Recent structural data revealed that the CP29 protein of higher plant photosystem II (PSII) contains 13 chlorophylls (Chl鈥檚) per complex (Pan et al.
Nat. Struct. Mol. Biol.
2011,
18, 309), i.e., five Chl鈥檚 more than in the predicted CP29 homology-based structure model (Bassi et al.
Proc. Natl. Acad. Sci. U.S.A.1999,
96, 10056). This lack of consensus presents a constraint on the interpretation of CP29 optical spectra and their underlying electronic structure. To address this problem, we present new low-temperature (5 K) absorption, fluorescence, and hole-burned (HB) spectra for CP29 proteins from spinach, which are compared with the previously reported data. We focus on excitation ene
rgy transfer (EET) and the nature of the lowest-ene
rgy state(s). We a
rgue that CP29 proteins previously studied by HB spectroscopy lacked at least one Chl
a molecule (i.e.,
a615 or
a611), which along with Chl
a612 contribute to the lowest ene
rgy state in more intact CP29, and one Chl
b (most likely
b607). This is why the low-ene
rgy state and fluorescence maxima reported by
Pieper et al. (
Photochem. Photobiol.2000,
71, 574) were blue-shifted by 1 nm, the low-ene
rgy state appeared to be highly localized on a single Chl
a molecule, and the position of the low-ene
rgy state was independent of burning fluence. In contrast, the position of the nonresonant HB spectrum shifts blue with increasing fluence in intact CP29, as this state is strongly contributed to by several pigments (i.e.,
a611,
a612,
a615, and
a610). Zero-phonon hole widths obtained for the Chl
b band at 638.5 nm (5 K) revealed two independent Chl
b 鈫?Chl
a EET times, i.e., 4 卤 0.5 and 0.4 卤 0.1 ps. The latter value is a factor of 2 faster than previously observed by HB spectroscopy and very similar to the one observed by Gradinaru et al. (
J. Phys. Chem. B2000,
104, 9330) in pump鈥損robe experiments. EET time from 650 nm Chl
b 鈫?Chl
a and downward EET from Chl(s)
a state(s) at 665 nm occurs in 4.9 卤 0.7 ps. These findings provide important constraints for excitonic calculations that are discussed in the accompanying paper (part II, DOI
rg/10.1021/jp4004278">10.1021/jp4004278).