RESUMEN
Aquaporins (AQPs) feature highly selective water transport through cell membranes, where the dipolar orientation of structured water wires spanning the AQP pore is of considerable importance for the selective translocation of water over ions. We recently discovered that water permeability through artificial water channels formed by stacked imidazole I-quartet superstructures increases when the channel water molecules are highly organized. Correlating water structure with molecular transport is essential for understanding the underlying mechanisms of (fast) water translocation and channel selectivity. Chirality adds another factor enabling unique dipolar oriented water structures. We show that water molecules exhibit a dipolar oriented wire structure within chiral I-quartet water channels both in the solid state and embedded in supported lipid bilayer membranes (SLBs). X-ray single-crystal structures show that crystallographic water wires exhibit dipolar orientation, which is unique for chiral I-quartets. The integration of I-quartets into SLBs was monitored with a quartz crystal microbalance with dissipation, quantizing the amount of channel water molecules. Nonlinear sum-frequency generation vibrational spectroscopy demonstrates the first experimental observation of dipolar oriented water structures within artificial water channels inserted in bilayer membranes. Confirmation of the ordered confined water is obtained via molecular simulations, which provide quantitative measures of hydrogen bond strength, connectivity, and the stability of their dipolar alignment in a membrane environment. Together, uncovering the interplay between the dipolar aligned water structure and water transport through the self-assembled I-quartets is critical to understanding the behavior of natural membrane channels and will accelerate the systematic discovery for developing artificial water channels for water desalting.
RESUMEN
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.
RESUMEN
The iconic helical structure of DNA is stabilized by the solvation environment, where a change in the hydration state can lead to dramatic changes to the DNA structure. X-ray diffraction experiments at cryogenic temperatures have shown crystallographic water molecules in the minor groove of DNA, which has led to the notion of a spine of hydration of DNA. Here, chiral nonlinear vibrational spectroscopy of two DNA sequences shows that not only do such structural water molecules exist in solution at ambient conditions but that they form a chiral superstructure: a chiral spine of hydration. This is the first observation of a chiral water superstructure templated by a biomolecule. While the biological relevance of a chiral spine of hydration is unknown, the method provides a direct way to interrogate the properties of the hydration environment of DNA and water in biological settings without the use of labels.
RESUMEN
Molecular monolayers exhibit structural and dynamical properties that are different from their bulk counterparts due to their interaction with the substrate. Extracting these distinct properties is crucial for a better understanding of processes such as heterogeneous catalysis and interfacial charge transfer. Ultrafast nonlinear spectroscopic techniques such as 2D infrared (2D IR) spectroscopy are powerful tools for understanding molecular dynamics in complex bulk systems. Here, we build on technical advancements in 2D IR and heterodyne-detected sum frequency generation (SFG) spectroscopy to study a CO2 reduction catalyst on nanostructured TiO2 with interferometric 2D SFG spectroscopy. Our method combines phase-stable heterodyne detection employing an external local oscillator with a broad-band pump pulse pair to provide the first high spectral and temporal resolution 2D SFG spectra of a transparent material. We determine the overall molecular orientation of the catalyst and find that there is a static structural heterogeneity reflective of different local environments at the surface.
