Articles you may be interested in Low-frequency collective dynamics in deep eutectic solvents of acetamide and electrolytes: A femtosecond Raman-induced Kerr effect spectroscopic study

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.

The time and frequency resolved optical response of wild-type green fluorescent protein (wt-GFP) has been measured at room temperature following 30 fs, 400 nm photo-excitation. In the wavelength range covering the stationary fluorescence spectrum of the protein, the stimulated emission rises on a time scale of roughly 20 ps due to excited-state proton-transfer (ESPT). The rise can be described phenomenologically by a sum of two exponentials. A long-time isosbestic behavior on the blue edge of the stationary emission implies a barrier for ESPT which is significantly larger than thermal excitations. In addition, an instantaneous component to the stimulated emission appears within the time resolution of our experiment. This observation is indicative of nonvertical cross-well transitions that prepare the proton-transferred configuration of the excited state directly from the equilibrium geometry of the ground-state neutral species during photo-excitation. Finally, transient absorptions around 500 nm and 650 nm can be observed, which are attributed to transitions from different protonated forms of the excited-state of GFP to higher lying electronic configurations, S n . The entire optical response of GFP is quantitatively simulated using a dynamic model that includes: (i) an energy-dependent rate coefficient for ESPT, (ii) intra-and intermolecular transfer of excess vibrational energy (IVR and VET), and (iii) an additional non-radiative decay pathway for the initially prepared Franck-Condon state leading to internal conversion via motion along a torsional coordinate. In particular, the nonexponential nature of the ESPT originates from overlapping time scales of reactive and non-reactive elementary processes following optical excitation.

A high-quality depolarized Raman-spectrum is obtained in the frequency range 0 p o p 600 cm À1 by Fouriertransformation of time-resolved dual-color heterodyne-detected optical Kerr-effect data of liquid water at 0 C. The time-resolution was sufficient to fully capture the restricted translational and part of the hindered rotational region of the Raman spectrum. This low-temperature spectrum is used to test the applicability of stochastic line broadening theories. A conventional Kubo line shape analysis indicates that restricted translational modes involving hydrogen-bond bending and stretching motions are predominantly in the slow modulation limit at temperatures close to the melting point. However, a pronounced residual fine structure exists which cannot be fully accounted for by the theory in its standard form. Instead, we propose to apply a modified Kubo model based on truncating its continued-fraction representation at a finite order N including a convolution with a quasi-static structural inhomogeneity in the liquid. In particular, a quantitative agreement of our experimental data with such an inhomogeneous N-state random-jump model is interpreted with a discrete size distribution of aggregates which can interconvert on a time scale of about 500 fs by breaking and making of hydrogen bonds.

Analysis of antigen dissociation provides insight into peptide presentation modes of folded human leukocyte antigen (HLA) molecules, which consist of a heavy chain, beta2-microglobulin (beta2m), and an antigenic peptide. Here we have monitored peptide-HLA interactions and peptide dissociation kinetics of two HLA-B27 subtypes by fluorescence depolarization techniques. A single natural amino-acid substitution distinguishes the HLA-B*2705 subtype that is associated with the autoimmune disease ankylosing spondylitis from the non-disease-associated HLA-B*2709 subtype. Peptides with C-terminal Arg or Lys represent 27% of the natural B*2705 ligands. Our results show that dissociation of a model peptide with a C-terminal Lys (GRFAAAIAK) follows a two-step mechanism. Final peptide release occurs in the second step for both HLA-B27 subtypes. However, thermodynamics and kinetics of peptide-HLA interactions reveal different molecular mechanisms underlying the first step, as indicated by different activation energies of 95+/-8 kJ/mol (HLA-B*2705) and 150+/-10 kJ/mol (HLA-B*2709). In HLA-B*2709, partial peptide dissociation probably precedes fast final peptide release, while in HLA-B*2705 an allosteric mechanism based on long-range interactions between beta2m and the peptide binding groove controls the first step. The resulting peptide presentation mode lasts for days at physiological temperature, and determines the peptide-HLA-B*2705 conformation, which is recognized by cellular ligands such as T-cell receptors.