Electron Nuclear Double Resonance of the Chlorophyll Triplet State in the Water-Soluble Chlorophyll Protein from Brassica oleracea: Investigation of the Effect of the Binding Site on the Hyperfine Couplings

An investigation of the photoexcited triplet state of chlorophyll (Chl) a in the water-soluble chlorophyll protein (WSCP) of Brassica oleracea has been carried out by means of electron-nuclear double resonance (ENDOR), achieving a complete assignment of the observed hyperfine couplings corresponding to methine protons and methyl groups of Chl a triplet state. The triplet-state properties, and in particular the hyperfine couplings, were found to be similar to those previously reported for Chl a in the WSCP of Lepidium virginicum. Therefore, the porphyrin ring deformation observed in Brassica oleracea WSCP seems to only slightly affect the spin density of 3Chl a. This may be relevant when considering the robustness of triplet–triplet energy transfer mechanisms, relying on wavefunction overlap, in systems, such as the photosynthetic light-harvesting complexes, in which Chl triplet states with distorted geometries are involved.