TY  - JOUR
AB  - Broadband transient absorption spectroscopy is used to study the photoisomerization of stiffened stilbenes in solution, specifically E/Z mixtures of bis(benzocyclobutylidene) (t4, c4) and (E)-1-(2,2-dimethyltetralinylidene)-2-2-dimethyltetraline (t6). Upon excitation to S 1 , all evolve to perpendicular molecular conformation P, followed by decay to S 0 , while the spectra and the kinetic behavior crucially depend on the size of the stiffening ring. In 4, contrary to all previously studied stilbenes, the trans and cis absorption and excited-state spectra are nearly indistinguishable, while the corresponding isomerization times are comparable: τ i = 166 ps for t4 and τ i = 64 ps for c4 in n-hexane, as opposed to 114 and 45 ps in acetonitrile, respectively. Faster isomerization in polar solvents agrees with the zwitterionic character of the P state. In t6, torsion to P is effectively barrier-less and completes within 0.3 ps, the S 1 → P evolution being directly traceable through the transient spectra of stimulated emission and that of excited-state absorption. In n-hexane, the P state is remarkably long-lived, τ P = 1840 ps, but the lifetime drops down to 35 ps in acetonitrile. The trans-to-cis photoisomerization yield for t6 is measured to be 20%, while for t4, it remains uncertain. We discuss the effects of stiffening and substitution on the formation and lifetime of the intermediate states through which the stilbene molecules evolve on the S 1 energy surface.
AU  - Krohn, O.A.*
AU  - Quick, M.*
AU  - Ioffe, I.N.*
AU  - Mazaleva, O.N.*
AU  - Lenoir, D.
AU  - Detert, H.*
AU  - Kovalenko, S.A.*
C1  - 56142
C2  - 46839
SP  - 4291-4300
TI  - Time-resolved photochemistry of stiffened stilbenes.
JO  - J. Phys. Chem. B
VL  - 123
IS  - 19
PY  - 2019
SN  - 1520-6106
ER  - 

TY  - JOUR
AB  - Ionic liquids are often considered as green alternatives of volatile organic solvents. The thermal behavior of the ionic liquids is relevant for a number of emerging large-scale applications at elevated temperature. Knowledge about the degradation products is indispensable for treatment and recycling of the used ionic liquids. The objective of this paper was an investigation of the short- and long-term stability of several 1-alkyl-3-methylimidazolium halides, determination of the degradation products, and the elucidation of their decomposition patterns and structure-stability relations. Short-term stability and mechanism of thermal degradation were investigated by a self-developed, innovative thermal analysis single-photon ionization timeof-flight mass spectrometry device with Skimmer coupling. The applied technology provides real-time monitoring of the forming species and allows tracing their change during the course of the decomposition. Therein, the almost fragment-free soft ionization with vacuum ultraviolet photons plays a crucial role. We have detected unfragmented molecules whose formation was only assumed by electron ionization. Nevertheless, the main decomposition products of the selected ionic liquids were alkyl imidazoles, alkenes, alkyl halides, and hydrogen halides. From the decomposition products, we have deduced the fragmentation patterns and discussed their interrelation with the length of the alkyl chain and the type of the halide anion. Our results did not suggest the evaporation of the investigated ionic liquids prior to their decomposition under atmospheric conditions. Long-term thermal stability and applicability were determined based on thermogravimetric analysis evaluated with a kinetic model. Thus, the time-dependent maximum operation temperature (MOT) for the respective ionic liquids has been calculated. As a rule, the short-term stability overestimates the long-term decomposition temperatures; the calculated MOT are significantly lower (at least 100 K) than the standardly obtained decomposition temperatures.
AU  - Efimova, A.*
AU  - Varga, J.
AU  - Matuschek, G.
AU  - Saraji-Bozorgzad, M.R.*
AU  - Denner, T.*
AU  - Zimmermann, R.
AU  - Schmidt, P.*
C1  - 54374
C2  - 45536
CY  - 1155 16th St, Nw, Washington, Dc 20036 Usa
SP  - 8738-8749
TI  - Thermal resilience of imidazolium-based ionic liquids-studies on short- and long-term thermal stability and decomposition mechanism of 1-Alkyl-3-methylimidazolium halides by thermal analysis and single-photon ionization time-of-flight mass spectrometry.
JO  - J. Phys. Chem. B
VL  - 122
IS  - 37
PB  - Amer Chemical Soc
PY  - 2018
SN  - 1520-6106
ER  - 

