Biomolecular hydration dynamics probed with 2D-IR spectroscopy: From dilute solution to a macromolecular crowd

Why are Vibrational Lines Narrow in Proteins?

The vibrational Stark effect in proteins yields line shifts indicative of strong internal electric fields up to a few volts per angstrom. These values are supported by numerical simulations of proteins. The simulations also show a significant breadth of field fluctuations translating to inhomogeneous broadening of vibrational lines. According to fluctuation-dissipation arguments, strong internal fields should lead to broad lines. Experimentally reported vibrational lines in proteins are, however, very narrow. This disconnect is explained here in terms of the insufficient (nonergodic) sampling of the protein's configurations on the lifetime of the vibrational probe. The slow component of the electric field fluctuations in proteins relaxes on the time scale of tens of nanoseconds and is dynamically frozen on the vibrational lifetime.

Toward comprehensive measurement of protein hydration dynamics: Facilitation of NMR-based methods by reverse micelle encapsulation

Protein-water interactions are a fundamental determinant of protein structure and function. Despite their importance, the molecular details of water orientations and dynamics near protein surfaces remain poorly understood, largely due to the difficulty of measuring local water mobility near the protein in a site-resolved fashion. Solution NMR-based measurement of water mobility via the nuclear Overhauser effect was presented as a method for performing comprehensive, site-resolved measurements of water dynamics many years ago. Though this approach yielded extensive insight on the dynamics and locations of waters buried within proteins, its promise for measuring surface hydration dynamics was impeded by various technical barriers. Over the past several years, however, this approach has been pursued anew with the aid of reverse micelle encapsulation of proteins of interest. The confined environment of the reverse micelle resolves many of these barriers and permits site-resolved measurement of relative water dynamics across much of the protein surface. Here, the development of this strategy for measuring hydration dynamics is reviewed with particular focus on the important remaining challenges to its widespread application.

Water at surfaces with tunable surface chemistries

Aqueous interfaces are ubiquitous in natural environments, spanning atmospheric, geological, oceanographic, and biological systems, as well as in technical applications, such as fuel cells and membrane filtration. Where liquid water terminates at a surface, an interfacial region is formed, which exhibits distinct properties from the bulk aqueous phase. The unique properties of water are governed by the hydrogen-bonded network. The chemical and physical properties of the surface dictate the boundary conditions of the bulk hydrogen-bonded network and thus the interfacial properties of the water and any molecules in that region. Understanding the properties of interfacial water requires systematically characterizing the structure and dynamics of interfacial water as a function of the surface chemistry. In this review, we focus on the use of experimental surface-specific spectroscopic methods to understand the properties of interfacial water as a function of surface chemistry. Investigations of the air-water interface, as well as efforts in tuning the properties of the air-water interface by adding solutes or surfactants, are briefly discussed. Buried aqueous interfaces can be accessed with careful selection of spectroscopic technique and sample configuration, further expanding the range of chemical environments that can be probed, including solid inorganic materials, polymers, and water immiscible liquids. Solid substrates can be finely tuned by functionalization with self-assembled monolayers, polymers, or biomolecules. These variables provide a platform for systematically tuning the chemical nature of the interface and examining the resulting water structure. Finally, time-resolved methods to probe the dynamics of interfacial water are briefly summarized before discussing the current status and future directions in studying the structure and dynamics of interfacial water.