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pH titration monitored by quantum cascade laser-based vibrational circular dichroism

: Rüther, Anja; Pfeifer, Marcel; Lórenz-Fonfría, Víctor A.; Lüdeke, Steffen


Journal of physical chemistry. B 118 (2014), No.14, pp.3941-3949
ISSN: 1089-5647
ISSN: 1520-6106
ISSN: 1520-5207
Journal Article
Fraunhofer IPM ()
spectroscopy; QCL; VCD

Vibrational 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 cm1. 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.