# Investigations of the Energy Transfer
## 3 Results
### Excitation Energy Transfer in the Phycobilisome Antenna
#### 3.3.2 Transient Absorption
[41], which reported a value of 700 fs for the energy transfer between \(\alpha\)-84 and \(\beta\)-84 chromophores in a PBS extracted from A. marina grown in low white light. The second EADS presented a minimum at 627 nm, which corresponded well to the 629 nm band resolved in the absorption spectrum of PBS from A. marina [37] and was assigned to phycocyanin. Moreover, it resembled well the second evolutionary spectrum obtained for PC, showing positive values above 652 nm with a peak at 670 nm. Therefore, we assign the 14 ps kinetic component to the energy transfer between PC disks in the rods. Similarly, [PERSON] et al. [41] obtained a time component of 14 ps, which was assigned to the energy transfer between spectral variants of \(\beta\)-84 PCB. Figure 8: (a) Three-dimensional differential absorption signal as a function of probe wavelength and delay time, obtained when the phycobilisome was pumped at 580 nm; (b) transient absorption difference spectra recorded at various delay times. The third EADS showed a local minimum at 634 nm, a positive band centered at 676 nm, and another weak negative band peaking at 713 nm. We assign the 122 ps kinetic component to the rod to core energy transfer. A comparable value was reported by [PERSON] et al. for the 640 nm excitation of a rod-core complex from Anabaena variablis [44]. Additionally, [PERSON] et al. [41] reported a kinetic component of 142 ps for the energy transfer between spectral variants of \(\beta\)-84 PCB in PC, based on the minimum observed at 637 nm in the resulted species associated spectrum; however, their analysis was performed on a PBS free of APC. The EADS associated to the last compartment presented negative values throughout the entire spectral region, showing a minimum at 660 nm close to where APC emits. This could indicate that the radiative and non-radiative decay of the final emitting chromophores from the excited state back to the ground state took place in 1.15 ns. Although the value is shorter than the fluorescence lifetime of 1.8 ns obtained from the TCSPC experiments on APC, the numbers are comparable. We should mention here that the decrease of EADS amplitude was more clearly observed for the PBS than for PC and APC because for the PBPs, the process of ground-state repopulation was not over in the 3600 ps delay scan used in the measurement. The situation was different in the PBS, because of the presence of linker proteins that maintain the PBS compact and interact with the chromophores, therefore influencing the decay by providing additional decay pathways. In the case of PC and APC, the linker proteins are not present, being removed during purification since they are unstable in aqueous solution outside the PBS complex. The difference absorption spectra recorded at long delay times (above 1000 ps) in the case of PC and APC presented bleaching bands around 630 nm for PC  
Figure 9: (**a**) Kinetic traces recorded at 623 and 665 nm with fits; (**b**) Global analysis of the absorption changes induced in the PBS after 580 nm excitation showing the resulted evolution-associated difference spectra; (**c**) time dependencies of the populations of different compartments; (**d**) the kinetic scheme used for the global analysis model. GS—ground state, 0 and 1—the sensitizing and respectively fluorescing chromophores in phycocyanin, 2—phycocyanin rods, and 3—allophycocyanin. and around 660 nm in the case of APC of rather significant intensities (\(-1\) mOD for both PC and APC compared to \(-0.17\) mOD for PBS), while the process in the PBS was already finished at a delay time of 1700 ps. The results obtained here are in good agreement with the data reported by [PERSON] et al. for a wild-type PBS consisting of a three-cylinder core composed of four APC disks and six rods made of up to three PC hexamers, respectively a mutant PBS comprised of rods with a single hexamer, both extracted from Synechocystis [42].