Band Shape Heterogeneity of the Low-Energy Chlorophylls of CP29: Absence of Mixed Binding Sites and Excitonic Interactions

Abstract

A number of spectroscopic characteristics of three almost isoenergetic, red-shifted chlorophylls (chls) in the PS II antenna complex CP29 are investigated with the aim of (i) determining whether their band shapes are substantially identical or not, (ii) addressing the topical problem of whether they are involved in excitonic interactions with other chls, and (iii) establishing whether their binding sites may be defined as "mixed" with respect to their capacity to bind chls a and b. The three chls A2-CHL612, A3-CHL613, and B3-CHL614 were analyzed after in vitro apoprotein-pigment reconstitution using the CP29 coding sequence from Arabidopsis thaliana for both the wild-type and mutant complexes. Difference spectra thermal broadening analyses indicated that the half-bandwidths varied between 12 and 15 nm (at room temperature), due mainly to differences in the optical reorganization energy (25-40 cm(-1)). Moreover, only the A2 chl displayed an intense vibrational band in the 300-600 cm(-1) interval from the 0-0 transition. We conclude that within the red absorbing (approximately 680 nm) antenna chls of a single chl-protein complex a marked spectral band shape heterogeneity exists. By analysis of the absorption and circular dichroism spectra no evidence was found of significantly strong excitonic interactions. The single gene mutation of the A3 and B3 binding sites causes absorption changes in both the long wavelength chl a absorbing region and in the chl b spectral region. This has previously been observed and was attributed to "mixed" chl a/b binding sites [Bassi, R., Croce, R., Cugini, D., and Sandona, D. (1999) Proc. Natl. Acad. Sci. U.S.A. 96,10056-10061]. This interpretation, while in principle not being unreasonable, is shown to be incorrect for these two chls.

Reconstituted CP29: multicomponent fluorescence decay from an optically homogeneous sample

“…The continuous line represents the spectrum with an uncorrected instrumental baseline, which is slightly sloping. The dashed curve represents the spectrum from which this slightly sloping instrumental baseline was subtracted Photosynth Res (2012) 111:53-62 57 Visual comparison indicates, as previously reported (Giuffra et al 1997;Belgio et al 2010), that close agreement exists between the Stepanov calculated emission and that measured. In the wavelength region around the maxima (±5 nm) the spectra are indistinguishable (Fig.…”

“…van Metter and Knox 1979;Zucchelli et al 1995;Jennings et al 2000Jennings et al , 2003Dau 1996;Tumino et al 2008). It has been also applied, with success, to both native and recombinant CP29 suspensions (Giuffra et al 1997;Belgio et al 2010). It is known to provide accurate data substantially in the absorption/fluorescence overlap interval which, in the present case of CP29, is between about 660 and 690 nm.…”

“…Giuffra et al 1996;Belgio et al 2010). This complex has a relatively simple decay, characterised by just two dominant decay processes of around 3 and 5 ns and a minor fast decay.…”

Higher Plant Photosystem II Light-Harvesting Antenna, Not the Reaction Center, Determines the Excited-State Lifetime—Both the Maximum and the Nonphotochemically Quenched

“…Previously, similar results were obtained using low temperature, a treatment that reduced RCII accumulation and increased LHCII but not CP29 (25). CP29 is closely associated with the RCII core complex and is one of the key complexes required to stabilize the PSII supercomplex structure (26) from the antenna to the RC (27). Therefore, a possible explanation for the different regulation is that its expression and incorporation into the photosynthetic membrane is at least partially dependent on the PSII assembly.…”

“…In the treated plants, the amount of major LHCII complex was increased, whereas in the minor CP29 complex, it was decreased. The latter is closely associated with the RCII core complex and is one of the key complexes that are required to stabilize the PSII supercomplex structure and have been suggested to have a crucial role in transferring energy from antenna to RCs (26,27). Interestingly, >90% of the RCII decrease was accompanied by a decline of only~40% of CP29.…”