Speaker
John Tatini Titantah
(University of Western Ontario)
Description
The structural and dynamic properties of solvation water of an
amphiphilic molecule, tetramethylurea (TMU), is studied using an ab initio molecular dynamic approach over the
wide range of temperature (220 K - 370 K) [1,2]. Unprecedented data collection times ranging from 100 to 450 ps
are performed. Comparison with the corresponding properties of bulk
water exposes important insights into the hydrophobic/hydrophilic effects. In bulk water,
we find a fragile to strong cross-over in the dynamics behaviour of supercooled water at a temperature of about
245 K.
This crossover is accompanied by the passage from relatively weaker hydrogen bonds in the
predominantly high density liquid (HDL) at high temperatures to stronger hydrogen bonds (HBs), low density liquid (LDL) at low temperatures, lending support to the possibility of the existence of a liquid-liquid critical point (LLCP) in supercooled water as was predicted by Peter Poole and collaborators using ST2 water model [3].
Computed rotational relaxation times, diffusivity, hydrogen-bond life-span and the dynamics of the local
tetrahedral order of the solvation water of TMU also reveal a cross-over at 256$\pm$4 K from high activation
energy dynamics at higher temperatures to weakly thermally activated dynamics at lower temperatures,
which compares very well with nuclear magnetic resonance study that reveals a similar cross-over for the solvation water of
this molecule and other similar amphiphilic molecules at 255$\pm$2 K [4]. We also locate a temperature of 265 $\pm$5 K
above which the water-carbonyl hydrogen bonds are less stable than water-water HBs. Below this temperature
the water-carbonyl HBs become more stable than water-water HBs. The findings of this work may help shed more light on the mechanism of cold denaturation of proteins.
[1] J. T. Titantah and M. Karttunen, Long-Time Correlations and Hydrophobe-Modified Hydrogen-Bonding Dynamics in Hydrophobic Hydration. J. Am. Chem. Soc. 134, 9362 (2012)
[2] J. T. Titantah and M. Karttunen, Water dynamics: Relation between hydrogen bond bifurcations, molecular jumps, local density & hydrophobicity. Sci. Rep. 3, 2991 (2013)
[3] P. H. Poole et al., Phase behaviour of metastable water. Nature 360, 324–328 (1992)
[4] J. Qvist and B. Halle, J. Am. Chem. Soc. 130, 10345 (2008).
Author
John Tatini Titantah
(University of Western Ontario)
Co-author
Prof.
Mikko Karttunen
(Department of Chemistry & Waterloo Institute of Nanotechnology, University of Waterloo, Waterloo, Ontario, Canada)