TY  - JOUR
AB  - The measurement of dipolar couplings among directly bonded nuclei yields direct information on the amplitude of dynamic processes in the solid-state. For a reliable motional analysis using, e.g., the model-free approach, a correct quantification of the absolute values of these order parameters is absolutely essential. In the absence of a reference value for the rigid limit, too low dipolar coupling values might be misinterpreted as motion. Therefore, a detailed understanding of the effects that influence the quantification of the experimental order parameters is necessary. We compare here REDOR and CPPI derived order parameters assessed in 1 H-detected experiments, and discuss the influence of remote protons and rf inhomogeneity on the extracted dipolar coupling constant for MAS rotation frequencies in the range 20-100 kHz. Experimental results are furthermore compared with the order parameter obtained from a molecular dynamics simulation. We find that fast magic-Angle spinning up to 100 kHz can yield artifact-free REDOR based 1 H, 15 N order parameters for perdeuterated and 100% amide back-exchanged proteins, and potentially even in uniformly protonated samples. We believe that awareness of systematic errors introduced by the measurement and in the analysis of order parameters will yield a better understanding of the dynamic properties of a protein derived from solid-state NMR observables.
AU  - Asami, S.*
AU  - Reif, B.
C1  - 52008
C2  - 43646
CY  - Washington
SP  - 8719-8730
TI  - Comparative study of REDOR and CPPI derived order parameters by 1H-detected MAS NMR and MD simulations.
JO  - J. Phys. Chem. B
VL  - 121
IS  - 37
PB  - Amer Chemical Soc
PY  - 2017
SN  - 1520-6106
ER  - 

TY  - JOUR
AB  - Photoinduced isomerization of 1,1′-bis-indanyliden (stiff-stilbene) in solution was studied with broadband transient absorption and femtosecond Raman spectroscopies, and by quantum-chemical calculations. Trans-to-cis S 1 isomerization proceeds over a 600 and 400 cm-1 barrier in n-hexane and acetonitrile, respectively. The reaction develops on multiple time scales with fast (0.3-0.4 ps) viscosity-independent and slower (2-26 ps) viscosity-dependent components. In the course of intramolecular torsion (which should be the main reaction coordinate) some excited molecules pass through the perpendicular conformation P and reach the cis geometry, to be temporarily trapped there. Subsequently they relax back to P and further to the ground state S0. The cis-to-trans isomerization reveals ultrafast (0.06 ps) oscillatory relaxation followed by 13 ps decay in n-hexane and 2 ps decay in acetonitrile, corresponding to barriers of 800 and 400 cm-1, respectively. Raman S0 and S1 spectra are reported and discussed. The perpendicular conformation P was not detected, possibly due to its low oscillator strength and short lifetime, or because of strong overlap with hot product spectra. XMCQDPT2 calculations locate a stationary S 1 point on the cis side and two perpendicular-pyramidalized stationary points, to be reached from the former over 300 and 680 cm -1 barrier. Implications for parent stilbene are discussed; in this case we also see evidence for the trans-to-cis adiabatic path, as in stiff-stilbene. Very similar viscosity dependence for the two compounds supports the common isomerization pathway: torsion about the central double bond.
AU  - Quick, M.*
AU  - Berndt, F.*
AU  - Dobryakov, A.L.*
AU  - Ioffe, I.N.*
AU  - Granovsky, A.A.*
AU  - Knie, C.*
AU  - Mahrwald, R.*
AU  - Lenoir, D.
AU  - Ernsting, N.P.*
AU  - Kovalenko, S.A.*
C1  - 30626
C2  - 33762
SP  - 1389-1402
TI  - Photoisomerization dynamics of stiff-stilbene in solution.
JO  - J. Phys. Chem. B
VL  - 118
IS  - 5
PY  - 2014
SN  - 1520-6106
ER  - 

