Controlling Interfacial Hydrogen Bonding at a Gold Surface: The Effect of Organic Cosolvents.

Water often serves as both a reactant and solvent in electrocatalytic reactions. Interfacial water networks can affect the transport and kinetics of these reactions, e.g., hydrogen evolution reaction and CO2 reduction reaction. Adding cosolvents that influence the hydrogen-bonding (H-bonding) environment, such as dimethyl sulfoxide (DMSO), has the potential to tune the reactivity of these important electrocatalytic reactions by regulating the interfacial local environment and water network. We investigate interfacial H-bonding networks in water-DMSO cosolvent mixtures on gold surfaces by using surface-enhanced infrared absorption spectroscopy and molecular dynamics simulations. Experiments and simulations show that the gold surface is enriched with dehydrated DMSO molecules and the mixture phase-separates to form water clusters. Simulations show a "buckled" water conformation at the surface, further constraining interfacial H-bonding. The small size of these water clusters and the energetically unfavorable H-bond conformations might inhibit H-bonding with bulk water, suppressing the proton diffusion required for efficient hydrogen evolution reaction processes.

Ultrafast Spectroscopy Reveals Slow Water Dynamics in Biocondensates.

Cells achieve high spatiotemporal control over biochemical processes through compartmentalization to membrane-bound as well as membraneless organelles that assemble by liquid-liquid phase separation. Characterizing the balance of forces within these environments is essential to understanding their stability and function, and water is an integral part of the condensate, playing an important role in mediating electrostatic and hydrogen-bonding interactions. Here, we investigate the ultrafast, picosecond hydrogen-bond dynamics of a model biocondensate consisting of a peptide poly-l-arginine (Poly-R) and the nucleic acid adenosine monophosphate (AMP) using coherent two-dimensional infrared (2D IR) spectroscopy. We investigated three vibrational modes: the arginine side-chain C═N stretches, an AMP ring mode, and the amide backbone carbonyl stretching modes. Dynamics slow considerably between the dilute phase and the condensate phase for each vibrational probe. For example, the arginine side-chain C═N modes slow from 0.38 to 2.26 ps due to strong electrostatic interactions. All-atom molecular dynamics simulations provide an atomistic interpretation of the H-bond network disruption resulting from electrostatic contributions as well as collapse within the condensate. Simulations predict that a fraction of water molecules are highly constrained within the condensate, explaining the observed slowdown in the H-bond dynamics.