Biomedical subjects

Thomas Lenzer

Publications and source records attributed to Thomas Lenzer.

7 recordsLinked to original sources

PubMed2006

Solvent effects on the S0(1(1)Ag-) --> S2(1(1)Bu+) transition of beta-carotene, echinenone, canthaxanthin, and astaxanthin in supercritical CO2 and CF3H.

Solvent-induced spectral shifts of the four C40 carotenoids, beta-carotene, echinenone, canthaxantin, and astaxanthin, have been studied in supercritical CO2 and CF3H. In situ absorption spectroscopic analysis was used to determine the maximum peak position of the electronic transitions from the ground state (1(1)Ag-) to the S2 state (1(1)Bu+) of the carotenoids. The medium polarizability function, R(n) = (n2 - 1)/(n2 + 2) of the refractive index of the solvent was varied over the range R(n) = 0.08-0.14, by changing the pressure of CO2 or CF3H between 90 and 300 bar at the temperature 308 K. For all the carotenoids studied here, a significant hypsochromic shift of ca. 20-30 nm was observed in supercritical fluids as compared to that in nonpolar liquids. The spectral shifts in supercritical fluids were compared with those in liquids and showed a clear linear dependence on the medium polarizability. The temperature-dependent shift of the absorption maxima was less significant. Interestingly, there was almost no difference in the energetic position of the absorption maxima in supercritical CO2 and CF3H at a given R(n) value. This is in contrast to previous extrapolations from studies in liquids at larger R(n) values, which yielded different slopes of the R(n)-dependent spectral shifts for polar and nonpolar solvents toward the gas-phase limit of R(n) = 0. The current experimental results in the gas-to-liquid range show that the polarity of the solvent has only a minor influence on the 1(1)Ag- --> 1(1)Bu+ transition energy in the region of low R(n). We also obtain more reliable extrapolations of this 0-0 transition energy to the gas-phase limit nu(0-0)(gas-phase) approximately (23,000 +/- 120) cm(-1) for beta-carotene.

Canthaxanthin↗

PubMed2006

Extremely strong solvent dependence of the S1--> S0 internal conversion lifetime of 12'-apo-beta-caroten-12'-al.

The ultrafast internal conversion (IC) dynamics of the carbonyl carotenoid 12'-apo-beta-caroten-12'-al has been investigated in solvents of varying polarity using time-resolved femtosecond transient absorption spectroscopy. The molecules were excited to the S(2) state by a pump beam of either 390 or 470 nm. The subsequent intramolecular dynamics were detected at several probe wavelengths covering the S(0)--> S(2) and S(1)--> S(n) bands. For the S(1)--> S(0) internal conversion process, a remarkably strong acceleration with increasing polarity was found, e.g., lifetimes of tau(1) = 220 ps (n-hexane), 91 ps (tetrahydrofuran) and 8.0 ps (methanol) after excitation at 390 nm. The observation can be rationalized by the formation of a combined S(1)/ICT (intramolecular charge transfer) state in the more polar solvents. The effect is even stronger than the strongest one reported so far in the literature for peridinin. Addition of lithium salts to a solution of 12'-apo-beta-caroten-12'-al in ethanol leads only to small changes of the IC time constant tau(1). In addition, we estimate an upper limit for the time constant tau(2) of the S(2)--> S(1) internal conversion process of 300 fs in all solvents.

Solvents↗

PubMed2006

Transient lens spectroscopy of ultrafast internal conversion processes in citranaxanthin.

The ultrafast internal conversion (IC) dynamics of the apocarotenoid citranaxanthin have been studied for the first time by means of two-color transient lens (TL) pump-probe spectroscopy. After excitation into the high-energy edge of the S2 band by a pump pulse at 400 nm, the subsequent intramolecular processes were probed at 800 nm. Experiments were performed in a variety of solvents at room temperature. Upper limits for the S2 lifetime tau2 on the order of 100-200 fs are estimated. The S1 lifetime tau1 varies only slightly between solvents (10-12 ps), and the only clear decrease is observed for methanol (8.5 ps). The findings are consistent with earlier results from transient absorption studies of other apocarotenoids and carotenoid ketones and transient lens experiments of C40 carbonyl carotenoids. Possible reasons for the observed weak solvent dependence of tau1 for citranaxanthin are discussed.

Carotenoids↗

PubMed2006

Kinetically controlled selective ionization study on the efficient collisional energy transfer in the deactivation of highly vibrationally excited trans-stilbene.

Direct measurements of the gas-phase collisional energy transfer parameters are reported for the deactivation of highly vibrationally excited trans-stilbene molecules, initially prepared with an average energy of about 40 000 cm(-1), in the bath gases argon, CO2, and n-heptane. The method of kinetically controlled selective ionization (KCSI) has been used. Complete experimental collisional transition probability density functions P(E',E) are determined, which are represented by a monoexponential form with a parametric exponent in the argument, P(E',E) proportional to exp[-{(E - E')/(C0 + C1E)}Y] (for downward collisions), well established from earlier KCSI studies. A comparison of the first moments of energy transfer rate constants, kE,1, or of resulting first moments of energy transfer, , for trans-stilbene with those for azulene and toluene clearly shows the considerably more efficient deactivation of trans-stilbene for all bath gases, presumably due to the much greater number of very low-frequency modes of trans-stilbene. However, on a relative scale this gain in deactivation rate of excited trans-stilbene is clearly collider dependent and decreases distinctly with the growing collision efficiency of the larger bath gas molecules.

