Optically Detected Magnetic Resonance of Chlorophyll Triplet States in Water-Soluble Chlorophyll Proteins from Lepidium virginicum: Evidence for Excitonic Interaction among the Four Pigments

Abstract

Optically detected magnetic resonance of triplet states populated by photoexcitation in water-soluble chlorophyll proteins (WSCPs) from Lepidium virginicum has been performed using both absorption and fluorescence detection. Well resolved triplet-singlet (T-S) spectra have been obtained and interpreted in terms of electronic interactions among the four chlorophylls (Chls), forming two dimers in the WSCP tetramer. Localization of the triplet state on a single Chl leads to a redistribution of the oscillator strength in the remaining three Chls of the complex. By comparing the spectra with those obtained on a substoichiometric WSCP complex containing only 2 Chls per protein tetramer, we proved that, to interpret the optical spectra of the WSCP fully loaded with 4 Chls, the interactions between the two dimers have to be taken into account and cannot be considered negligible. The results show that the WSCP may well be considered as an ideal model system to study Chl-Chl interactions, also in view of the possibility to modify the number and molecular structure of the bound porphyrin chromophores.

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

“…2. The positions of the |D| −|E| and |D| +|E| transitions (721 and 947 MHz, respectively; note that, as previously reported for 3 Chls, the 2|E| transition was too weak to be detected) allowed a precise calculation of the values of the ZFS parameters |D| and |E| (|D|= 0.028 cm −1 and |E|= 0.0038 cm −1 ), which are close to those previously reported for Lv WSCP (|D|= 0.029 cm −1 and |E|= 0.0037 cm −1 [20]). The similarity points towards a similar electronic distribution on the single Chl a on which the triplet state is localized, as previously reported for Lv WSCP [11].…”

“…ADMR spectra were acquired in a home-built set-up which has been previously described in detail [20]. Namely, a 250 W halogen lamp (Philips) was employed as a light source, filtered through a 10 cm water filter, and focused on the sample cell immersed in a helium bath cryostat.…”

“…The four symmetry-related chlorophyll (Chl) binding sites are identical, and therefore, the four bound chromophores experience identical protein surroundings and are spectroscopically equivalent, contrarily to chlorophyll (Chl)-binding complexes involved in photosynthesis, in which tens to hundreds of chromophores are bound in finely tuned individual binding sites [12], resulting in highly complex spectra. It follows that WSCP is an ideal model system for detailed spectroscopic investigation focused on understanding Chl-protein and Chl-Chl interactions [5,[13][14][15][16][17][18][19][20].…”

Stability of Water-Soluble Chlorophyll Protein (WSCP) Depends on Phytyl Conformation

“…In the same time interval of illumination, Chl a as a control dissolved in 2% OG (w/v) has lost almost all (98%) of its initial fluorescence, as has been shown previously. 18 Obviously, the exchange of leucine at position 91 with proline has a drastic effect on the photostability, leading to the question of whether this is a consequence of the less bulky side chain of P91 with respect to that of L91. To address this question, photobleaching measurements with Lv mutants exhibiting different side-chain lengths in position 91 (L91V, L91A, and L91G) were performed under identical experimental conditions.…”

“…15−17 A recombinant version can be reconstituted with either 4 or 2 Chls per protein tetramer. 17,18 The biological function of WSCP is still enigmatic, 14,19−21 but it is reasonable to assume that it is related to the remarkable stability of the pigment–protein complex. Tetrameric WSCP resists dissociation and denaturation even at 100 °C.…”