Near Infra‐Red Absorption Spectra of the Chlorophyll <i>a</i> Cations and Triplet State in vitro and in vivo

Abstract

Abstract Absorption spectra in the near infra‐red have been obtained for transient species obtained from chlorophyll a , by flash absorption spectroscopy. The triplet state of Chl a presents an absorption band around 760 nm (ϵ = 7,500 M −1 cm −1 ) and a very broad band (ϵ = 1,700 M −1 cm −1 ) around 1100 nm. At low temperature, evidence was obtained for the formation of the triplet state of aggregated Chl a . Its t 1/2 of decay is around 0.5 ms instead of 1 ms for the monomeric species. The radical‐cation Chl a + was formed by reaction of 3 Chl a with a quinone; it has an absorption band at 840 nm (in cyclohexanol) with a shoulder around 740 nm and no absorption between 1000 and 1650 nm. The cation has also been formed in vivo, as the oxidized state of the photosynthetic primary electron donors, P 680 and P 700 . The spectrum of P + 680 resembles that of Chl a + ; its absorption maximum is at 820 nm. The spectrum of P + 680 , maximum at 810 nm, is significantly broader. P + 700 has no absorption between 1000 and 1650 nm. These spectra are in favor of recent suggestions on a monomeric nature of P 680 and of a dimeric nature for P 700 .