Excitation Wavelength-Dependent Electron−Phonon and Electron−Vibrational Coupling in the CP29 Antenna Complex of Green Plants

Electron−phonon and electron−vibrational coupling strengths of a weakly (excitonically) coupled chlorophyll a S₁ → S₀ transition of the CP29 antenna complex of plant photosystem II were studied by difference fluorescence-line-narrowing spectroscopy at 4.5 K. A strong, almost linear increase of the electron−phonon coupling strength toward longer wavelengths was observed, with Huang−Rhys factors S_ph increasing from 0.41 ± 0.05 at 680 nm to about 0.66 ± 0.07 at 688 nm. The former and latter wavelengths are located close to the peak and on the red edge of the inhomogeneous site distribution function, respectively. The experimentally obtained wavelength dependence of S_ph may originate either from an alteration of the electron−phonon coupling strength by the local environment of the fluorescing chromophore and/or from the presence of two isoforms of CP29, which are characterized by different coupling strengths to the protein environment. The one-phonon profile peaks at ω_m = 22 cm^-1 and is described by an asymmetric function composed of a Gaussian low-energy wing and a Lorentzian high-energy tail with half-widths at half-maximum of 10 ± 1 and 60 ± 10 cm^-1, respectively. Thirty-nine individual vibrational modes between 90 and 1665 cm^-1 were resolved, and their Huang−Rhys factors were determined, which fall in the range between 0.0004 and 0.032. The broad feature present in the overlap region of phonon and vibrational modes at about 90 cm^-1 is characterized by S = 0.048. An integral value of vibrational coupling strengths S_vib = 0.36 ± 0.05 was determined, which is similar to that observed earlier for the trimeric LHC II complex.