ANTI-COOPERATIVITY OF THE 2 WATER OH GROUPS

This paper presents the results concerning anti-cooperativity effects between two H-bonds of a water molecule. The IR OH stretching band shi fts Delta nu-as a measure of the H-bond energy-are compared for HOD wi th different bases B of 1:1 complexes B...HOD...CCl4 (Delta nu(11)) an d 1:2 complexes B...HOD...B (Delta nu(12)) We found that Delta nu(12) of the 1:2 complexes for different bases B are 25% smaller than Delta nu(11) for the 1:1 complexes. Corrections for the solvent shifts are i ntroduced. This effect is in line with different observations concerni ng cooperativity effects of OH H-bonds by polarization with neighbouri ng molecules. The reduction of Delta nu(12) in 1:2 complexes can be un derstood on the assumption of a negative polarization by the first H-b ond to the second OH or OD group and is called anti-cooperativity. Thi s anti-effect has been already detected by NMR observation on NH2. We already observe a decrease of the CCl4 solvent shift by van der Waals forces OH...CCl4 of 1:1 complexes, induced by the H-bond of the other OH or OD group. This decrease is measured by the CCl4 solvent effect f or monomers. This indicates a real negative polarization by the H-bond in 1:1 complexes on the second OH/OD. This experiment establishes the real polarization and excludes the importance of repulsions of the ba ses as the cause. The dependence of intermolecular forces is known on the polarizability. Our method demonstrates directly the polarization by interactions. The anti-cooperativity of symmetric complexes B-1...H OD...B-1 by a strong base B-1 can be reduced in unsymmetric 1:2 comple xes B-1...HOD...B-2 by weaker bases B-2. This weakening of the anti-co operativity of the stronger base could be predicted quantitatively. Si milarly, the anti-cooperativity of the weaker base B-2 is strengthened in unsymmetric 1.2 complexes by stronger bases B-1. It is known that H-bonds XH...B can be strengthened by cooperativity with a second H-bo nd XH...XH...B. They can be weakened for water by anti-cooperativity o f two H-bonds B...HOH...B. The H-bond B (1)...HO of 1:2 complexes B-1. ..HOH...B-2 can be weakened if the base strength of B-2 is stronger th an of B-1 or strengthened if B-2 is weaker than B-1. Nature may use th ese possibilities in biochemistry. (C) 1997 Elsevier Science B.V.