pH Titration Monitored by Quantum Cascade Laser-Based Vibrational Circular Dichroism

• Published in: The journal of physical chemistry. B Vol. 118; no. 14; pp. 3941 - 3949
• Main Authors: Rüther, Anja, Pfeifer, Marcel, Lórenz-Fonfría, Víctor A, Lüdeke, Steffen
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 10.04.2014
• Subjects: Anions - chemistry; Aqueous solutions; Cations - chemistry; Circular Dichroism; Circularity; Dichroism; Fittings; Hydrogen-Ion Concentration; Infrared; Lasers, Semiconductor; Proline - chemistry; Protonation; Spectra; Spectroscopy, Fourier Transform Infrared; Water - chemistry
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp4122886

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–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.