High Pressure Hole-Burning Studies of Transport Dynamics in Photosynthetic Protein Complex

Hole burning spectroscopies have proven to be powerful tools for the elucidation of the excited state (Qy) electronic structure and transport (energy, charge) dynamics of photosynthetic protein-chlorophyll complexes.1-3 Furthermore, hole burning has proven that such complexes are glass-like on a microscopic scale, which results in inhomogeneous broadenings of ~50-150 cm-1 (ΓI) for the Qy←S0 Chi absorption transitions. Importantly, it has also been shown that the zero-phonon line (ZPL) frequency distribution functions for different Qy-states of the same complex are uncorrelated, meaning that the widths of the distribution functions for energy gaps relevant to energy and electron transfer are ~21/2 ΓI. This raises the possibility that the kinetics for transport could be dispersive.4 Whether or not they are turns out to be dependent on the .strength of the electron-phonon coupling associated with the transport process. Fortunately, hole burning has proven to be capable of characterizing the nature and strength of this coupling. Required was the development of an accurate theory for entire hole profile (zero-phonon hole [ZPH] plus phonon sideband holes) applicable for arbitrarily strong electron-phonon coupling in the low temperature limit. More recently, this theory5 has been extended to arbitrary temperature.6 A key point is that the ZPH is only one small part of the entire profile. It is the entire hole profile that is important to the problem of transport dynamics in the photosynthetic unit. Thus it is that single molecule (complex) detection would be of little consequence to the problem of energy and electron transfer.