Femtosecond Spectroscopy of Calcium Dipicolinate-A Major Component of Bacterial Spores.

Bacterial spores are rich in calcium dipicolinate (CaDPA). The role of this compound in the high UV resistance of spore DNA and their unique DNA photochemistry is not yet clarified. Here, the photophysical properties of CaDPA dissolved in water are studied by means of steady-state and time-resolved spectroscopy as well as quantum chemistry. Upon 255 nm excitation, a fluorescence emission with a yield of 1.7 × 10(-5) is detected. This low yield is in line with a measured fluorescence lifetime of 110 fs. Transient absorption experiments point to further transitions with time constants of 92 ps and 6.8 μs. The microsecond time constant is assigned to the decay of a triplet state. The yield of this state is close to unity. With the aid of quantum chemistry (TD-DFT, DFT-MRCI), the following transitions are identified. The primarily excited (1)ππ* state depletes within 110 fs. The depletion results in the population of an energetically close lying (1)nπ* state. An El-Sayed allowed intersystem crossing process with a time constant of 92 ps ensues. Implications of these findings on the interaction between photoexcited CaDPA and spore DNA are discussed.