Ultrafast Hydration Dynamics Near Extended Macromolecular Interfaces

Abstract

Liquid water is arguably the most complex, but interesting chemical, enabling development and sustenance of life. The tetrahedral geometry of the water molecule allows the formation of a dense network of hydrogen bonds which undergoes rapid fluctuations at picosecond timescales. This ultrafast making and breaking of hydrogen bonds near an extended macromolecular interface may govern various biochemical kinetics, such as enzymatic activity, protein folding and membrane formation and disruption. Having a better understanding of interfacial hydration dynamics has implications to tune enzymatic activity, design targeted drugs and develop efficient desalination techniques. This dissertation elucidates the complex origin of the slowdown in hydration dynamics near the interfaces of micelle, protein and polymer.

To completely capture the timescale and perturbation of hydration dynamics by an extended interface, surface charge cannot be excluded. Using the thiocyanate anion (SCN–) as a vibrational probe in the infrared and in conjunction with magnetic resonance spectroscopy, we find that the thiocyanate anion strongly associates with an interfacial model system of dodecyltrimethylammonium bromide (DTAB) micelles. Ultrafast two-dimensional infrared (2D-IR) spectroscopy of the SCN– probe in a range of DTAB micelle sizes shows little if any size dependence to the time scale for spectral diffusion, which is found to be ~3.5 times slower compared to bulk water (both D2O and H2O). We conclusively find that the SCN– spectral dynamics in cationic micelles is largely dominated by hydration contributions and offers a promising probe for interfacial hydration near positively charged interfaces.

Graph theoretical analysis of water hydrogen bond network is implemented to map its network topology obtained using molecular dynamic simulations in confined protein (Hen Egg White Lysozyme) geometries. The observed power-law dependence for average path length on system size reveals that the bulk hydrogen bond networks cannot be considered random, but rather consists of a giant lattice-like component. At small protein separations (5-10 Å) with reduced hydrogen bond connectivity, similar global network structures are observed, indicating the maintenance of a completely unperturbed network topology. A Monte Carlo simulation on square lattices devoid of surface heterogeneity of real proteins reveals that the slowdown in hydration dynamics falls off exponentially near flat interfaces and converges within 2-3 shells with no evidence of cooperative effects. However, we conclusively find that protein surface residues become significantly slow when crowded and remains decoupled with interfacial hydration dynamics. The long-range collective influences by an interface may be due to complex chemical patterning of the surface.

Poly(ethylene oxide) is well known for its water structuring ability and bio-compatibility by forming strong a rigid hydration shell. In small poly(ethylene oxide) polymers, high charge density cations slaves PEG-200 to adopt a cyclic conformation, even at low salt concentrations. Probing the CN stretching frequency of the thiocyanate counter anion shows significantly slow spectral diffusion (~5-fold) time scale offering evidence for direct interactions between the polymer and cations contrary to currently accepted water mediated mechanism. The lack of correlation with the Hofmeister ordering of the cations implies that PEG-cation interactions are highly specific. While complete maintenance of bulk-like dynamics in concentrated DNA duplex confirms weak DNA-water interactions.

The diverse range of dynamical timescales for water fluctuations near macromolecular interfaces may require simultaneous probing of chemical groups present on macromolecular interfaces and water directly, a feat that is now possible using the recently developed broadband mid-infrared light source.