Controlling Vibronic Coupling in Chlorophyll Proteins:\nThe Effects of Excitonic Delocalization and Vibrational Localization

Vibrational–electronic (vibronic)\ncoupling plays a critical\nrole in excitation energy transfer in molecular aggregates and pigment–protein\ncomplexes (PPCs). But the interplay between excitonic delocalization\nand vibronic interactions is complex, often leaving even qualitative\nquestions as to what conceptual framework (e.g., Redfield versus Förster\ntheory) should be used to interpret experimental results. To shed\nlight on this issue, we report here on the interplay between excitonic\ndelocalization and vibronic coupling in site-directed mutants of the\nwater-soluble chlorophyll protein (WSCP), as reflected in 77 K fluorescence\nspectra. Experimentally, we find that in PPCs where excitonic delocalization\nis disrupted (either by mutagenesis or heterodimer formation), the\nrelative intensity of the vibrational sideband (VSB) in fluorescence\nspectra is suppressed by up to 37% compared to that of the native\nprotein. Numerical simulations reveal that this effect results from\nthe localization of high-frequency vibrations in the coupled system;\nwhile excitonic delocalization suppresses the purely electronic transition\ndue to H-aggregate-like dipole–dipole interference, high-frequency\nvibrations are unaffected, leading to a relative enhancement of the\nVSB. By comparing VSB intensities of PPCs both in the presence and\nabsence of excitonic delocalization, we extract a set of “local”\nHuang–Rhys (HR) factors for Chl <i>a</i> in WSCP.\nMore generally, our results suggest a significant role for geometric\neffects in controlling energy-transfer rates (which depend sensitively\non absorption/fluorescence line shapes) in molecular aggregates and\nPPCs.