Heteronuclear double resonance in nuclear magnetic resonance spectroscopy: Relaxation of multiple-quantum coherences.

Theoretical and experimental investigations of the relaxation rates of multiple-quantum coherences during heteronuclear double resonance (HDR) pulse sequences are presented. Average Liouvillian theory yields analytical expressions to describe the effective relaxation rates of multiple-quantum coherences during HDR irradiation. Experiments were carried out on a (13)C-(1)H pair in glycerol to measure the effective auto- and cross-relaxation rates of multiple-quantum coherences during HDR schemes. The experimental results exhibit a very good agreement with theoretical predictions, even when the average Liouvillian expansion is truncated to zeroth order.

Preservation of heteronuclear multiple-quantum coherences in NMR by double-resonance irradiation.

A heteronuclear double-resonance (HDR) method based on MLEV-32 or WALTZ-32 pulse sequences has been designed for the investigation of relaxation of heteronuclear multiple-quantum (MQ) coherences. The theoretical analysis of this technique uses average Hamiltonian theory (AHT) to treat the effects of coherent evolution associated with scalar couplings, offsets, and inhomogeneous radiofrequency (rf) fields during the pulse sequence. Under most conditions, the dynamics of the MQ coherences during the HDR sequence is not affected by rf inhomogeneities and scalar couplings for offsets as large as the nutation frequency. The predictions drawn from AHT are supported by numerical simulations and experiments.

Evidence of chemical exchange in recombinant Major Urinary Protein and quenching thereof upon pheromone binding.

The internal dynamics of recombinant Major Urinary Protein (rMUP) have been investigated by monitoring transverse nitrogen-15 relaxation using multiple-echo Carr-Purcell-Meiboom-Gill (CPMG) experiments. While the ligand-free protein (APO-rMUP) features extensive evidence of motions on the milliseconds time scale, the complex with 2-methoxy-3-isobutylpyrazine (HOLO-rMUP) appears to be much less mobile on this time scale. At 308 K, exchange rates k (ex) = 500-2000 s(-1) were typically observed in APO-rMUP for residues located adjacent to a beta-turn comprising residues 83-87. These residues occlude an entry to the binding pocket and have been proposed to be a portal for ligand entry in other members of the lipocalin family, such as the retinol binding protein and the human fatty-acid binding protein. Exchange rates and populations are largely uncorrelated, suggesting local 'breathing' motions rather than a concerted global conformational change.