Femtosecond Time-Resolved Absorption Spectroscopy of Astaxanthin in Solution and in α-Crustacyanin

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 S0 --> S2 and S1 --> 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 S1 lifetime of astaxanthin was found to be approximately 5 ps and independent of solvent. In alpha-crustacyanin, the lifetime was noticeably shorter at approximately 1.8 ps. Both fitting procedures led to the conclusion that the lifetime of the S2 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 S0 --> S2 and S1 --> S(n) transitions. Planarization of the astaxanthin molecule, which leads to a longer effective pi-electron conjugated chain and a lower S1 energy, accounts for the shorter tau1 in the protein.

Spectrally resolved femtosecond photon echo spectroscopy of astaxanthin

“…However, for Astaxanthin in acetonitrile, we find the S 1 and S 2 timescales are about 5 ps and 180 fs respectively. This is in contrast to the earlier analysis of the dynamics of the excited-state decay processes which indicated that the S 1 and S 2 lifetimes of Astaxanthin are independent of solvent [13].…”

Self-assembly and energy transfer in artificial light-harvesting complexes of bacteriochlorophyll c with astaxanthin

“…A peak at similar position, although with a significantly shorter lifetime (1.9 ps) was observed for astaxanthin dissolved in carbon disulfide (Ilagan et al 2005) and it was suggested that the signal is due to either a S* state or excited state solvation. Though we observed much longer lifetime, it is worth noting that S* lifetime of 7 ps was measured in a water-soluble analog of astaxanthin, astalysine ,…”

“…Such a lifetime and spectral profile are typical for vibrational relaxation within the S 1 state of carotenoids. For instance, the 0.6 ps component was attributed to vibrational relaxation in the S 1 state of astaxanthin bound to α-crustacyanin (Ilagan et al 2005). This and all subsequent components have also a minor contribution from the PB/SE of the BChl c Q y band.…”

“…Support for the suggested energy transfer pathway in the BChl c assemblies with astaxanthin can be found from the comparison of our transient data with some results derived from measurements of the S1 and S2 states lifetimes of astaxanthin in several solutions. Ilagan et al (2005) determined the S 1 state lifetime of astaxanthin in methanol, acetonitrile and carbon disulfide to be ~5ps, and the lifetime of S 2 state to be between 105 fs (methanol) and 165 fs (acetonitrile). Kopczynski et al (2005) also reported a lifetime of ~5ps for the S 1 state of astaxanthin in various solutions ranging from toluene to methanol and a lifetime of ≤120fs for S 2 state of astaxanthin in acetone and dimethyl sulfoxide.…”

Computational Studies of Photobiological Keto-Enol Reactions and Chromophores

“…While AXT has a UV-vis absorption maximum (λ max ) around 630 nm in α-crustacyanin (580 − 590 nm in β-crustacyanins), the free chromophore in solution absorbs in the 475 − 500 nm region, giving the well-known orange-red color of cooked lobster. [81][82][83] Thus, the protein environment induces a > 0.5 eV bathochromic shift of the absorption by AXT, which is one of the largest proteininduced spectral shifts recorded in nature.…”