Abstract

Our previous paper showed that certain infrared (IR) peaks, e.g., the peak at 1739 cm<sup>−1</sup>, are due to varying (trace) amounts of vegetative cells amongst the <i>Bacillus</i> spores and that these and other vegetative bands are associated with lipid-soluble compounds, likely an ester or phospholipid. This work investigates the infrared spectra of eight different sporulated <i>Bacillus</i> bacteria. For the endospores it is observed that peaks at 1441, 1277, and 1015 cm<sup>−1</sup> along with a distinct quartet of peaks at 766, 725, 701, and 659 cm<sup>−1</sup> are clearly associated with calcium dipicolinate trihydrate, CaDP·3H<sub>2</sub>O. It is emphasized that the spore peaks, especially the quartet, arise from the calcium dipicolinate trihydrate and not from dipicolinic acid or other dipicolinate hydrate salts. The CaDP·3H<sub>2</sub>O infrared peaks and the effects of hydration are studied using quantum chemistry in the PQS software package. The quartet is associated with many modes including contributions from the Ca<sup>2+</sup> counterion and hydration waters including Ca–O–H bends, H<sub>2</sub>O–Ca–O torsions, and O–C–O bends. The 1441 and 1015 cm<sup>−1</sup> modes are planar pyridine modes with the 1441 cm<sup>−1</sup> mode primarily a ring C–N stretch and the 1015 cm<sup>−1</sup> mode primarily a ring C–C stretch.

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