Assignment of IR bands of isolated and protein-bound Peridinin in its fundamental and triplet state by static FTIR, time-resolved step-scan FTIR and DFT calculations

The vibrational properties of Peridinin in its fundamental state and in the excited triplet state have been investigated by DFT calculations and static and time-resolved FTIR spectroscopy. The infrared spectrum of Peridinin in its fundamental state has been explored in the whole 2000–600 cm−1 range, and interpreted in term of molecular vibrations. In particular, new infrared bands have been identified and assigned to specific molecular vibrations. 3Peridinin molecular vibrations have also been investigated by DFT calculations. In addition, putative IR bands belonging to Peridinin and 3Peridinin have been identified in the step-scan FTIR difference spectrum of the Peridinin–Chlorophyll a–Protein from Amphidinium carterae, where light induce formation of a triplet state localized on one or more Peridinins. The exact nature of the triplet state formed in Peridinin–Chlorophyll a–Protein from dinoflagellates, in particular the possible involvement in this triplet state of 3Chlorophyll a, has been largely debated in the last few years (see Carbonera et al., 2014 [3]); time-resolved differential FTIR experiments have played a key role in this debate. Identification of IR marker bands for the main molecule (Peridinin) implicated in this photophysical process is therefore particularly important and makes this study a significant step towards the full understanding of Peridinin–Chlorophyll-a–Proteins photophysics.