Ionic Hydrogen Bonds in Bioenergetics. 3. Proton Transport in Membranes, Modeled by Ketone/Water Clusters

Hydrogen bond networks in protonated acetone/water clusters are stabilized by H3O+(Me2CO)2 centers, and the stabilizaton increases with further acetone content. For example, proton transfer from neat water (H2O)6H+ clusters to form mixed (Me2CO)3(H2O)3H+ clusters is exothermic by 80 kJ/mol (19 kcal/mol), due to strong hydrogen bonding of the carbonyl groups; in a series of mixed clusters B3(H2O)3H+, the stability of the hydrogen bond network correlates with the proton affinities PA(B). In diketone models of adjacent peptide links, the proton is stabilized by internal hydrogen bonds between the carbonyl groups. The internal bonds can be significant, for example, 31 kJ/mol (7 kcal/mol) in (MeCOCH2CH2COMe)H+, but proton transfer through the internal bond has a high barrier. However, water molecules can bridge between the CO groups. In these bridges, the proton remains on an H3O+ center, in both acetone/water and diketone/water systems. With a further H2O molecule, the diketone/water cluster (MeCOCH2CH2COMe)(H2O)2H+ and diamide/water clusters form two-water H3O+···H2O bridges, which allow proton transfer between the CO groups with a small barrier of