Water in a Crowd

Abstract

In many situations, form biology to geology, water occurs not as the pure bulk liquid but rather in nanoscopic environments, in contact with interfaces, interacting with ionic species, and interacting with large organic molecules. In such situations, water does not behave in the same manner as it does in the pure bulk liquid. Water dynamics are fundamental to many processes such as protein folding and proton transport. Such processes depend on the dynamics of water's hydrogen bonding network. Here, the results of ultrafast infrared experiments are described that shed light on the influences of nanoconfinement, interfaces, ions, and organic molecules on water hydrogen bond dynamics.

Self-aggregation of the pyrene labeled poly(acrylic acid) in nanoscopically crowded environments

“…2B). In this case, a strong association takes place between the sulfonate heads of surfactant and Na + counterions, and the interfacial water primarily hydrates the counterions [25]. This interaction confers a strong stiffness to the micellar interface and polymer coil, so that pyrenes are in close proximity favoring the static excimer formation.…”

“…For RMs with larger water pools (w 0 = 15), a small red shift occurs from 342 to 343 nm. In the larger RMs, the sulfonate groups of AOT are fully hydrated and the remaining water forms hydrogen bonding networks like in bulk water [25]. Now, the polymer coil adopts a less rigid but compact conformation.…”

“…In RMs, the surfactant molecules cluster with the polar head groups toward the nano-sized water droplets and the hydrocarbon tails delimit the organic phase [24]. For RMs with spherical morphology, the size of aqueous core depends on the molar ratio of water molecules per surfactant molecule or the hydration degree (w 0 ) [25][26][27]. The micellar system formed by self-assembly of anionic surfactant sodium bis(2-ethylhexyl) sulfosuccinate (AOT) in non-polar solvents has been used to highlight the unique properties of water at the micellar interface [28][29][30].…”

Ultrafast 2D IR microscopy

“…During the past 15 years, ultrafast timeresolved IR spectroscopy-two-dimensional infrared (2D IR) spectroscopy in particular-has experienced tremendous advances in both new methods and applications. For example, 2D IR has found applications in the study of processes such as membrane protein dynamics [1,2], water dynamics and confinement [3,4], protein-drug interactions [5], fast protein folding [6], transient photochemical dynamics [7,8], or charge separation in organic photovoltaic materials [9]. Indeed, 2D IR spectra offer a wealth of molecular information, including: 1. molecular structure, as observed in the off-diagonal peaks that arise from vibrational coupling or relative dipole orientations that are extracted from polarization-dependent spectra [10,11]; 2. molecular dynamics, which are obtained from vibrational dynamics measurements such as spectral diffusion, orientational relaxation rate, and vibrational lifetime; 3. rates of chemical exchange between different species [12]; and 4.…”

Infrared spectroscopy of proteins in reverse micelles

“…These microenvironments are subject to macromolecular crowding, with high concentrations of dissolved substances that alter the bulk properties of water, and lead to changes in the structure of proteins and the activity of enzymes [5,10,13,18,32,34,35]. The effects of confinement and crowding may be mimicked in reverse micelles, making them useful models of biological microenvironments [15]. …”