  The influence of dimethyl sulfoxide (DMSO) on membrane thickness,
multilamellar repeat distance, and phase transitions of
1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC) was investigated by
X-ray diffraction and small-angle neutron scattering (SANS). The differential
scanning calorimetry (DSC) study of water freezing and ice melting was
performed in the ternary DPPC /DMSO /water and binary DMSO /water systems. The
methods applied demonstrated the differences in membrane structure in three
sub-regions of the DMSO mole fraction (X_dmso): from 0.0 to 0.3 for the first,
from 0.3 to 0.8 for the second, and from 0.9 to 1.0 for the third sub-region.
The thickness of the intermembrane solvent at T =20C decreases from 14.4 +/-
1.8 A at X_dmso =0.0 to 7.8 +/- 1.8 A at X_dmso =0.1. The data were used to
determine the number of free water molecules in the intermembrane space in the
presence of DMSO. The results for 0.0 < X_dmso < 0.3 were explained in the
framework of DMSO-induced dehydration of the intermembrane space.
