Molecular architecture of light-harvesting complexes of chromophytes

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Fucoxanthin-Chl a/c protein (FCP), the major light harvesting protein of brown algae and diatoms, is very unstable and its excitation energy transfer between pigments was very readily injured when exposed to detergents conventionally used for isolating various photoactive protens from green plants. We studied on the conditions of FCP isolation from the thalli of brown algae, and revealed that the assemblies of FCP are bound with photochemical reaction center to form complexes which are embedded in thylakoids, like LHCP in green algae. Monomeric form of FCP consisted of a protein with MW 20,226 (in the case of Ectocarpus), 4 Chl a, 1 Chl c and 4 fucoxanthin, and its native form showed an highly efficient energy transfer (higher than 95%) between fucoxanthin*Chl a, Chl c*Chl a. Fucoxanthin bound in FCP showed more red-shifted absorbance than liberated one, showing an absorbance spectrum similar to that dissoved in imidoN-containing, polarsolvets. Exposed to warm temperature (40ﾟC), the red-shifted FCP turned to green reflecting the blue-shift of fucoxanthin with concomitant loss of CD signals, indicating the disarrangement pigment molecules. However, the resonance Raman spectrum of blue-shifted FCP stayd unaltered from the native FCP,suggesting that fucoxanthin active in excitation energy transfer is highly interacted with Chl a in FCP,and this interaction is responsible to the red shift of fucoxanthin.