Excited-State Properties of Canthaxanthin in Cyanobacterial Carotenoid-Binding Proteins HCP2 and HCP3.

Recently a new family of carotenoproteins, homologues of the N-terminal domain of the orange carotenoid protein (NTD-OCP), have been identified in cyanobacteria. These homologues are called helical carotenoid proteins (HCPs) as they are all predicted to maintain the all-helical structure of the NTD-OCP and to bind carotenoids. Here, HCP2 and HCP3 isolated from the cyanobacterium Tolypothrix PCC 7601 were studied by ultrafast transient absorption spectroscopy to explore the excited-state dynamics of the bound carotenoid, canthaxanthin. The lowest excited state, S1, of canthaxanthin in both HCPs yields a lifetime of 3.5 ps; it is thus shorter than for canthaxanthin in solution (4.5 ps). This is because of the longer effective conjugation of canthaxanthin in HCPs, as one of the terminal rings is in an s-trans configuration. Use of two different excitation wavelengths, 470 and 570 nm, revealed excitation wavelength dependent spectroscopic response. Additional excited-state absorption bands are observed after excitation at 470 nm for both HCPs, proving the presence of more than one ground state conformer.