Effects of homonuclear scalar couplings during multiple refocusing sequences in nuclear magnetic resonance

Multiple refocusing cycles can be used to extract transverse relaxation rates, R2, while homonuclear scalar couplings do not interfere. In this work, I have demonstrated the usefulness of a hybrid sequence, which is a compromise between single and multiple refocusing sequences, in order to investigate homonuclear scalar coupled spin systems, and to find a regime where the decays used to extract transverse relaxation rates are not modulated by cosine functions due to J-couplings. It was possible to measure the transverse relaxation rates of carbon-13 in doubly-enriched glycine and compare them with those of carbon-13 in singly-enriched glycine samples, which were in good agreement, and confirmed the feasibility of extracting R2 rates in coupled spin systems. I also investigated spin systems with more than one J-coupling interaction, and showed by experiments and numerical simulations that the increased complexity of the system reduces the possibilities of extracting unmodulated decays.

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

Geoffrey Bodenhausen, Fabien Ferrage, Simone Ulzega, Mariachiara Verde

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.

2009

Extending Timescales and Narrowing Linewidths in NMR

Geoffrey Bodenhausen, Aurélien Bornet, Marc Anthony Caporini, Diego Carnevale, Sami Jannin, Pascal Miéville, Martial Rey, Nicola Salvi, Simone Ulzega, Paul Romeo Vasos

Among the different fields of research in nuclear magnetic resonance (NMR) which are currently investigated in the Laboratory of Biomolecular Magnetic Resonance (LRMB), two subjects that are closely related to each other are presented in this article. On the one hand, we show how to populate long-lived states (LLS) that have long lifetimes T_LLS which allow one to go beyond the usual limits imposed by the longitudinal relaxation time T_1. This makes it possible to extend NMR experiments to longer time-scales. As an application, we demonstrate the extension of the timescale of diffusion measurements by NMR spectroscopy. On the other hand, we review our work on long-lived coherences (LLC), a particular type of coherence between two spin states that oscillates with the frequency of the scalar coupling constant J_IS and decays with a time constant T_LLC. Again, this time constant T_LLC can be much longer than the transverse relaxation time T_2. By extending the coherence lifetimes, we can narrow the linewidths to an unprecedented extent. J-couplings and residual dipolar couplings (RDCs) in weakly-oriented phases can be measured with the highest precision.

Schweizerische Chemische Gesellschaft2011

Dynamic Studies through Control of Relaxation in NMR Spectroscopy

Nicola Salvi

In numerous biological processes that constitute the base of living organisms, protein function is fundamentally related to internal dynamics occurring on μs-ms time scales that can give rise to chemical exchange contributions to relaxation. In a heteronuclear two-spin system (e.g., 1H-15N), correlated motions of the two nuclei induce cross-relaxation between multiple-quantum coherences that can be quantified using new Heteronuclear Double Resonance (HDR) techniques. An analytical model describes the effect of an applied radio-frequency field on the relaxation rate of interest in the presence of fast exchange, providing accurate information on the kinetics of correlated processes. Numerical simulations and experimental results confirm the validity of the model. HDR and more conventional relaxation dispersion techniques are used to characterize the internal dynamics in several biological systems. Motions on similar time scales are detected in the two different binding surfaces in human ubiquitin and in the linker between them (Phe45), suggesting the presence of a possible global motion. Additional applications of HDR give insights into the dynamics occurring in the KID-binding domain (KIX) of the CREB-binding protein. We identified the presence of exchange processes, faster than the one reported by Konrat and coworkers, outside the main helices of KIX. These fast motions are most likely the sign of conformational disorder within the native state, which may promote the transition to the unfolded ensemble. A combination of 15N relaxation rates and magnetization transfer rates are used to provide a qualitative characterization of internal dynamics in Engrailed 2. While contribution of exchange processes in the ms time scale are small, fluctuations at sub-ms timescales are found to occur both in the unstructured part that contains important binding sites for other transcription factors, and at a few restricted locations in the structured homeodomain. Such motions are characterized by 15N R1ρ relaxation dispersion. Our results reveal that the hexapeptide motif is characterized by complex dynamics that may be linked to its physiological function. Moreover, the time scale of motions in the homeodomain is reasonably close to that of the hexapeptide region. It is therefore tempting to suppose that transient contacts occur between the structured and unstructured regions.

EPFL2013

Dynamic Studies through Control of Relaxation in NMR Spectroscopy

Nicola Salvi

EPFL2013