Elucidating mechanisms of thermostabilization of poliovirus by D2O and MgCl2.

To understand a significant reduction in the loss of poliovirus infectivity by D2O and a combination of D2O and MgCl2 at 37-45 degrees C, this paper attempts to elucidate the mechanisms underlying the thermostabilization of poliovirus. Three serotypes of Sabin oral poliovirus vaccine strains were investigated. Temperature-dependent fluorescence emission intensity studies showed that the effects of D2O and MgCl2 on the stability and conformation of poliovirus are correlated with those of the infectivity of poliovirus. Fluorescence steady-state polarization revealed that the conformation of poliovirus capsid is sensitive to D2O medium and MgCl2 salt, and that the rigidity of poliovirus conformation is increased in their presence. The exposure of poliovirus tryptophan residues to water is modified by D2O and MgCl2, as evidenced by changes in fluorescence emission intensity excited at 295 nm. The involvement of hydrogen bonding in the D2O effect was demonstrated by the greatly increased value of relative fluorescence intensity. Conformational alteration was also shown by changes in the positive band (193-230 nm) of circular dichroism spectra. D2O and MgCl2 were also found to reduce the interaction of virus with water as examined by differential scanning microcalorimetry, leading to a decline in the extent of water penetration into the poliovirus capsid. All these observations were found to be more profound in a combination of D2O with MgCl2 than D2O or MgCl2 alone. By inducing a conformation favorable to maintaining the poliovirus assembly and by reducing virus-water interaction to decrease water penetration into the poliovirus capsid, D2O, MgCl2, or D2O-MgCl2 is able to exert its thermostabilization effect. Thus, to maintain the virus assembly and conformation of the virus and to reduce the swelling of the virus capsid are key factors in increasing the thermostability of poliovirus. These two factors are mutually complementary. The latter can provide a favorable environment for the formers and the formers, in turn, lead to the latter.