High-Resolution NMR Probe for Experiments at High Pressures

Unfolding under Pressure: An NMR Perspective

This review aims to analyse the role of solution nuclear magnetic resonance spectroscopy in pressure-induced in vitro studies of protein unfolding. Although this transition has been neglected for many years because of technical difficulties, it provides important information about the forces that keep protein structure together. We first analyse what pressure unfolding is, then provide a critical overview of how NMR spectroscopy has contributed to the field and evaluate the observables used in these studies. Finally, we discuss the commonalities and differences between pressure-, cold- and heat-induced unfolding. We conclude that, despite specific peculiarities, in both cold and pressure denaturation the important contribution of the state of hydration of nonpolar side chains is a major factor that determines the pressure dependence of the conformational stability of proteins.

Molecular Dynamics and Nuclear Magnetic Resonance Studies of Supercritical CO2 Sorption in Poly(Methyl Methacrylate)

The study of supercritical carbon dioxide sorption processes is an important and urgent task in the field of "green" chemistry and for the selection of conditions for new polymer material formation. However, at the moment, the research of these processes is very limited, and it is necessary to select the methodology for each polymer material separately. In this paper, the principal possibility to study the powder sorption processes using ¹³C nuclear magnetic resonance spectroscopy, relaxation-relaxation correlation spectroscopy and molecular dynamic modeling methods will be demonstrated based on the example of polymethylmethacrylate and supercritical carbon dioxide. It was found that in the first nanoseconds and seconds during the sorption process, most of the carbon dioxide, about 75%, is sorbed into polymethylmethacrylate, while on the clock scale the remaining 25% is sorbed. The methodology presented in this paper makes it possible to select optimal conditions for technological processes associated with the production of new polymer materials based on supercritical fluids.

Refined high-pressure tube design for improved resolution in high-pressure NMR spectroscopy

A simple, low-cost, and easy-to-replicate high-pressure sapphire tube for NMR spectroscopy experiments is introduced. The significant improvement in spectral resolution enables, e.g., in situ quantification of the H2 ingress in green solvents.

Combining High-Pressure Perturbation with NMR Spectroscopy for a Structural and Dynamical Characterization of Protein Folding Pathways

High-hydrostatic pressure is an alternative perturbation method that can be used to destabilize globular proteins. Generally perfectly reversible, pressure exerts local effects on regions or domains of a protein containing internal voids, contrary to heat or chemical denaturant that destabilize protein structures uniformly. When combined with NMR spectroscopy, high pressure (HP) allows one to monitor at a residue-level resolution the structural transitions occurring upon unfolding and to determine the kinetic properties of the process. The use of HP-NMR has long been hampered by technical difficulties. Owing to the recent development of commercially available high-pressure sample cells, HP-NMR experiments can now be routinely performed. This review summarizes recent advances of HP-NMR techniques for the characterization at a quasi-atomic resolution of the protein folding energy landscape.

A low cost, portable NMR probe for high pressure, MR relaxometry

A low cost, portable, high volume, stainless steel pressure reactor is modified to easily perform magnetic resonance relaxometry at industrially relevant pressures. Unlike existing pressurization strategies common to nuclear magnetic resonance (NMR) spectroscopy, this approach is amenable to realistic samples that feature heterogeneity and have traditionally escaped NMR study at pressure. This pressure reactor/NMR probe combination is easily accommodated by most single-sided and other low magnetic field permanent magnet assemblies. The performance of the probe is demonstrated by accomplishing NMR relaxometry on polydimethylsiloxane at different pressures with two types of unilateral magnets.

Practical aspects of high-pressure NMR spectroscopy and its applications in protein biophysics and structural biology

Pressure and temperature are the two fundamental variables of thermodynamics. Temperature and chemical perturbation are central experimental tools for the exploration of macromolecular structure and dynamics. Though it has long been recognized that hydrostatic pressure offers a complementary and often unique view of macromolecular structure, stability and dynamics, it has not been employed nearly as much. For solution NMR applications the limited use of high-pressure is undoubtedly traced to difficulties of employing pressure in the context of modern multinuclear and multidimensional NMR. Limitations in pressure tolerant NMR sample cells have been overcome and enable detailed studies of macromolecular energy landscapes, hydration, dynamics and function. Here we review the practical considerations for studies of biological macromolecules at elevated pressure, with a particular emphasis on applications in protein biophysics and structural biology.

Monitoring protein folding through high pressure NMR spectroscopy

High-pressure is a well-known perturbation method used to destabilize globular proteins. It is perfectly reversible, which is essential for a proper thermodynamic characterization of a protein equilibrium. In contrast to other perturbation methods such as heat or chemical denaturant that destabilize protein structures uniformly, pressure exerts local effects on regions or domains of a protein containing internal cavities. When combined with NMR spectroscopy, hydrostatic pressure offers the possibility to monitor at a residue level the structural transitions occurring upon unfolding and to determine the kinetic properties of the process. High-pressure NMR experiments can now be routinely performed, owing to the recent development of commercially available high-pressure sample cells. This review summarizes recent advances and some future directions of high-pressure NMR techniques for the characterization at atomic resolution of the energy landscape of protein folding.

