Cryoprotective mechanism of using Ficoll for cell cryopreservation at non-cryogenic temperatures: A molecular dynamics study

Molecular dynamics simulations were carried out to investigate the cryoprotective mechanism of using Ficoll, a highly compact spherical polysucrose, as a non-permeating cryoprotectant for practices of cell cryopreservation at temperatures higher than −80 °C. Three types of simulation boxes were prepared for Ficoll-dimethyl sulfoxide (DMSO)-water, sucrose-DMSO-water, and DMSO-water systems, respectively, and depicted with the Optimized Potentials for Liquid Simulations (OPLS-all) as potential function for molecular systems. The entire molecular system for each scenario was firstly fully equilibrated into a state with known concentration, density and temperature, in agreement with the corresponding existing or newly measured phase-diagrams. Thereafter, molecular dynamics simulations were performed to characterize the behavior of liquid water molecules surrounding a pre-sited ice nucleus that was placed at the center of each molecular system by calculating the radial density distribution (RDF) and root-mean-square distance (RMSD) of atomic positions. The results showed that the system with Ficoll molecules present behaved significantly different from the other two systems at various non-cryogenic temperatures (−10 °C, −20.3 °C, −33.9 °C, and −80 °C). The Ficoll molecules obviously prevent the water molecules from approaching the ice nuclei, and simultaneously lower the activities of the water molecules. These results agree well with previous thermal studies that demonstrate the effect of using Ficoll to minimize recrystallization of such solutions, and also provide a qualitative explanation on a molecular level for why Ficoll facilitates long-term storage of cells at non-cryogenic temperatures.