Abstract
Proteins are built of amino acid residues that differ from each other only by their side chain. The pH-dependent response of these side chains on protein surface is vital for various biological processes. Here we have investigated the aqueous interface in the presence of different amino acid side chains at varying pH (bulk) using heterodyne-detected vibrational sum-frequency generation (HD-VSFG) spectroscopy. It is observed that amine/imidazolic (e.g., lysine/histidine) and alcoholic (e.g., serine and threonine) side chains preferentially orient the interfacial water as “H-down” (i.e., the water hydrogens are pointed toward the aqueous bulk) in acidic solution (pH ∼2). At physiological pH (7.4), the interfacial water takes “H-up” orientation (i.e., the water hydrogens are pointed away from the aqueous bulk) for the alcoholic/imidazolic side chains but remains “H-down” for the amine containing side chain. On further increasing the pH (up to 12.0), the interfacial water becomes increasingly H-up oriented, r...
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