Spectral Hole Burning Studies of Photosystem II Reaction Centers: Excited State Structure, Charge Separation and Energy Transfer Dynamics

Abstract

Spectral hole burning spectroscopy [1] has led to an improvement in the resolution of the optical spectra of photosynthetic protein-chlorophyll (Chl) complexes of 2–4 orders of magnitude [2]. As a result the level of detail available on the coupling of Chl optical transitions to intramolecular Chl modes and phonons (both delocalized and localized) is at an unprecedented level. Hole burning can also provide information on the early time (picosecond) events of the photosynthetic unit which are inaccessible by time domain techniques. For example, the primary charge separation process in bacterial RC was shown to occur subsequent to thennalization of relevant intramolecular and phonon modes [2]. Rapid thennalization had been assumed in theories for charge separation [3]. It was proven that ultra-fast (femtosecond) decay of the primary electron donor (PED) state, P*, of bacterial RC does not occur prior to primary charge separation [2,4]. Hole burning has unraveled the structure underlying the relatively broad PED state absorption profiles, P870 and P960, of Rb. sphaeroides and Rps. viridis; e.g., a lengthy Franck-Condon progression in an intennolecular special pair “marker” mode was identified and the coupling to low frequency (≈ 25 cm⁻¹ phonons characterized [2]. On the basis of the total electron-phonon coupling strength (S_T0TAL ~ 3.5) it was concluded that P* possesses significant charge-transfer character, in agreement with Stark measurements [4].