jp4122886_si_001.pdf (903.69 kB)
pH Titration Monitored by Quantum Cascade Laser-Based Vibrational Circular Dichroism
journal contribution
posted on 2014-04-10, 00:00 authored by Anja Rüther, Marcel Pfeifer, Víctor A. Lórenz-Fonfría, Steffen LüdekeVibrational circular dichroism (VCD)
spectra of aqueous solutions
of proline were recorded in the course of titrations from basic to
acidic pH using a spectrometer equipped with a quantum cascade laser
(QCL) as an infrared light source in the spectral range from 1320
to 1220 cm–1. The pH-dependent spectra were analyzed
by singular value decomposition and global fitting of a two-pK Henderson–Hasselbalch model. The analysis delivered
relative fractions of the three different protonation species. Their
agreement with the relative fractions obtained from performing the
same analysis on pH-dependent Fourier transform infrared (FT-IR) and
QCL-IR spectra validates the quantitative results from QCL-VCD. Global
fitting of the pH-dependent VCD spectra of l-proline allowed
for extraction of pure spectra corresponding to anionic, zwitterionic,
and cationic l-proline. From a static experiment, only pure
spectra of the zwitterion would be accessible in a straightforward
way. A comparison to VCD spectra calculated for all three species
led to assignment of vibrational modes that are characteristic for
the respective protonation states. The study demonstrates the applicability
of QCL-VCD both for quantitative evaluation and for qualitative interpretation
of dynamic processes in aqueous solutions.