Rüther, Anja Pfeifer, Marcel Lórenz-Fonfría, Víctor A. Lüdeke, Steffen pH Titration Monitored by Quantum Cascade Laser-Based Vibrational Circular Dichroism 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 cm<sup>–1</sup>. The pH-dependent spectra were analyzed by singular value decomposition and global fitting of a two-p<i>K</i> 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. VCD spectra;vibrational modes;quantum cascade laser;protonation species;protonation states;QCL;pH Titration Monitored;light source;acidic pH;value decomposition 2014-04-10
    https://acs.figshare.com/articles/journal_contribution/pH_Titration_Monitored_by_Quantum_Cascade_Laser_Based_Vibrational_Circular_Dichroism/2306656
10.1021/jp4122886.s001