| 1. |
- Ilagan, R P, et al.
(författare)
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Femtosecond time-resolved absorption spectroscopy of astaxanthin in solution and in alpha-crustacyanin
- 2005
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Ingår i: The Journal of Physical Chemistry Part A: Molecules, Spectroscopy, Kinetics, Environment and General Theory. - : American Chemical Society (ACS). - 1520-5215. ; 109:14, s. 3120-3127
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Tidskriftsartikel (refereegranskat)abstract
- Steady-state absorption and femtosecond time-resolved spectroscopic studies have been carried out on astaxanthin dissolved in CS2, methanol, and acetonitrile, and in purified alpha-crustacyanin. The spectra of the S-0 -> S-2 and S-1 -> S-n transitions were found to be similarly dependent on solvent environment. The dynamics of the excited-state decay processes were analyzed with both single wavelength and global fitting procedures. In solution, the S-1 lifetime of astaxanthin was found to be similar to 5 ps and independent of solvent. In alpha-crustacyanin, the lifetime was noticeably shorter at similar to 1.8 ps. Both fitting procedures led to the conclusion that the lifetime of the S-2 state was either comparable to or shorter than the instrument response time. The data support the idea that dimerization of astaxanthin in alpha-crustacyanin is the primary molecular basis for the bathochromic shift of the S-0 -> S-2 and S-1 - S-n transitions. Planarization of the astaxanthin molecule, which leads to a longer effective pi-electron conjugated chain and a lower S-1 energy, accounts for the shorter tau(1) in the protein.
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| 2. |
- Polivka, Tomas, et al.
(författare)
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Spectroscopic properties of the carotenoid 3 '-hydroxyechinenone in the orange carotenoid protein from the cyanobacterium Arthrospira maxima
- 2005
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Ingår i: Biochemistry. - : American Chemical Society (ACS). - 0006-2960 .- 1520-4995. ; 44:10, s. 3994-4003
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Tidskriftsartikel (refereegranskat)abstract
- The cyanobacterial water-soluble orange carotenoid binding protein (OCP) is an ideal system for study of the effects of protein environment on photophysical properties of carotenoids. It contains a single pigment, the carotenoid 3'-hydoxyechinenone (hECN). In this study, we focus on spectroscopic properties of hECN in solution and in the OCP, aiming to elucidate the spectroscopic effects of the carotenoid-protein interaction in the context of the function(s) of the OCP. The noncovalent binding of hECN to the OCP causes a conformational change in the hECN, leading to a prolongation of the effective conjugation length. This change is responsible for shortening of the S, lifetime from 6.5 ps in solution to 3.3 ps in the OCP. The conformational change and the hydrogen bonding via the carbonyl group of hECN result in stabilization of an intramolecular charge-transfer (ICT) state. No signs of the ICT state were found in hECN in solution, regardless of the solvent polarity; spectral bands in transient absorption spectra of OCP-bound hECN exhibit features typical for the ICT state. Application of global fitting analysis revealed further effects of binding hECN in the OCP. The S, state of hECN in the OCP decays with two time constants of 0.9 and 3.3 ps. Modeling of the excited-state processes suggests that these two components are due to two populations of hECN in the OCP that differ in the hydrogen bonding via the carbonyl group. These results support the hypothesis that the OCP functions as a photoprotective shield under excess light. Mechanistically, the broadening of the hECN absorption spectrum upon binding to OCP enhances filtering, effect of hECN. Furthermore, the binding-induced conformational change and activation of the ICT state that leads to a shortening of hECN lifetime effectively makes the protein-bound hECN a more effective energy dissipator.
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