CO Hydration Shell Structure and Transformation.

The hydration-shell of CO is characterized using Raman multivariate curve resolution (Raman-MCR) spectroscopy combined with ab initio molecular dynamics (AIMD) vibrational density of states simulations, to validate our assignment of the experimentally observed high-frequency OH band to a weak hydrogen bond between water and CO. Our results reveal that while the hydration-shell of CO is highly tetrahedral, it is also occasionally disrupted by the presence of entropically stabilized defects associated with the CO-water hydrogen bond. Moreover, we find that the hydration-shell of CO undergoes a temperature-dependent structural transformation to a highly disordered (less tetrahedral) structure, reminiscent of the transformation that takes place at higher temperatures around much larger oily molecules. The biological significance of the CO hydration shell structural transformation is suggested by the fact that it takes place near physiological temperatures.