1H-NMR conformational studies of biomolecules and their complexes with diamagnetic metal ions: solvent exposure delineations of proton nuclei by using stable nitroxides

Abstract

Nitroxide induced perturbations of proton relaxation rates of compounds of established solution structure has been shown to be mainly correlated to the hydrogen solvent exposure and, hence, to the molecular conformation [1]. The solution dynamics and relaxation mechanisms involved in the nitroxide-biomolecule interactions have been analyzed and experimental data suggest that solvent exposed amide groups are hydrogen bonded with nitroxide N-oxyl moieties. This interaction and random biomolecule-nitroxide collisions determine the extent of observed paramagnetic effects on proton relaxation rates. Dipolar relaxation occurs between the unpaired electron of the nitroxide and proton nuclei of the biomolecule, but a quantitation of relaxation enhancements in terms of exposure factors is not straightforward, due to the complexity of biosystems where many simultaneous interacting sites have to be taken into account. Nevertheless, a qualitative insight into solvent accessibility to protons attached to complex molecular systems and the conformation of their diamagnetic metal complexes is possible. Relaxation data obtained for the 2,2,6,6-tetramethyl piperidine-1-oxyl/gramicidine S system in DMSO, shown in Table I, confirm that hydrogen bonded and/or solvent shielded protons have the smallest molar paramagnetic relaxation S 1p. t001 Relaxation Data. ppm a S 1p(sec −1 M −1 Leu NH 8.31 100 ± 6 Leu H 4.64 107 ± 7 Orn NH 8.62 380 ± 13 Orn H 4.94 40 ± 10 Phe NH 9.11 350 ± 15 Phe H 4.47 235 ± 10 Pro H 4.42 130 ± 6 Methyls 1.04 370 ± 14 Aromatics 7.20 410 ± 16 a ppm from internal TMS.