NMR at Low and Ultralow Temperatures

Abstract

Conspectus Solid state nuclear magnetic resonance (NMR) measurements at low temperatures have been common in physical sciences for many years, and are becoming increasingly important in studies of biomolecular systems. This article reviews a diverse set of projects from my laboratory, dating back to the early 1990s, that illustrate the motivations for low-temperature solid state NMR, the types of information that are available from the measurements, and likely directions for future research. These projects include NMR studies of both physical and biological systems, performed at low (cooled with nitrogen, down to 77 K) and very low (cooled with helium, below 77 K) temperatures, and performed with and without magic-angle spinning (MAS). In NMR studies of physical systems, the main motivation is to study phenomena that occur only at low temperatures. Two examples from my laboratory are studies of molecular rotation and an orientational ordering in solid C60 at low temperatures and studies of unusual electronic states, called skyrmions, in two-dimensionally confined electron systems within semiconductor quantum wells. NMR measurements on quantum wells were facilitated by optical pumping of nuclear spin polarizations, a signal enhancement phenomenon that exists at very low temperatures. In studies of biomolecular systems, motivations for low-temperature NMR include suppression of molecular tumbling (thereby permitting solid state NMR measurements on soluble proteins), suppression of conformational exchange (thereby permitting quantitation of conformational distributions), and trapping of transient intermediate states in a non-equilibrium kinetic process (by rapid freeze-quenching). Solid state NMR measurements on AIDS-related peptide/antibody complexes, chemically denatured states of the model protein HP35, and a transient intermediate in the rapid folding pathway of HP35 illustrate these motivations. NMR sensitivity generally increases with decreasing sample temperature. It is therefore advantageous to go as cold as possible, particularly in studies of biomolecular systems in frozen solutions. However, solid state NMR studies of biomolecular systems generally require rapid MAS. A novel MAS NMR probe design that uses nitrogen gas for sample spinning and cold helium only for sample cooling allows a wide variety of solid state NMR measurements to be performed on biomolecular systems at 20-25 K, where signals are enhanced by factors of 12-15 relative to measurements at room temperature. MAS NMR at very low temperatures also facilitates dynamic nuclear polarization (DNP), allowing sizeable additional signal enhancements and large absolute NMR signal amplitudes to be achieved with relatively low microwave powers. Current research in my laboratory seeks to develop and exploit DNP-enhanced MAS NMR at very low temperatures, for example in studies of transient intermediates in protein folding and aggregation processes and studies of peptide/protein complexes that can be prepared only at low concentrations.

NMR of sedimented, fibrillized, silica-entrapped and microcrystalline (metallo)proteins

“…Furthermore, on frozen solutions, a large increase of the signal-to-noise ratio can be obtained working from low to very low temperatures (90-25 K), by cooling the sample at with evaporated nitrogen or helium [37,38], thus increasing the Boltzmann population of the lowest energy state [39,40,37,38]. Sample manipulation in the case of frozen solutions is intrinsically simple.…”

Fluid flow dynamics in MAS systems

“…Even at cryogenic temperature below 80 K the properties of helium gas (e.g., condensation point at 4.2 K at standard conditions, its density and speed of sound) are notably different from nitrogen or air at ambient temperature. This has an impact on the development of suitable heat exchanger systems as well as the features of the MAS turbine itself [13,33,[83][84][85][86][87][88][89][90]. Early attempts to work with helium for achieving very low temperatures under MAS go back to Yannoni et al [83,84].…”

“…Early attempts to work with helium for achieving very low temperatures under MAS go back to Yannoni et al [83,84]. Apart from pure helium systems [13,33,85,86,89], where all MAS gas flows -bearing, drive, VT -are helium flows, a system following a different approach has been proposed which works with nitrogen gas for bearing and drive and uses cryogenic helium for VT [87,88].…”

Ultra-Low Temperature Nuclear Magnetic Resonance

“…The construction of a device to access this "ultra"-low temperature spinning (ULTMAS) regime is far from straightforward and specific limitations as cost and loss of (precious) He or low kinematic viscosity of He gas inhibiting stable MAS [25]. However, the benefits are numerous, embracing an increased Boltzmann distribution (cf., ~0.1% 1H polarization at 10 K, ~0.003% at 300 K, all at 10 T), reduced Johnson-Nysquist noise in the radio frequency (RF) circuit, in addition of being able to study low-temperature phenomena at an atomic scale [22,26]. Hardware for ULT-MAS combined with DNP at temperatures <<100 K, has been developed in the teams of R. Griffin at the Massachusetts Institute of Technology (MIT), USA [12], R. Tycko in Bethesda (NIH), USA [14], and T. Fujiwara in Osaka, Japan [16].…”