TY  - JOUR
AB  - The multidomain ser/thr kinase "target of rapamycin" (TOR) centrally controls eukaryotic growth and metabolism. The C-terminal FATC domain is important for TOR regulation and was suggested to directly mediate TOR-membrane interactions. Here, we present a detailed characterization of the membrane immersion properties of the oxidized and reduced yeast TOR1 FATC domain (2438-2470 = y1fatc). The immersion depth was characterized by NMR-monitored interaction studies with DPC micelles containing paramagnetically tagged 5-or 16-doxyl stearic acid (5-/16-SASL) and by analyzing the paramagnetic relaxation enhancement (PRE) from Mn2+ in the solvent. Complementary MD-simulations of micellar systems in the absence and presence of protein showed that 5-/16-SASL can move in the micelle and that 16-SASL can bend such that the doxyl group is close to the headgroup region and not deep in the interior as commonly assumed. Based on oriented CD (OCD) data, the single α-helix of oxidized/reduced y1fatc has an angle to the membrane normal of ∼30-60°/ ∼ 35-65°in neutral and ∼5-35°/∼0-30°in negatively charged bilayers. The presented experimentally well-founded models help to better understand how this redox-sensitive peripheral membrane anchor may be part of a network of protein-protein and protein-membrane interactions regulating TOR localization at different cellular membranes. Moreover, the presented work provides a good methodological reference for the structural characterization of other peripherally membrane associating proteins.
AU  - Sommer, L.A.*
AU  - Janke, J.*
AU  - Bennett, W.F.*
AU  - Bürck, J.*
AU  - Ulrich, A.S.*
AU  - Tieleman, D.P.*
AU  - Dames, S.A.
C1  - 31355
C2  - 34749
CY  - Washington
SP  - 4817-4831
TI  - Characterization of the immersion properties of the peripheral membrane anchor of the FATC domain of the kinase "Target of rapamycin" by NMR, oriented CD spectroscopy, and MD simulations.
JO  - J. Phys. Chem. B
VL  - 118
IS  - 18
PB  - Amer Chemical Soc
PY  - 2014
SN  - 1520-6106
ER  - 

TY  - JOUR
AB  - The space accessed by a folding macromolecule is vast, and how to best project computer simulations of protein folding trajectories into an interpretable sequence of discrete states is an open research problem. There are numerous alternative ways of associating individual configurations into collective states, and in deciding on the number of such clustered states there is a trade-off between human interpretability (smaller number of states) and accuracy of representation (larger number of states). Here we introduce a trajectory likelihood measure for assessing alternative discrete state models of protein folding. We find that widely used rmsd-based clustering methods require large numbers of initial states and a second agglomeration step based on kinetic connectivity to produce models with high predictive power; this is the approach taken in elegant recent work with Markov State Models of protein folding. In contrast, we find that grouping of states based on secondary structure pairings or contact maps, when refined with K-means clustering, yields higher likelihood models with many fewer states. Using the most predictive contact map representation to study the folding transitions of the WW domain in very long molecular dynamics simulations, we identify new states and transitions. The methods should be generally useful for investigating the structural transitions in protein folding simulations for larger proteins.
AU  - Kellogg, E.H.*
AU  - Lange, O.F.
AU  - Baker, D.*
C1  - 10626
C2  - 30348
SP  - 11405-11413
TI  - Evaluation and optimization of discrete state models of protein folding.
JO  - J. Phys. Chem. B
VL  - 116
IS  - 37
PB  - American Chemical Society
PY  - 2012
SN  - 1520-6106
ER  -