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

Anja Rüther1, Marcel Pfeifer, Víctor A Lórenz-Fonfría

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Vibrational circular dichroism (VCD) spectroscopy using a quantum cascade laser (QCL) quantitatively tracks proline protonation states in water. This method successfully extracts unique spectra for each species, aiding dynamic process analysis.

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Area of Science:

  • Analytical Chemistry
  • Spectroscopy
  • Physical Chemistry

Background:

  • Proline exists in different protonation states depending on pH.
  • Understanding these states is crucial for biochemical and chemical studies.
  • Vibrational Circular Dichroism (VCD) is a powerful spectroscopic technique.

Purpose of the Study:

  • To investigate the protonation states of proline in aqueous solutions using VCD.
  • To demonstrate the quantitative capabilities of Quantum Cascade Laser (QCL)-VCD.
  • To extract characteristic VCD spectra for different proline protonation states.

Main Methods:

  • Recording VCD spectra of aqueous proline solutions across a pH titration range.
  • Utilizing a Quantum Cascade Laser (QCL) as the infrared source.
  • Analyzing pH-dependent spectra with singular value decomposition and global fitting of a Henderson-Hasselbalch model.

Main Results:

  • Successfully determined the relative fractions of anionic, zwitterionic, and cationic proline species.
  • Validated QCL-VCD quantitative results against FT-IR and QCL-IR data.
  • Extracted pure VCD spectra for each protonation state of L-proline.

Conclusions:

  • QCL-VCD is applicable for quantitative analysis of dynamic processes in aqueous solutions.
  • The study successfully assigned vibrational modes characteristic of each proline protonation state.
  • This technique provides insights beyond static measurements, enabling dynamic process interpretation.