Carotenoids in Photosynthesis quenching triplet (B)Chl excited states direct role in scavenging of singlet oxygen deactivation of Chl excited states (NPQ) redox activity in PSII reaction centers antenna chromophores for the mid- visible part of the solar spectrum LHCII trimer (2BHW.pdb) Standfuss et al. EMBO J. 2005, 24, 919. β-carotene Frank, H. A.; Brudvig, G. W. Biochemistry 2004, 43,

Excitation Energy Transfer from Carotenoids to Chlorophylls Optical transitions between the carotenoid S₀ and S₁ states are forbidden by symmetry (C 2h point group). Birge, R. R. Acc. Chem. Res. 1986, 19, Carotenoids(B)Chls S2S2 S1S1 S0S0 11Bu+11Bu+ 21Ag−21Ag− 11Ag−11Ag− A F T1T1 QxQx QyQy S0S0 T1T1 F A

Peridinin Peridinin–Chlorophyll a Protein (1PPR) Hofmann et al. Science 1996, 272, 1788.

Intramolecular Charge Transfer in Peridinin Hypothesis: The S₁ state of peridinin enhances energy transfer to the Q y state of chlorophyll a by obtaining an intramolecular charge-transfer (ICT) character. Ilagan et al. Biochemistry 2006, 45,

Third-Order Nonlinear Spectroscopy Stimulated photon-echo and transient-grating spectroscopy with optical heterodyne detection. DO Sample ND WP (τ) 1 and 2 3 and LO signal Miller and coworkers, Proc. Natl. Acad. Sci. U.S.A. 2001, 98, Fleming and coworkers, J. Chem. Phys. 2004, 121, Scherer and coworkers, J. Chem. Phys. 2006, 124, β-carotene

Fourier Transform Spectral Interferometry Isolation of the third-order nonlinear signal as the interference term at t LO Phasing Signal (Real) Window at t LO FT -1 Complex FT Complex Real component = Absorption Imaginary component = Dispersion

β-carotene Heterodyne transient-grating signal in benzonitrile S2S2 IS1S1 S0S0 16 fs142 fs9 ps / 12 ps

β-carotene Heterodyne transient-grating signal in benzonitrile An intermediate state is formed from the initially formed S₂ state in 16 fs. S2S2 IS1S1 S0S0 16 fs142 fs9 ps / 12 ps

β-carotene Heterodyne transient-grating signal in benzonitrile An intermediate state is formed from the initially formed S₂ state in 16 fs. The intermediate decays to the S₁ state in 142 fs, which is comparable to the fluorescence lifetime detected in upconversion experiments. S2S2 IS1S1 S0S0 16 fs142 fs9 ps / 12 ps

β-carotene Heterodyne transient-grating signal in benzonitrile The dispersion and absorption recovery time constants show that a "hot" or conformationally displaced ground-state ensemble is produced by nonradiative decay of the S₁ state. Xu et al. J. Chem. Phys. 2002, 116, S2S2 IS1S1 S0S0 16 fs142 fs9 ps / 12 ps

Peridinin Heterodyne transient-grating signal in methanol S2S2 IS1S1 S0S0 31 fs615 fs7 ps / 11 ps

Peridinin Heterodyne transient-grating signal in methanol The decay of the intermediate is strongly dependent on the polar solvation timescale. Solventτ polar S₂IS₁ ethyl acetate2.5 ps23 fs160 fs85 ps / 150 ps methanol6.2 ps31 fs615 fs7 ps / 11 ps 2-propanol220 ps35 fs1600 fs29 ps / 49 ps S2S2 IS1S1 S0S0 31 fs615 fs7 ps / 11 ps

Peridinin Heterodyne transient-grating signal in methanol The decay of the intermediate is strongly dependent on the polar solvation timescale. But the S₁ state lifetime is dramatically lengthened as the polarity of the solvent decreases. Bautista et al. J. Phys. Chem. B 1999, 103, SolventPolarityS₂IS₁ ethyl acetate fs160 fs85 ps / 150 ps methanol fs615 fs7 ps / 11 ps 2-propanol fs1600 fs29 ps / 49 ps S2S2 IS1S1 S0S0 31 fs615 fs7 ps / 11 ps

Peridinin–Chlorophyll a Protein Heterodyne transient-grating signal The intermediate state is the energy-transfer donor to the Q x state of Chl a. 31 fs500 fs16 ps / 16 ps S2S2 IS1S1 S0S0 QxQx QyQy 200 fs 3 ps

Peridinin–Chlorophyll a Protein Heterodyne transient-grating signal The intermediate state is the energy-transfer donor to the Q x state of Chl a. The absorption and dispersion components for the decay of S₁ have the same time constant, so conformational relaxation does not make a strong contribution to the recovery to the photoselected ground state. 31 fs500 fs16 ps / 16 ps S2S2 IS1S1 S0S0 QxQx QyQy 200 fs 3 ps

β-carotene Time-resolved transient grating spectra in benzonitrile The S₂ state decays as the stimulated emission contribution to the signal shifts irreversibly to the red. S2S2 IS1S1 S0S0 16 fs142 fs9 ps / 12 ps

β-carotene Time-resolved transient grating spectra in benzonitrile The S₂ state decays as the stimulated emission contribution to the signal shifts irreversibly to the red. The ESA spectrum shifts to the red as the intermediate forms, and then it shifts back to the blue as the S₁ state forms. S2S2 IS1S1 S0S0 16 fs142 fs9 ps / 12 ps

