THE RELATIONSHIP BETWEEN THE BINDING OF DICYCLOHEXYLCARBODIIMIDE AND QUENCHING OF CHLOROPHYLL FLUORESCENCE IN THE LIGHT-HARVESTING PROTEINSOF PHOTOSYSTEM-II

The relationship between the binding of dicyclohexylcarbodiimide (DCCD ) to isolated light-harvesting proteins of photosystem LT and the inhi bition of chlorophyll fluorescence quenching by DCCD have been investi gated. For a range of different complexes an approximately linear rela tionship was obtained between the efficiency of DCCD binding and the D CCD-dependent reversal of fluorescence quenching. The most efficient l abeling was found for the minor light-harvesting complexes, CP29 and C P26, In the case of the former, five different preparations were compa red including two reconstituted complexes in which a putative DCCD-bin ding site had been mutagenized. Again, an approximately linear relatio nship between DCCD binding and the extent of reversal of fluorescence quenching was found. However, the binding of DCCD was found to occur a t least an order of magnitude faster than the change In fluorescence. The results are discussed in terms of the multiplicity of DCCD-binding sites and the influence of protein structure on both the binding of D CCD and the fluorescence quenching mechanism.