title: Circularly polarized luminescence spectroscopy reveals low-energy excited states and dynamic localization of vibronic transitions in CP43.
abstract: Circularly polarized luminescence (CPL) spectroscopy is an established but relatively little-used technique that monitors the chirality of an emission. When applied to photosynthetic pigment assemblies, we find that CPL provides sensitive and detailed information on low-energy exciton states, reflecting the interactions, site energies and geometries of interacting pigments. CPL is the emission analog of circular dichroism (CD) and thus spectra explore the optical activity only of fluorescent states of the pigment-protein complex and consequently the nature of the lowest-energy excited states (trap states), whose study is a critical area of photosynthesis research. In this work, we develop the new approach of temperature-dependent CPL spectroscopy, over the 2-120 K temperature range, and apply it to the CP43 proximal antenna protein of photosystem II. Our results confirm strong excitonic interactions for at least one of the two well-established emitting states of CP43 named "A" and "B". Previous structure-based models of CP43 spectra are evaluated in the light of the new CPL data. Our analysis supports the assignments of Shibata et al. [Shibata et al. J. Am. Chem. Soc. 135 (2013) 6903-6914], particularly for the highly-delocalized B-state. This state dominates CPL spectra and is attributed predominantly to chlorophyll a's labeled Chl 634 and Chl 636 (alternatively labeled Chl 43 and 45 by Shibata et al.). The absence of any CPL intensity in intramolecular vibrational sidebands associated with the delocalized "B" excited state is attributed to the dynamic localization of intramolecular vibronic transitions.
authors.affiliation: Research School of Chemistry, Australian National University, Canberra, Australia.
authors.forename: Jeremy
authors.initials: J
authors.lastname: Hall
authors.affiliation: Institut für Theoretische Physik, Johannes Kepler Universität, Linz, Austria.
authors.forename: Thomas
authors.initials: T
authors.lastname: Renger
authors.affiliation: Estacion Experimental de Aula Dei (CSIC), Avda. Montañana, Zaragoza, Spain.
authors.forename: Rafael
authors.initials: R
authors.lastname: Picorel
authors.affiliation: Research School of Chemistry, Australian National University, Canberra, Australia. Electronic address: elmars.krausz@anu.edu.au.
authors.forename: Elmars
authors.initials: E
authors.lastname: Krausz
mesh_terms: D002942:Circular Dichroism; D005453:Fluorescence; D049449:Luminescence; D008958:Models, Molecular; D045332:Photosystem II Protein Complex / Q000737:chemistry*; D013057:Spectrum Analysis; D013696:Temperature; D014732:Vibration
chemical_list: D045332:Photosystem II Protein Complex; C083100:photosystem II, chlorophyll binding protein, CP-43
keywords: Circular dichroism; Circularly polarized luminescence; Dynamic localization; Fluorescence; Lowest excited states; Photosystem II