β-carotene Time-resolved transient grating spectra in benzonitrile The S₂ state decays as the stimulated emission contribution to the signal shifts irreversibly to the red. The ESA spectrum shifts to the red as the intermediate forms, and then it shifts back to the blue as the S₁ state forms. Similar ESA dynamics (and the same time constants) are observed in the ESA in the red and near-IR part of the spectrum. Cerullo et al. Science 2002, 298, S2S2 IS1S1 S0S0 16 fs142 fs9 ps / 12 ps

Four-Mode Dynamics Four coordinates control the nonradiative decay of β-carotene and peridinin in solution. Sanchez-Galvez, A.; Hunt, P.; Robb, M. A.; Olivucci, M.; Vreven, T.; Schlegel, H. B. J. Am. Chem. Soc. 2000, 122,

Four-Mode Dynamics Four coordinates control the nonradiative decay of β-carotene and peridinin in solution. 1.Bond-alternation (C–C, C=C) coordinates in the Franck–Condon region Sanchez-Galvez, A.; Hunt, P.; Robb, M. A.; Olivucci, M.; Vreven, T.; Schlegel, H. B. J. Am. Chem. Soc. 2000, 122,

Four-Mode Dynamics Four coordinates control the nonradiative decay of β-carotene and peridinin in solution. 1.Bond-alternation (C–C, C=C) coordinates in the Franck–Condon region 2.Torsional coordinates (φ) over the S₂-state barrier and to a twisted minimum Sanchez-Galvez, A.; Hunt, P.; Robb, M. A.; Olivucci, M.; Vreven, T.; Schlegel, H. B. J. Am. Chem. Soc. 2000, 122,

Four-Mode Dynamics Four coordinates control the nonradiative decay of β-carotene and peridinin in solution. 1.Bond-alternation (C–C, C=C) coordinates in the Franck–Condon region 2.Torsional coordinates (φ) over the S₂-state barrier and to a twisted minimum 3.Out-of-plane motions (α) that produce a pyramidal structure in the S₁ state with enhanced ICT character. Sanchez-Galvez, A.; Hunt, P.; Robb, M. A.; Olivucci, M.; Vreven, T.; Schlegel, H. B. J. Am. Chem. Soc. 2000, 122, Levine, B.; Martínez, T. J. Annu. Rev. Phys. Chem. 2007, 58, 613.

Four-Mode Dynamics Four coordinates control the nonradiative decay of β-carotene and peridinin in solution. 1.Bond-alternation (C–C, C=C) coordinates in the Franck–Condon region 2.Torsional coordinates (φ) over the S₂-state barrier and to a twisted minimum 3.Out-of-plane motions (α) that produce a pyramidal structure in the S₁ state with enhanced ICT character. 4.Solvent motions that are coupled to formation of intramolecular charge- transfer character near the the twisted minimum. Sanchez-Galvez, A.; Hunt, P.; Robb, M. A.; Olivucci, M.; Vreven, T.; Schlegel, H. B. J. Am. Chem. Soc. 2000, 122, Levine, B.; Martínez, T. J. Annu. Rev. Phys. Chem. 2007, 58, 613. Malhado, J. P.; Spezia, R.; Hynes, J. T. J. Phys. Chem. A 2011, 115,

Peridinin–Chlorophyll a Protein Time-resolved transient grating spectra The earliest PCP spectra are comparable in shape to the 50 fs peridinin/methanol spectrum. The structure of peridinin in PCP is like that for the twisted S₂ state in solution. S2S2 IS1S1 S0S0 31 fs500 fs16 ps / 16 ps

Binding-Site Control of Dynamics in PCP The X-ray crystal structure suggests that a twisted and kinked structure is favored by the peridinin binding site in PCP. Hofmann et al. Science 1996, 272, Shima et al., J. Phys. Chem. A 107, 8052 (2003).

Conclusions and Next Steps

1.In PCP, the twisted S₂ state intermediate serves as the energy-transfer donor to the Chl a Q x state.

Conclusions and Next Steps 1.In PCP, the twisted S₂ state intermediate serves as the energy-transfer donor to the Chl a Q x state. 2.The S₁ state of peridinin may obtain an enhanced ICT character by forming a kinked (pyramidal) conformation.

Conclusions and Next Steps 1.In PCP, the twisted S₂ state intermediate serves as the energy-transfer donor to the Chl a Q x state. 2.The S₁ state of peridinin may obtain an enhanced ICT character by forming a kinked (pyramidal) conformation. 3.The structure of the peridinin-binding pocket favors a twisted and kinked ground-state conformer.

Conclusions and Next Steps 1.In PCP, the twisted S₂ state intermediate serves as the energy-transfer donor to the Chl a Q x state. 2.The S₁ state of peridinin may obtain an enhanced ICT character by forming a kinked (pyramidal) conformation. 3.The structure of the peridinin-binding pocket favors a twisted and kinked ground-state conformer. Broadband 2D spectra to characterize the energy correlation of the twisted/pyramidal intermediates with the S₂ state and with the Chl a acceptors in PCP.

Acknowledgment Amy LaFountain (Uconn) Professor Andrew Moran (UNC Chapel Hill) Professor Ben Levine (MSU) NSF MCB DOE BES DE-SC