2002). In line with these results, Kim and colleagues studied a carotenoid-free mutant of BChl c containing C. tepidum and found that a significant fraction of the BChls forms a long-lived, triplet-like state that does not interact with oxygen and it was proposed that these states are triplet excitons formed by triplet–triplet interaction between BChls that are lower in energy

than the singlet oxygen state (but also than the triplet energy level of carotenoids) (Kim et al. 2007). Light spectroscopy and structure The large excitonic see more red shift of the chlorosomes requires an arrangement of the pigments that is reminiscent of the organization in J-aggregates (Moll et al. 1995), i.e. head-to-tail or head-to-head organization and many possibilities have been provided in literature over the years (for an “early” overview see, for instance, Blankenship et al. 1995). Most of these proposed aggregates were linear but to account for the relatively pronounced circular dichroism (CD) helical and cylindrical models were introduced (Lin et al. 1991; Prokhorenko et al. 2003; Somsen et al. 1996; Linnanto and Korppi-Tommola 2008) in which the J-type organization selleckchem was kept intact. Over

the years also many linear-dichroism (LD) measurements have been performed and these all demonstrated that the transition dipole moment corresponding to the long-wavelength Q y transition dipoles make a relatively small angle with the long axis of the chlorosomes (for more details see below). Also U0126 in vivo polarized transient absorption measurements (Lin et al. 1991; Pšenčík et al. 2003) Methocarbamol and polarized fluorescence measurements on non-oriented chlorosomes (Ma et al. 1996; Van Dorssen et al. 1986) and chlorosomes

in intact cells of C. limicola (Fetisova et al. 1988) indicated a high degree of ordering, that was more or less consistent with the LD results. As LD measurements provide spectroscopic information that may be used to verify structural models we will briefly address the LD of chlorosomes. The LD (ΔA) is defined as the difference in absorption (A) of light polarized parallel (v) and perpendicular (h) to the orientation axis of the sample (expansion direction of a squeezed gel containing the chlorosomes or the direction of an orienting electric field): ΔA = A v  − A h (see also Garab and Van Amerongen 2009). LD measurements provide the angle θ between a transition dipole moment and the long axis of the chromosome. Values between 15° and 27° were obtained for the transition dipole moment of the main Q y band and the long axis of the chlorosomes from Cf. aurantiacus (Frese et al. 1997; Griebenow et al. 1991; Matsuura et al. 1993; Van Amerongen et al. 1988, Van Amerongen et al. 1991). Single molecule experiments on chlorosomes from Cf. aurantiacus also showed preferential orientation of the Q y dipole moment along the long axis, and from these results an average angle of around 29° can be inferred. Recent experiments on chlorosomes from C.