Spectroscopic Studies of Cryptophyte Light Harvesting Proteins: Vibrations and Coherent Oscillations

Abstract

The first step of photosynthesis is the absorption of light by antenna complexes. Recent studies of light-harvesting complexes using two-dimensional electronic spectroscopy have revealed interesting coherent oscillations. Some contributions to those coherences are assigned to electronic coherence and therefore have implications for theories of energy transfer. To assign these femtosecond data and to gain insight into the interplay among electronic and vibrational resonances, we need detailed information on vibrations and coherences in the excited electronic state compared to the ground electronic state. Here, we used broad-band transient absorption and femtosecond stimulated Raman spectroscopies to record ground- and excited-state coherences in four related photosynthetic proteins: PC577 from Hemiselmis pacifica CCMP706, PC612 from Hemiselmis virescens CCAC 1635 B, PC630 from Chroomonas CCAC 1627 B (marine), and PC645 from Chroomonas mesostigmatica CCMP269. Two of those proteins (PC630 and PC645) have strong electronic coupling while the other two proteins (PC577 and PC612) have weak electronic coupling between the chromophores. We report vibrational spectra for the ground and excited electronic states of these complexes as well as an analysis of coherent oscillations observed in the broad-band transient absorption data.

Estimation of damped oscillation associated spectra from ultrafast transient absorption spectra

“…The presence of such phase jumps and sharp nodes at the position of the transitions is of special interest, as these have been reported before. 18,[25][26][27] Our results correspond to those in Fig. 4(c) Finally, let us compare the DOAS features in the experimental, Figure 6, and simulated, Figure 8, data.…”

“…Vibrations can be assigned to the ground or excited state based on their amplitude and phase profiles, where a node in the amplitude, accompanied by a corresponding phase flip, marks the minimum of the electronic potential energy surface that the wavepacket is propagating on. [5][6][7][25][26][27][28][29] Vibronic coherences are relevant to the study of photosynthetic energy transfer, 25,27,[30][31][32][33][34][35][36][37][38] photosynthetic charge separation, 39 exciton coherence, 40 electa) Electronic mail: i.h.m.van.stokkum@vu.nl. Telephone: +31205987868.…”

“…Time traces at single detection wavelengths are either Fourier transformed, or fitted with damped cosines. 1,2,4,25,27,41,47 Next, for a few dominant frequencies the amplitude and phase can be plotted as function of the detection wavelength. No attempt has been made yet to globally analyze the complete data.…”

“…In a case study, new high quality data from the cryptophyte photosynthetic antenna complex PC612 25,48 will be analyzed. When structural information is available, the dynamics of complex molecular systems can be studied with quantum mechanical methods, and time resolved spectra can be predicted, which has been done for the photosynthetic antenna complexes PE545 49,50 and PC577.…”

“…We will demonstrate the applicability of the methodology and compare with results from published high-resolution measurements of light harvesting proteins. 25,27 …”