Studying Local Electrostatics by Terahertz Spectroscopy Using Amines as a Probe.

In a previous study we could show that a large amplitude mode of the zwitterion glycine can serve as a sensitive probe for protonation and allows to deduce local pK values. Here we show that the underlying concept is more general: We present the results of a pH dependent measurement of Terahertz-FTIR (THz-FTIR) spectra of solvated amines, i. e. Diethylamine (DEA), Triethylamine (TEA), and Diisopropylamine (DiPA). We show that amines serve as a sensitive, label free probe for local protonation. Protonation of the amines yield intensity changes which can be quantified by precise THz spectroscopy (30 cm -450 cm ). A detailed analysis allows us to correlate the titration spectra of solvated amines in the THz range with pK values. This demonstrates the potential of THz spectroscopy to probe the charge state of biomolecules in water in a label free manner.