Tuning the Intrinsic Photophysical Properties of Chlorophyll a

In nature, the finely tuned photophysical properties of chlorophyll a (Chla) are vital to the capture and transfer of sunlight during photosynthesis. In order to better understand how these properties are influenced by the molecular environment, we have examined the intrinsic spectroscopy of Chla in vacuo. Visible photodissociation action spectra (an indirect measure of absorption) of gaseous protonated Chla and Chla complexed with metal cations are reported. These show that spectral features within the Soret band (∼350-445 nm) have markedly different intensities depending on the identity of the cation. In contrast, fluorescence emission spectra of metalated Chla complexes show only small dependences on the identity of the metal ion, with emission maxima shifting from 661 to 654 nm. Remarkably, replacing the metal ion with a proton turns off the fluorescence of this key pigment. Density functional theory geometry-optimized structures indicate that the most favorable site of protonation differs from that of metal cationization, and may help explain the surprising on/off behavior of Chla's intrinsic fluorescence.