Chlorophyll, triplet, lifetime

As opposed to P-700 in PS I and to the cation radicals of the bacterial reaction centers, P-680 is difficult to trap in its oxidized state - even at low temperatures its lifetime following photogeneration is only 3-4 ms [90] - and chemical oxidation so far has not been possible owing to the high P-680 midpoint potential [1]. Consequently the battery of techniques, particularly magnetic resonance, which has proven fruitful in unraveling the structures of the other reaction center chlorophylls has not been applied to P-680. Its spin-polarized triplet has been detected [61,91] and its unexpected parallel orientation with respect to the membrane plane postulated. The zero-field splitting parameters are almost identical to those of... 

Theoretical calculations have been made for photosynthetic pigments . An extensive review of models of energy and electron transfer events of synthetic molecules for photosynthesis has been prepared by Wasielewski . Other studies have made on tetraphenylporphyrin-polypeptide pigments , photosensitization of triplet carotenoids , fluorescence yields and lifetimes for bacteriochlorophyll c , triplet yields and ESR of chlorophyll 3 8 and quenching processes of pheophytin 539 ... 

Note that the newly estimated lifetime of 32 5 ps for the photoreduced primary electron acceptor is shorter than the 200 ps reported previously This difference has been attributed to the different nature of the PS-I particles used in the two studies. As the PS-I particle used in the earlier studies had only 26 Chi per P700 v. a ratio of 75-100 in the particles used in the present studies, the extensive detergent treatment used to achieve the high P700-to-chlorophyll ratio might have affected the electron-transfer rate of Ao" and consequently its lifetime. The origin of the additional shoulder at 675 nm in the [Ao -Aq] difference spectrum has not yet been established it was tentatively explained by Sbuvalov et al. as possibly due to some contribution from antenna-cbloropbyll triplet state, in spite of tbe use of 710-nm excitation. 

The 1ms lifetime of the P680 triplet state is similar to the lifetime of chlorophyll-a in solution, which is 1.5-1.7ms. This indicates that the carotenoid present in this preparation is not quenching the P680 triplet state in the majority of reaction centres. 

---