin-lattice relaxation to the determination of the intramolecular motions in peptides is discussed, and results are presented on the application of I3C 2' 1, to peptide microdynamics in solution. The effective molecular rotational reorientation times a t the amide and amino nitrogens may be derived from appropriate data on 2' 1, of the carbons adjacent to them. We also show by theoretical calculations that lH and I3C 2'1, experiments on suitable 2H and 15N substituted peptides will allow intramolecular main-and sidechain motions, characterized by times in the range 10-1-10-6 sec, to be investigated.