PARENT SESSION
Posters P2B Light, redox and metabolic regulation: Light Reactions. Abstracts (444-478)

Excited state dynamics of the orange carotenoid protein from cyanobacteria. Tomáš Polívka^*,1, Cheryl Kerfeld², Villy Sundström¹, ¹ Chemical Physics, Lund University, Sweden² Molecular Biology Institute, University of California, USA

ABSTRACT- The water soluble orange carotenoid binding protein (OCP) from cyanobacteria is an ideal model system for study of the effects of protein environment on photophysical properties of carotenoids. The three-dimensional structure of the OCP has been determined (Kerfeld et al. Structure 11, 1-20; 2003). The OCP contains a single pigment, the carotenoid 3′-hydoxyechinenone (hECN). This carotenoid belongs to the family of carotenoids containing a conjugated carbonyl group, which provides unique properties of their excited states. Due to charge-transfer character of the lowest excited state, excited state properties depend on solvent polarity. Since the OCP appears to play a role in photoprotective processes, excited state dynamics of hECN in both solution and OCP is important for understanding the biological function of the OCP. Absorption spectra of hECN in n-hexane and methanol reveals 0-0 origin of the S₂ state at ∼475 nm; this is shifted to ∼495 nm in OCP. Interestingly, the vibrational structure of the S₂ state is better resolved in OCP, suggesting that confinement of hECN in within the protein prevents a wider distribution of conformers. In solution, the S₁ lifetime of hECN is ∼6 ps in methanol, n-hexane and CS₂, suggesting that neither polarity nor polarizability affects the S₁ lifetime. While the S₁ lifetime is the same in all used solvents, it is significantly shortened to 2.6 ps in OCP, demonstrating a dramatic effect of protein environment on the S₁ lifetime. The large protein-induced changes are further emphasized in transient absorption spectra. While in solution the S₁-S_N band of hECN peaks at 570 nm in both n-hexane and methanol, this band is shifted to nearly 650 nm in OCP, making the protein-induced shift of the S₁-S_N band in OCP the largest observed so far. The possible origin of these protein-induced changes and their relation to possible roles of OCP protein will be discussed.

KEY WORDS: photoprotection, femtosecond, carotenoids, excited state