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The impact of pore geometry and functionality on the dynamics of water nanoconfined in porous media are the subject of some debate. We report the synthesis and small-angle X-ray scattering (SAXS) characterization of a series of perdeuterated gemini surfactant lyotropic liquid crystals (LLCs), in which convex, water-filled nanopores of well-defined dimensions are lined with carboxylate functionalities. Quasielastic neutron scattering (QENS) measurements of the translational water dynamics in these dicarboxylate LLC nanopores as functions of the surfactant hydration state and the charge compensating counterion (Na+, K+, NMe4+) reveal that the measured dynamics depend primarily on surfactant hydration, with an unexpected counterion dependence that varies with hydration number. We rationalize these trends in terms of a balance between counterion–water attractions and the nanopore volume excluded by the counterions. On the basis of electron density maps derived from SAXS analyses of these LLCs, we directly show that the volume excluded by the counterions depends on both their size and spatial distribution in the water-filled channels. The translational water dynamics in the convex pores of these LLCs are also slower than those reported in the concave pores of AOT reverse micelles, implying that water dynamics also depend on the nanopore curvature.

Optimal enzyme activity depends on a number of factors, including structure and dynamics. The role of enzyme structure is well recognized; however, the linkage between protein dynamics and enzyme activity has given rise to a contentious debate. We have developed an approach that uses an aqueous mixture of organic solvent to control the functionally relevant enzyme dynamics (without changing the structure), which in turn modulates the enzyme activity. Using this approach, we predicted that the hydride transfer reaction catalyzed by the enzyme dihydrofolate reductase (DHFR) from Escherichia coliin aqueous mixtures of isopropanol (IPA) with water will decrease by ∼3 fold at 20% (v/v) IPA concentration. Stopped-flow kinetic measurements find that the pH-independent khydriderate decreases by 2.2 fold. X-ray crystallographic enzyme structures show no noticeable differences, while computational studies indicate that the transition state and electrostatic effects were identical for water and mixed solvent conditions; quasi-elastic neutron scattering studies show that the dynamical enzyme motions are suppressed. Our approach provides a unique avenue to modulating enzyme activity through changes in enzyme dynamics. Further it provides vital insights that show the altered motions of DHFR cause significant changes in the enzymeʼs ability to access its functionally relevant conformational substates, explaining the decreased khydriderate. This approach has important implications for obtaining fundamental insights into the role of rate-limiting dynamics in catalysis and as well as for enzyme engineering.

The diffusive motions of covalently tethered 1,3-diphenylpropane (DPP) via a silyl-aryl-ether linkage in the mesopores of MCM-41 were studied by quasielastic neutron scattering. The geometric effect of pore radius was investigated with samples having pores that ranged from 1.6 to 3.0 nm in diameter and highest achievable DPP grafting density. The effect of molecular crowding was investigated in 3.0 nm diameter pores for surface coverage ranging from 0.60 to 1.61 DPP/nm2. Temperature dependence was determined for large pore diameter samples from 240 to 370 K. As the DPP molecules remain attached over this entire temperature range, data were analyzed in terms of a model of localized diffusion inside a sphere. Only the motions of the DPP hydrogen atoms were considered because of the high sensitivity of neutron scattering to the presence of hydrogen. As atoms far from the attachment point have a greater range of motion than those nearer the tether, the radius of the sphere limiting the motion of individual hydrogen atoms was allowed to increase based on the atom’s distance from the tether point. Both smaller pore diameters and higher DPP grafting density resulted in larger amplitude motion while the diffusion coefficient was greatest in the largest pores at highest DPP density. These observations support a model where the DPP molecules prefer an orientation allowing close proximity to the MCM-41 pore surface and are forced into the pore interior by either the steric effect of small pore diameter or by increased competition for surface area at high molecule surface coverage.

Neutron scattering is exquisitely sensitive to the position, concentration, and dynamics of hydrogen atoms in materials and is a powerful tool for the characterization of structure‐function and interfacial relationships in biological systems. Modern neutron scattering facilities offer access to a sophisticated, nondestructive suite of instruments for biophysical characterization that provides spatial and dynamic information spanning from Ångstroms to microns and from picoseconds to microseconds, respectively. Applications in structural biology range from the atomic‐resolution analysis of individual hydrogen atoms in enzymes through to meso‐ and macro‐scale analysis of complex biological structures, membranes, and assemblies. The large difference in neutron scattering length between hydrogen and deuterium allows contrast variation experiments to be performed and enables H/D isotopic labeling to be used for selective and systematic analysis of the local structure, dynamics, and interactions of multi‐component systems. This overview describes the available techniques and summarizes their practical application to the study of biomolecular systems. Curr. Protoc. Protein Sci. 72:17.16.1‐17.16.34. © 2013 by John Wiley & Sons, Inc.