Different protonation equilibria of 4-methylimidazole and acetic acid.
Dynamic protonation equilibria in water of one 4-methylimidazole molecule as well as for pairs and groups consisting of 4-methylimidazole, acetic acid and bridging water molecules are studied using Q-HOP molecular dynamics simulation. We find a qualitatively different protonation behavior of 4-methylimidazole compared to that of acetic acid. On one hand, deprotonated, neutral 4-methylimidazole cannot as easily attract a freely diffusing extra proton from solution. Once the proton is bound, however, it remains tightly bound on a time scale of tens of nanoseconds. In a linear chain composed of acetic acid, a separating water molecule and 4-methylimidazole, an excess proton is equally shared between 4-methylimidazole and water. When a water molecule is linearly placed between two acetic acid molecules, the excess proton is always found on the central water. On the other hand, an excess proton in a 4-methylimidazole-water-4-methylimidazole chain is always localized on one of the two 4-methylimidazoles. These findings are of interest to the discussion of proton transfer along chains of amino acids and water molecules in biomolecules.
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Wei
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Zentrum für Bioinformatik, Universität des Saarlandes, 66041 Saarbrücken, Germany.
Helms
Volkhard
V
D019342:Acetic Acid / Q000737:chemistry*; D007093:Imidazoles / Q000737:chemistry*; D008958:Models, Molecular; D008968:Molecular Conformation; D011522:Protons; D012997:Solvents; D014867:Water / Q000737:chemistry
D007093:Imidazoles; D011522:Protons; D012997:Solvents; D014867:Water; D019342:Acetic Acid; C023439:4-methylimidazole