Annihilation of Excess Excitations along Phycocyanin Rods Precedes Downhill Flow to Allophycocyanin Cores in the Phycobilisome of Synechococcus elongatus PCC 7942.

Cyanobacterial phycobilisome complexes absorb visible sunlight and funnel photogenerated excitons to the photosystems where charge separation occurs. In the phycobilisome complex of Synechococcus elongatus PCC 7942, phycocyanin protein rods that absorb bluer wavelengths are assembled on allophycocyanin cores that absorb redder wavelengths. This arrangement creates a natural energy gradient toward the reaction centers of the photosystems. Here, we employ broadband pump-probe spectroscopy to observe the fate of excess excitations in the phycobilisome complex of this organism. We show that excess excitons are quenched through exciton-exciton annihilation along the phycocyanin rods prior to transfer to the allophycocyanin cores. Our observations are especially relevant in comparison to other antenna proteins, where exciton annihilation primarily occurs in the lowest-energy chlorophylls. The observed effect could play a limited photoprotective role in physiological light fluences. The exciton decay dynamics is faster in the intact phycobilisome than in isolated C-phycocyanin trimers studied in earlier work, confirming that this effect is an emergent property of the complex assembly. Using the obtained annihilation data, we calculate exciton hopping times of 2.2-6.4 ps in the phycocyanin rods. This value agrees with earlier FRET calculations of exciton hopping times along phycocyanin hexamers by Sauer and Scheer.

Time-Domain Line-Shape Analysis from 2D Spectroscopy to Precisely Determine Hamiltonian Parameters for a Photosynthetic Complex.

Optical signals come from coherences between quantum states, with spectral line widths determined by the coherences' dephasing dynamics. Using a 2D electronic spectrometer, we observe weak coherence- and rephasing-time-domain signals persisting to 1 ps in the Fenna-Matthews-Olson complex at 77 K. These are coherences between the ground and excited states prepared after the complex interacts once or three times with light, rather than zero-quantum coherences that are more frequently investigated following two interactions. Here, we use these small but persistent signal components to isolate spectral contributions with narrowed peaks and reveal the system's eigenenergies.

The spectroscopy and photochemistry of quinioline structural isomers: (E)- and (Z)-phenylvinylnitrile.

In Titan's atmosphere, photochemical pathways that lead to nitrogen heteroaromatics may incorporate photoisomerization of their structural isomers as a final step. (E)- and (Z)-phenylvinylnitrile ((E)- and (Z)-PVN, C6H5-CH=CHCN) are structural isomers of quinoline that themselves possess extensive absorptions in the ultraviolet, and thus may engage in such photoisomerization pathways. The present study explores the vibronic spectroscopy and photo-induced isomerization of gas-phase (E)- and (Z)-PVN in the 33,600-35,850 cm(-1) region under jet-cooled conditions. The S0-S1 origins for (E)- and (Z)-PVN have been identified at 33 827 cm(-1) and 33 707 cm(-1), respectively. Isomer-specific UV-UV hole-burning and UV depletion spectra reveal sharp vibronic structure that extends over almost 2000 cm(-1), with thresholds for fast non-radiative decay identified by a comparison between hole-burning and UV depletion spectra. Dispersed fluorescence spectra of the two isomers enable the assignment of many low frequency transitions in both molecules, aided by harmonic frequency calculations (B3LYP/6-311++G(d,p)) and a comparison with the established spectroscopy of phenylvinylacetylene, the ethynyl counterpart to PVN. Both isomers are proven to be planar in both the S0 ground and S1 electronic excited states. (E)-PVN exhibits extensive Duschinsky mixing involving out-of-plane modes whose frequencies and character change significantly in the ππ* transition, which modulates the degree of single- and double-bond character along the vinylnitrile substituent. This same mixing is much less evident in (Z)-PVN. The spectroscopic characterization of (E)- and (Z)-PVN served as the basis for photoisomerization experiments using ultraviolet hole-filling spectroscopy carried out in a reaction tube affixed to the pulsed valve. Successful interconversion between (E) and (Z)-PVN was demonstrated via ultraviolet hole-filling experiments. Photoexcitation of (E)- and (Z)-PVN at their respective S0-S1 origins failed to produce quinoline, a simple polycyclic aromatic nitrogen heterocylcle, within the detection sensitivity of our experiments. Stationary points along the potential energy surface associated with (Z)-PVN → quinoline isomerization showed a barrier of 93 kcal/mol associated with the first step in the isomerization process, slowing the interconversion process at the excitation energies used (96 kcal/mol) to timescales beyond those probed in the present experiment.

Ultraviolet spectroscopy of fundamental lignin subunits: guaiacol, 4-methylguaiacol, syringol, and 4-methylsyringol.

Ultraviolet spectroscopy of the G- and S-type lignin subunits, guaiacol (G) and syringol (S), along with their para-methylated derivatives 4-methylguaiacol (4-MG) and 4-methylsyringol (4-MS), has been carried out in the cold, isolated environment of a supersonic jet. The excitation spectra and dispersed fluorescence (DFL) spectra of G and 4-MG show strong S0-S1 origins and Franck-Condon activity involving both the ring modes typical of aromatic derivatives, and the four lowest frequency out-of-plane modes (a") and lowest in-plane mode (a') involving the OH and OCH3 groups. The four low-frequency out-of-plane modes undergo extensive Duschinsky mixing between the ground and excited state. In 4-MG, combination bands involving methyl rotor levels with out-of-plane modes appeared with surprisingly high intensity, indicating a high degree of hindered rotor-vibration coupling in both S0 and S1. These mixing effects accompany the change in geometry upon π-π∗ electronic excitation going from a planar ground state to a non-planar excited state. Time-dependent density functional theory (TDDFT M05-2X∕6-311++G(d,p)) calculations predict a geometric distortion along the out-of-plane oxygen flapping coordinate, yielding a double minimum potential in S1 with a barrier to planarity of 195 cm(-1) in G. The excitation spectrum of S and 4-MS showed a much higher degree of spectral congestion and a larger geometry change evident by a shifted intensity distribution peaking ∼300 cm(-1) above the electronic origin. TDDFT calculations predict a larger geometry change in S compared with G, with the OH and H-bonded methoxy groups displaced in opposite directions above∕below the ring plane. Dispersed fluorescence from all S1 excited state levels in S∕4-MS yield only broad emission peaking far to the red of the excitation wavelength (-4500 cm(-1)). Several hypotheses regarding the source of this broad, redshifted emission were tested, but the cause remains unclear. p-Methylation was found to significantly redshift the UV absorption in both 4-MG and 4-MS, and methyl rotor transitions were assigned in both allowing for the determination of the shape and barrier heights of their respective potentials. These results provide a foundation for the discrimination of G- and S-chromophores in lignin oligomers, and demonstrate the potential for site-selective absorption.