title: pH titration monitored by quantum cascade laser-based vibrational circular dichroism.
abstract: 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.
authors.affiliation: Institute of Pharmaceutical Sciences, Albert-Ludwigs-Universität Freiburg , Albertstr. 25, 79104 Freiburg, Germany.
authors.forename: Anja
authors.initials: A
authors.lastname: Rüther
authors.forename: Marcel
authors.initials: M
authors.lastname: Pfeifer
authors.forename: Víctor A
authors.initials: VA
authors.lastname: Lórenz-Fonfría
authors.forename: Steffen
authors.initials: S
authors.lastname: Lüdeke
mesh_terms: D000838:Anions / Q000737:chemistry; D002412:Cations / Q000737:chemistry; D002942:Circular Dichroism; D006863:Hydrogen-Ion Concentration; D054023:Lasers, Semiconductor; D011392:Proline / Q000737:chemistry*; D017550:Spectroscopy, Fourier Transform Infrared; D014867:Water / Q000737:chemistry
chemical_list: D000838:Anions; D002412:Cations; D014867:Water; D011392:Proline