Triple bond carotenoid alloxanthin in solution and in antenna complex from Rhodomonas salina

Tomáš Polívkaa, Robert Westa, Gürkan Keşana, Václav Šebelíka, Radek Kaňab, Eliška Trskováb, Marcel Fucimana, Roman Sobotkab
aFaculty of Science, University of South Bohemia, České Budějovice, Czech Republic; bInstitute of Microbiology, CAS, Centrum ALGATECH, Opatovický mlýn, 379 81, Třeboň, Czech Republic

Alloxanthin, a carotenoid containing two triple bonds was studied by steady-state and femtosecond transient absorption spectroscopies in solution and bound to an antenna complex from the cryptophyte Rhodomonas salina. In solution, alloxanthin has an S2 energy comparable to its non-triple bond homolog, zeaxanthin, while the S1 lifetime of 19 ps is markedly longer than that of zeaxanthin (9 ps). Along with corroborating quantum chemistry calculations, the results show that the long-lived S1 state of alloxanthin, implies the triple bond isolates the conjugation of the backbone, increasing the S1 state energy and diminishing the S1-S2 energy gap. This is reminiscent of the effect observed in some keto-carotenoids, which are usually very effective in transferring energy to Chl in antenna complexes.

Surprisingly, however, energy transfer from alloxanthin to Chl in the antenna complex from R. salina, in which alloxanthin is the major carotenoid, is rather ineffective. In contrast to other antenna complexes, there is almost no energy transfer from the S2 state of alloxanthin to Chl. The S1 pathway is active but energy transfer is slow, in the 5-7 ps range. Possible photoprotective role of alloxanthin in the antenna complex is also explored.