Nuclear magnetic relaxation in the slow tumbling regime
This thesis is primarily concerned with relaxation and the study of the molecular dynamics behind it. Molecular motions occur on a variety of timescales, for instance, site specific internal motions occur within nano-to-picoseconds, and the relative movement of secondary structure elements occur within milli-to-microseconds. Relaxation also occurs between two spins close in space, and gives rise to NOEs. The size of the NOE is determined by the cross-relaxation rate, which is inversely proportional to the distance between the two spins. The determination of accurate cross relaxation rate constants can be difficult; due to both spin diffusion and imperfect selective excitation. One particular spin diffusion suppression method is studied in detail by a series of simulations. This is then applied to Amphotericin-B, a medium sized molecule, and several NOEs are measured free of spin diffusion contributions. Motions on the milli-to-microsecond timescale can have a dramatic affect on the R2 relaxation rate constant. Previously, R2 has been measured as a function of field strength using either CPMG or spin lock experiments. Each experiment is only suited to measuring a limited range of timescales based on the field strengths available for use. Recent experiments have combined CPMG and off-resonance spin-lock experiments to measure R2 over a wide range of field strengths, from 2-40 krad/s. These experiments are applied to the unstructured domain of RhoGDI. RhoGDI is interesting as it consists of two domains: one domain is rigid, whilst the other is unstructured in solution, but contains transient elements of secondary structure. Motions on the pico-to-nanosecond timescale can be mapped by measuring the R1, R2 and heteronuclear steady state NOE. A reduced spectral density function analysis was performed on the structured domain of RhoGDI. A comparison is also shown between the RSDF of the protonated and deuterated forms of RhoGDI.