The hydration number n of calcium dipicolinate trihydrate, CaDP·nH2O, and its effect on the IR spectra of sporulated Bacillus bacteria

Abstract Previous results have shown a unique “quartet” of peaks in the infrared spectra of the sporulated phase of Bacillus bacteria, the four peaks being observed reproducibly for many different species of Bacillus endospores. We consistently observe bands at 766, 725, 701, and 659 cm −1 with the same relative amplitudes, as well as other spore peaks at 1441, 1277, and 1015 cm −1 . We have suggested that the peaks arise from calcium dipicolinate, not the conjugate acid. In this paper we conduct a theoretical and experimental study to show that the IR peaks not only arise from the calcium dipicolinate, a known spore component, but specifically the trihydrate salt, CaDP·3H 2 O. This is shown by calculating the absolute IR frequencies and intensities of the lone dipicolinate dianion, the calcium salt, as well as the mono-, di- and tri-hydrate salts of calcium dipicolinate. The quartet peaks arise from the crystalline trihydrate salt as verified both experimentally and using quantum chemistry methods. When the calculated intensities are not normalized, only the trihydrate spectrum shows low frequency modes (below 1000 cm −1 , including the quartet) having intensities comparable to those of the pyridine ring. The vibrational modes in this region are associated with many internal coordinate motions including contributions from the Ca 2+ counterion and the three waters including Ca–O–H bends, H 2 O–Ca–O torsions and O–C–O bends.