STUDIES ON CHROMOPHORE COUPLING IN ISOLATED PHYCOBILIPROTEINS. IV. FEMTOSECOND TRANSIENT ABSORPTION STUDY OF ULTRAFAST EXCITED STATE DYNAMICS IN TRIMERIC PHYCOERYTHROCYANIN COMPLEXES*

Abstract

Abstract The excited state dynamics of trimeric phycoerythrocyanin has been studied by two‐color femtosecond transient absorption spectroscopy with a time‐resolution better than 200 fs. Upon selective excitation of the short‐wavelength phycobiliviolin chromophore at 575 nm absorption bleachings are observed. An isotropic ultrafast decay of the initial bleaching of 585 ± 40 fs has been resolved at short detection wavelength (578 nm). Upon stepwise increase of the detection wavelength up to 617 nm, the bleaching showed a delayed rise above 593 nm with r rice =380–580 fs. All other isotropic kinetic components in this wavelength range were longer than 100 ps. The ultrafast component is discussed in terms of an energy transfer process from the α‐84 phycobiliviolin chromophore to the β‐84 chromophore in adjacent monomer subunits of the trimer. It is concluded that the β‐155 chromophore is the longest‐wavelength chromophore. Exciton relaxation between closely spaced chromophore pairs is discussed as an alternative interpretation for the ultrafast component.