Energy Transfer to and from Carotenoids in Photosynthesis

Excited carotenoid molecules can transfer energy rapidly to nearby chlorophyll molecules in photosynthetic antenna complexes [57]. It has been suggested that this process involves exchange coupling, because the radiative transitions from the lowest excited singlet states of the carotenoids to the ground states are forbidden by molecular symmetry (Box 4.12). However, the rate could possibly be explained by considering the full expression for the direct interaction (Eq. 7.14) instead of just dipole-dipole interactions [58]. 

Graph 1 below replots the absorption and emission spectra considered in Chaps. 4 and 5. Suppose these spectra apply to a chromophore bound to a protein (A). Graph 2 shows similar spectra for a different chromophore bound to a second protein (B), which can combine with A and a third protein (C) to form a heterotrimer, ABC. The table gives the amplitudes of fluorescence emission at 335 and 365 nm when the individual chromophore-protein complexes and the heterotrimer were excited at 315 nm. Assume that the absorption spectrum of ABC is simply the sum of those of A and B. 

Calculate the emission-absorption overlap integral (7) for A and B. (Be sure to give the units.) 

Using your results from Exercise 4.2 and the fluorescence yield given in Exercise 5.2, calculate the Eorster radius (/ ,) for resonance energy transfer from Ato B. 

Estimate the distance between the chromophores bound to A and B in the ABC complex, assuming that the change in the fluorescence yield of A relative to that for monomeric A is due solely to resonance energy transfer. Assume also that the transition dipoles of both chromophores rotate rapidly and isotropically on the timescale of fluorescence. 

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