Jianguo Zuo

Molecular dynamics study on water self-diffusion in aqueous mixtures of methanol, ethylene glycol and glycerol: investigations from the point of view of hydrogen bonding

Molecular dynamics simulations have been performed to investigate the aqueous binary mixtures of alcohols, including methanol, ethylene glycol (EG) and glycerol of molalities ranging from 1 to 5 m at the temperatures of 273, 288 and 298 K, respectively. The primary purpose of this paper is to investigate the mechanism of water self-diffusion in water-alcohol mixtures from the point of view of hydrogen bonding. The effects of temperature and concentration on water self-diffusion coefficient are evaluated quantitatively in this work. Temperature and concentration to some extent affect the hydrogen bonding statistics and dynamics of the binary mixtures. It is shown that the self-diffusion coefficient of water molecules decreases as the concentration increases or the temperature decreases. Moreover, calculations of mean square displacements of water molecules initially with different number n of H-bonds indicate that the water self-diffusion coefficient decreases as n increases. We also studied the aggregation of alcohol molecules by the hydrophobic alkyl groups. The largest cluster size of the alkyl groups clearly increases as the concentration increases, implying the emergence of a closely connected network of water and alcohols. The clusters of water and alcohol that interacted could block the movement of water molecules in binary mixtures. These findings provide insight into the mechanisms of water self-diffusion in aqueous binary mixtures of methanol, EG and glycerol.

Kinetics of osmotic water flow across cell membranes in non-ideal solutions during freezing and thawing

Cryopreservation requires quantitatively analytical models to simulate the biophysical responses of biomaterials during cryopreservation. The Mazur model and other improved ones, such as Karlsson model concerning solutions containing cryoprotectants (CPA), are somehow precluded by some minor points, particularly, the assumption of ideal solutions. To avoid the ideal solution assumption, in this study a new method is developed to simulate water transport across cell membranes in non-ideal solutions during cooling and thawing. The comparison between osmolalities calculated by the linear freezing-point depression used in this new method and other non-ideal ones is conducted and a good agreement is achieved. In addition, in an ideal case, besides a theoretical agreement, this new approach has been validated by its numerical simulation results. Comparisons between this new approach and the traditional ones with an ideal solution assumption have been conducted based on a spherical hypothetical cell. The main results are (1) the predicted non-ideal intracellular water content is larger than the ideal results; (2) the concentration of CPA solutions is directly proportional to the deviation between the non-ideal and ideal curves. In the end, this study presents a direct description of the degree of subcooling of the protoplasm during dynamic cooling. This study demonstrates that our experimental data-based method is a valid one with clear physical interpretations, convenient expressions and a more extensive application room than traditional ones.

Kinetics of coupling water and cryoprotectant transport across cell membranes and applications to cryopreservation.

Thermodynamic and kinetic models can provide a wealth of information on the physical response of living cells and tissues experiencing cryopreservation procedures. Both isothermal and nonisothermal models have been proposed so far, accompanied by experimental verification and cryoapplications. But the cryoprotective solution is usually assumed to be dilute and ideal in the models proposed in the literature. Additionally, few nonisothermal models are able to couple the transmembrane transport of water and cryoprotectant during cooling and warming of cells. To overcome these limitations, this study develops a whole new set of equations that can quantify the cotransport of water and cryoprotectant across cell membranes in the nondilute and nonideal solution during the freezing and thawing protocols. The new models proposed here can be simplified into ones consistent with the classic models if some specific assumptions are included. For cryobiological practice, they are applied to predict the volumetric change for imprinting control region (ICR) mouse spermatozoa and human corneal keratocytes in the freezing protocol. The new models can determine the intracellular concentration of cryoprotectant more precisely than others by abandoning the assumptions such as dilute and ideal solutions and nonpermeability of membranes to cryoprotectant. Further, the findings in this study will offer new insights into the physical response of cells undergoing cryopreservation.