The Infrared Spectra of Bacillus Bacteria Part II: Sporulated Bacillus —The Effect of Vegetative Cells and Contributions of Calcium Dipicolinate Trihydrate,CaDP-3H 2 O

Abstract

Our previous paper showed that certain infrared (IR) peaks, e.g., the peak at 1739 cm⁻¹, are due to varying (trace) amounts of vegetative cells amongst the Bacillus 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 Bacillus bacteria. For the endospores it is observed that peaks at 1441, 1277, and 1015 cm⁻¹ along with a distinct quartet of peaks at 766, 725, 701, and 659 cm⁻¹ are clearly associated with calcium dipicolinate trihydrate, CaDP·3H₂O. 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-3H₂O 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 Ca²⁺ counterion and hydration waters including Ca–O–H bends, H₂O–Ca–O torsions, and O–C–O bends. The 1441 and 1015 cm⁻¹ modes are planar pyridine modes with the 1441 cm⁻¹ mode primarily a ring C–N stretch and the 1015 cm⁻¹ mode primarily a ring C–C stretch.