Wasserstruktur und Permeation: Die Aktivierungsenergie und der molekulare Mechanismus der Wasserpermeation
Zeitschrift für Naturforschung. B, A journal of chemical sciences
The exchange of tissue water of corn roots with ambient water was investigated using D2O, a gaschromatographic method, and a technique which avoids prolonged contact of tissue water with atmospheric water. The exchange experiments were performed at 10°, 20°, and 25 °C and the activation energies for different exchange phases were calculated by a method involving a graphical determination of the relative exchange rates at certain H2O/D2O gradients. Log10 of the rates were plotted versus 1/T as
... ted versus 1/T as usual and the activation energies were calculated from the slopes of the straight lines. The energy of activation of the exchange process increased from 4.4 kcal-mol-1 in an initial phase (exchange of surface water and free space water) to 6.3 kcal·mol-1 in later phases which represent the processes of permeation through plasma and plasma membranes. This suggests that the hydrogen bonds of permeating water have a mean energy of 6 —7 kcal·mol-1 resulting from interaction with membrane (and plasma) constituents. The theory is proposed that cell membranes contain water phases with hydrogen bonds stronger than those in pure liquid water. These water phases are assumed to be located mainly within apolar portions of globular membrane proteins. Not solely a continous lipid layer, but a specific arrangement of polar and apoiar portions of globular membrane proteins is regarded to be essential for semipermeability and other membrane properties. Results from various authors were considered in establishing the general working hypothesis that agents like apoiar compounds which increase water structure decrease water permeability, and agents like salts which disrupt water structure increase water permeability.