Volumetric properties of polyols (ethylene glycol, glycerol, meso-erythritol, xylitol and mannitol) in relation to their membrane permeability: group additivity and estimation of the maximum radius of their molecules.

The relationship of molecular volume and maximum molecular radii to the ability of some polyols, ethylene glycol, glycerol and meso-erythritol, to permeate the Chara cell membrane and to the inability of one of the polyols, mannitol, to permeate the Chara cell membrane, was examined by measuring the partial molar volumes of the polyols, Vm(2). Analysis of Vm(2) at infinite dilution showed that group additivity is maintained for all the groups, i.e., CH2OH and CHOH, of the polyols tested. However, as the permeability and impermeability could not be related to the geometrical properties of the polyol molecules based only on the thermodynamic quantities, molecular models of the polyol molecules were constructed using the CPK(Corey-Pauling Koltum) molecular model, which is designed to have the van der Waals radius of 1 A equivalent to 1.25 cm. The results showed that the maximum radius of the water-filled pore (hydrophilic channel) should be 3.2-3.3 A, and the longer the axial length and maximum radius of the polyol molecule, the lesser was its permeative ability. All the experimental and analytical results and inferences support the idea that water molecules pass across the cell membrane through a narrow pore in a single-file fashion.