A nanosecond pH drop produced by a photoacid is used to protonate aminoacids in aqueous solutions at temperatures close to that of the solvent maximum density. The ensuing volume changes are followed by photoacoustic calorimetry (PAC). The processes of proton release from the photoacid and protonation of the aminoacid are discriminated by their time scales. The volume changes in the protonation of the negatively charged carboxylate group of acetyl-L-isoasparagine or acetyl-L-isoglutamine are opposite to that observed in the protonation of the uncharged imidazole ring of acetyl-L-histidine methylamide. Time-resolved PAC at different temperatures also provides the Arrhenius parameters for these protonation reactions, which have rate constant that differ by one order of magnitude at room temperature. This provides the basis to distinguish between carboxylate or imidazole protonation in systems containing aminoacids with both groups.The time scales of proton release, aminoacid protonation and early unfolding events are used to distinguish between these processes using photoacoustic calorimetry. A synthetic peptide known to form an alpha-helix was studied with the pH jump technique associated with PAC and revealed a concentration-independent volume expansion in the sub-microsecond range that is tentatively assigned to the unfolding of the alpha-helix.

Early events in protein folding followed by photoacoustic calorimetry after a pH jump

248th National Meeting (2014)

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