Journal Article↗

PubMed2005

Gas-phase collisional relaxation of the CH2I radical after UV photolysis of CH2I2.

Transient UV absorption spectra and kinetics of the CH(2)I radical in the gas phase have been investigated at 313 K. Following laser photolysis of 1-3 mbar CH(2)I(2) at 308 nm, transient spectra in the wavelength range 330-390 nm were measured at delay times between 60 ns and a few microseconds. The change of the absorption spectra at early times was attributed to vibrational cooling of highly excited CH(2)I radicals by collisional energy transfer to CH(2)I(2) molecules. From transient absorption decays measured at specific wavelengths, time-dependent concentrations of vibrationally "hot" and "cold" CH(2)I and CH(2)I(2) were extracted by kinetic modeling. In addition, the transient absorption spectrum of CH(2)I radicals between 330 and 400 nm was reconstructed from the simulated concentration-time profiles. The evolution of the absorption spectra of CH(2)I radicals and CH(2)I(2) due to collisional energy transfer was simulated in the framework of a modified Sulzer-Wieland model. Additional master equation simulations for the collisional deactivation of CH(2)I by CH(2)I(2) yield DeltaE values in reasonable agreement with earlier direct studies on the collisional relaxation of other systems. In addition, the simulations show that the shape of the vibrational population distribution of the hot CH(2)I radicals has no influence on the measured UV absorption signals. The implications of our results with respect to spectral assignments in recent ultrafast spectrokinetic studies of the photolysis of CH(2)I(2) in dense fluids are discussed.

Journal Article↗

PubMed2005

Ultrafast transient lens spectroscopy of various C40 carotenoids: lycopene, beta-carotene, (3R,3'R)-zeaxanthin, (3R,3'R,6'R)-lutein, echinenone, canthaxanthin, and astaxanthin.

The ultrafast internal conversion (IC) dynamics of seven C(40) carotenoids have been investigated at room temperature in a variety of solvents using two-color transient lens (TL) pump-probe spectroscopy. We provide comprehensive data sets for the carbonyl carotenoids canthaxanthin, astaxanthin, and-for the first time-echinenone, as well as new data for lycopene, beta-carotene, (3R,3'R)-zeaxanthin and (3R,3'R,6'R)-lutein in solvents which have not yet been investigated in the literature. Measurements were carried out to determine, how the IC processes are influenced by the conjugation length of the carotenoids, additional substituents and the polarity of the solvent. TL signals were recorded at 800 nm following excitation into the high energy edge of the carotenoid S2 band at 400 nm. For the S2 lifetime solvent-independent upper limits on the order of 100-200 fs are estimated for all carotenoids studied. The S1 lifetimes are in the picosecond range and increase systematically with decreasing conjugation length. For instance, in the sequence canthaxanthin/echinenone/beta-carotene (13/12/11 double bonds) one finds tau1 approximately 5, 7.7 and 9 ps for the S1-->S0 IC process, respectively. Hydroxyl groups not attached to the conjugated system have no apparent influence on tau1, as observed for canthaxanthin/astaxanthin (tau1 approximately 5 ps in both cases). For all carotenoids studied, tau1 is found to be insensitive to the solvent polarity. This is particularly interesting in the case of echinenone, canthaxanthin and astaxanthin, because earlier measurements for other carbonyl carotenoids like, e.g., peridinin partly showed dramatic differences. The likely presence of an intramolecular charge transfer state in the excited state manifold of C40 carbonyl carotenoids, which is stabilized in polar solvents, has obviously no influence on the measured tau1.

Antioxidants↗

PubMed2005

PECT model analysis and predictions of experimental collisional energy transfer probabilities P(E',E) and moments for azulene and biphenylene.

Experimental collisional energy transfer data from kinetically controlled selective ionization (KCSI) and ultraviolet absorption (UVA) experiments are analyzed in the framework of the partially ergodic collision theory (PECT). Collisions of azulene and biphenylene with different colliders are investigated as case studies. The downward wings of the P(E',E) energy transfer distributions obtained from the PECT model are fitted to the recently introduced "variable-shape"-exponential 3-parameter functional form of P(E',E) obtained from KCSI experiments, P(E',E) proportional, variant exp[-{(E - E')/(C(0) + C(1)E)}(Y)]. The PECT model is able to reproduce the characteristic dependence of the KCSI "shape parameter" Y on the choice of collider, the energy dependent width of the KCSI P(E',E) distributions, described by alpha(E) = C(0) + C(1)E, and the temperature dependence of the UVA data above room temperature. The statistical approach of PECT obviously captures the essence of large molecule energy transfer at chemically significant energies without the need of knowing specific features of the detailed collision dynamics. It therefore shows promise for predicting the shape of P(E',E) in master equation kernels for larger molecules.

Journal Article↗