Volume 130, Issue 6, 14 February 2009
Index of content:
130(2009); http://dx.doi.org/10.1063/1.3077008View Description Hide Description
We present fluorescence-detected measurements of the temperature-jump relaxation kinetics of the designed three-helix bundle protein taken under solvent conditions identical to previous infrared-detected kinetics. The fluorescence-detected rate is similar to the IR-detected rate only at the lowest temperature where we could measure it (326 K). The fluorescence-detected rate decreases by a factor of 3 over the 326–344 K temperature range, whereas the IR-detected rate remains nearly constant over the same range. To investigate this probe dependence, we tested an extensive set of physically reasonable one-dimensional (1D) free energysurfaces by Langevin dynamics simulation. The simulations included coordinate- and temperature-dependent roughness, diffusion coefficients, and IR/fluorescence spectroscopic signatures. None of these can reproduce the IR and fluorescence data simultaneously, forcing us to the conclusion that a 1D free energysurface cannot accurately describe the folding of . This supports the hypothesis that has a multidimensional free energysurface conducive to downhill folding at 326 K, and that it is already an incipient downhill folder with probe-dependent kinetics near its melting point.
130(2009); http://dx.doi.org/10.1063/1.3078267View Description Hide Description
We report on rate-dependent fractureenergymeasurements over three decades of steady crackvelocities in alginate and gelatin hydrogels. We evidence that irrespective of gel thermoreversibility, thermally activated “unzipping” of the noncovalent cross-link zones results in slow crack propagation, prevailing against the toughening effect of viscoussolvent drag during chain pull-out, which becomes efficient above a few . We extend a previous model [T. Baumberger et al., Nat. Mater.5, 552 (2006)] to account for both mechanisms and estimate the microscopic unzipping rates.
130(2009); http://dx.doi.org/10.1063/1.3078516View Description Hide Description
A new extension of the lattice cluster theory is developed to describe the influence of monomerstructure and local correlations on the free energy of strongly interacting and self-assemblingpolymer systems. This extension combines a systematic high dimension and high temperature expansion (that is appropriate for weakly interacting systems) with a direct treatment of strong interactions. The general theory is illustrated for a binary polymer blend whose two components contain “sticky” donor and acceptor groups, respectively. The free energy is determined as an explicit function of the donor-acceptor contact probabilities that depend, in turn, on the local structure and both the strong and weak interactions.
130(2009); http://dx.doi.org/10.1063/1.3079544View Description Hide Description
A principal weakness of the Herman–Kluk (HK) semiclassical approximation is its failure to provide a reliably accurate description of tunneling between different classically allowed regions. It was previously shown that semiclassical corrections significantly improve the HK treatment of tunneling for the particular case of the one-dimensional Eckart system. Calculations presented here demonstrate that the lowest-order correction also substantially improves the HK description of tunneling across barriers in two-dimensional systems. Numerical convergence issues either do not arise or are easily overcome, so that the calculations require only a moderate number of ordinary, real, classical trajectories.
130(2009); http://dx.doi.org/10.1063/1.3080809View Description Hide Description
Absolute total photodetachment cross sections of and anions stored in a multipole radio frequency trap have been measured using a novel laser depletion tomography method. For the total cross sections of and , measured at 662 and 632 nm, respectively, were found constant in the temperature range of 8–300 K. The cross sections and measured at 170 K at 662 and 532 nm, respectively, agree within error estimations with preceding experiments and increase the accuracy of the widely used calibration standard for relative photodetachment measurements of diverse atomic and molecular species.