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.

The lowest-energy chlorophyll of photosystem II is adjacent to the peripheral antenna: Emitting states of CP47 assigned via circularly polarized luminescence

“…Any conclusions need to be consistent with other results available, such as the spectral effects associated with site-directed mutagenesis, which can modify the binding site associated with specific chlorophylls [22]. Recently, the two approaches of spectral modeling [23] and direct calculations [24] have been applied successfully to the CP43 protein. The former study [23] incorporated, for the first time, parallel measurements of lowtemperature CD and circularly polarized luminescence (CPL).…”

“…It is well known that energetic disorder (non-correlated site energies) can give rise to a fraction of the complexes in a sample having lowest energy states of a different character to others [42,43]. This is evident for the A and B states of CP43 [23,24]. In the case of CP47 an additional complication arises, because steady-state fluorescence spectra at low temperature were reported with maxima at slightly varying wavelengths between 690 and 693 nm [29,35,41,44].…”

“…[13]. Our recent work on the CP43 antenna [23] has introduced CPL as an additional experimental quantity to check the predictive power of exciton Hamiltonians in the literature. Here, we will show that the CPL technique, along with lowtemperature CD, allows us to exclude Chl 627(29) from the three possible candidates, identified by their negative linear dichroism (discussed above), for the lowest energy state of CP47.…”

“…For these reasons, available data for the CD of the lowest excited state of CP47 has not been of sufficient quality to justify a high weighting in previous modeling studies. In the present work we have avoided artifactual signals that can arise due to strain in the low-temperature samples, using the methods described previously [23], and present CD spectra that are sufficiently conservative to justify inclusion in the analysis, using exciton theory of coupled Q y transitions.…”

“…Recently, the two approaches of spectral modeling [23] and direct calculations [24] have been applied successfully to the CP43 protein. The former study [23] incorporated, for the first time, parallel measurements of lowtemperature CD and circularly polarized luminescence (CPL). For CP43, the analysis was largely in agreement with earlier assignments [13,19] for the low-energy states.…”