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.