Pressure Dependence of Carbonate Exchange with [NpO2(CO3)3]4- in Aqueous Solutions

The rates of ligand exchange into the geochemically important [NpO₂(CO₃)₃]^4- aqueous complex are measured as a function of pressure in order to complement existing data on the isostructural [UO₂(CO₃)₃]^4- complex. Experiments are conducted at pH conditions where the rate of exchange is independent of the proton concentration. Unexpectedly, the experiments show a distinct difference in the pressure dependencies of rates of exchange for the uranyl and neptunyl complexes.

A reactor for high-throughput high-pressure nuclear magnetic resonance spectroscopy

The design of a reactor for operando nuclear magnetic resonance (NMR) monitoring of high-pressure gas-liquid reactions is described. The Wisconsin High Pressure NMR Reactor (WiHP-NMRR) design comprises four modules: a sapphire NMR tube with titanium tube holder rated for pressures as high as 1000 psig (68 atm) and temperatures ranging from -90 to 90 °C, a gas circulation system that maintains equilibrium concentrations of dissolved gases during gas-consuming or gas-releasing reactions, a liquid injection apparatus that is capable of adding measured amounts of solutions to the reactor under high pressure conditions, and a rapid wash system that enables the reactor to be cleaned without removal from the NMR instrument. The WiHP-NMRR is compatible with commercial 10 mm NMR probes. Reactions performed in the WiHP-NMRR yield high quality, information-rich, and multinuclear NMR data over the entire reaction time course with rapid experimental turnaround.

High-pressure denaturation of apomyoglobin

The pressure denaturation of wild type and mutant apomyoglobin (apoMb) was investigated using a high-pressure, high-resolution nuclear magnetic resonance and high-pressure fluorescence techniques. Wild type apoMb is resistant to pressures up to 80 MPa, and denatures to a high-pressure intermediate, I(p), between 80 and 200 MPa. A further increase of pressure to 500 MPa results in denaturation of the intermediate. The two tryptophans, both in the A helix, remain sequestered from solvent in the high-pressure intermediate, which retains some native NOESY cross peaks in the AGH core as well as between F33 and F43. High-pressure fluorescence shows that the tryptophans remain inaccessible to solvent in the I(p) state. Thus the high-pressure intermediate has some structural properties in common with the apoMb I(2) acid intermediate. The resistance of the AGH core to pressures up to 200 MPa provides further evidence that the intrinsic stability of these alpha-helices is responsible for their presence in a number of equilibrium intermediates as well as in the earliest kinetic folding intermediate. Mutations in the AGH core designed to disrupt packing by burying a charge or increasing the size of a hydrophobic residue significantly perturbed the unfolding of native apoMb to the high-pressure intermediate. The F123W and S108L mutants both unfolded at lower pressures, while retaining some resistance to pressures below 50 MPa. The charge burial mutants, A130K and S108K, are not stable at very low pressures and both denature to the intermediate by 100 MPa, half of the pressure required for wild type apoMb. Thus a similar intermediate state is created independent of the method of perturbation, and mutations have similar effects on native state destabilization for both methods of denaturation. These data suggest that equilibrium intermediates that can be formed through different means are likely to resemble a kinetic intermediate.

High-resolution, high-pressure NMR studies of proteins

Advanced high-resolution NMR spectroscopy, including two-dimensional NMR techniques, combined with high pressure capability, represents a powerful new tool in the study of proteins. This contribution is organized in the following way. First, the specialized instrumentation needed for high-pressure NMR experiments is discussed, with specific emphasis on the design features and performance characteristics of a high-sensitivity, high-resolution, variable-temperature NMR probe operating at 500 MHz and at pressures of up to 500 MPa. An overview of several recent studies using 1D and 2D high-resolution, high-pressure NMR spectroscopy to investigate the pressure-induced reversible unfolding and pressure-assisted cold denaturation of lysozyme, ribonuclease A, and ubiquitin is presented. Specifically, the relationship between the residual secondary structure of pressure-assisted, cold-denatured states and the structure of early folding intermediates is discussed.

A high-pressure, high-resolution NMR probe for experiments at 500 MHz

A novel high-pressure, high-resolution NMR probe is described which operates at a frequency of 500 MHz. The design features an alternative RF coil (8 mm sample tube) for high frequency, sensitivity, probe power, and resolution (< 3.0 x 10(-9)). The probe is capable of pressures to at least 5 kbar over a temperature range of -30 to 80 degreesC, and has a double-tuned 1H/2H circuit which can tune at 1H frequencies of either 300 or 500 MHz. The sensitivity of the 300-MHz circuit is over twice that of previous 10-mm high-pressure NMR probe designs, while at 500 MHz the sensitivity is nearly five times that of previous 300-MHz pressure probes. Potential biochemical applications are demonstrated by 2D NOESY spectra of a Troponin C mutant.

Pressure-induced changes in the folded structure of lysozyme

We demonstrate, for the first time in solution, that pressure induces changes in the overall folded structure of a protein (lysozyme). This was made possible by using a home-developed, on-line continuously variable pressure cell on a high resolution NMR spectrometer operating at 750 MHz. We could follow pressure-induced diamagnetic chemical shifts of more than 26 protons of lysozyme at variable pressure in the range of 1 to 2000 bar. The results indicate that the main effect of the pressure is a compaction of the hydrophobic core part of the protein consisting of bulky side-chains. The technique introduced here provides a general method with which one can probe microscopic internal flexibility of a protein in solution.