pH-induced changes in Raman, UV-vis absorbance, and fluorescence spectra of dipicolinic acid (DPA)

Dipicolinic acid (DPA) is an essential component for the protection of DNA in bacterial endospores and is often used as a biomarker for spore detection. Depending upon the pH of the solution, DPA exists in different ionic forms. Therefore, it is important to understand how these ionic forms influence spectroscopic response. In this work, we characterize Raman and absorption spectra of DPA in a pH range of 2.0-10.5. We show that the ring breathing mode Raman peak of DPA shifts from 1003 cm(-1) to 1017 cm (-1) and then to 1000 cm(-1) as pH increases from 2 to 5. The relative peak intensities related to the different ionic forms of DPA are used to experimentally derive the pK(a) values (2.3 and 4.8). We observe using UV-vis spectroscopy that the changes in the absorption spectrum of DPA as a function of pH correlate with the changes observed in Raman spectroscopy, and the same pK(a) values are verified. Lastly, using fluorescence spectroscopy and exciting a DPA solution at between 210-330 nm, we observe a shift in fluorescence emission from 375 nm to 425 nm between pH 2 and pH 6 when exciting at 320 nm. Our work shows that the different spectral responses from the three ionic forms of DPA may have to be taken into account in, e.g., spectral analysis and for detection applications. (C) 2022 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Automated summary

DPA's Raman and absorption spectra exhibit significant changes with varying pH, and the pKa values were determined experimentally. Fluorescence emission of DPA also shows a shift with pH, indicating the need to consider the different spectral responses from the three ionic forms of DPA in spectral analysis and detection applications.