Strong field coherent control of molecular torsions--Analytical models.

We introduce analytical models of torsional alignment by moderately intense laser pulses that are applicable to the limiting cases of the torsional barrier heights. Using these models, we explore in detail the role that the laser intensity and pulse duration play in coherent torsional dynamics, addressing both experimental and theoretical concerns. Our results suggest strategies for minimizing the risk of off-resonant ionization, noting the qualitative differences between the case of torsional alignment subject to a field-free torsional barrier and that of torsional alignment of a barrier-less system (equivalent to a 2D rigid rotor). We also investigate several interesting torsional phenomena, including the onset of impulsive alignment of torsions, field-driven oscillations in quantum number space, and the disappearance of an alignment upper bound observed for a rigid rotor in the impulsive torsional alignment limit.

Enhanced T-cell signaling in cells bearing linker for activation of T-cell (LAT) molecules resistant to ubiquitylation.

Linker for activation of T cells (LAT) plays a central role in T-cell activation by nucleating signaling complexes that are critical for the propagation of T-cell signals from the plasma membrane to the cellular interior. The role of phosphorylation and palmitoylation in LAT function has been well studied, but not much is known about other strategies by which the cell modulates LAT activity. We have focused on LAT ubiquitylation and have mapped the sites on which LAT is ubiquitylated. To elucidate the biological role of this process, we substituted LAT lysines with arginines. This resulted in a dramatic decrease in overall LAT ubiquitylation. Ubiquitylation-resistant mutants of LAT were internalized at rates comparable to wild-type LAT in a mechanism that required Cbl family proteins. However, these mutants displayed a defect in protein turnover rates. T-cell signaling was elevated in cells reconstituted with LAT mutants resistant to ubiquitylation, indicating that inhibition of LAT ubiquitylation enhances T-cell potency. These results support LAT ubiquitylation as a molecular checkpoint for attenuation of T-cell signaling.

Laser-driven torsional coherences.

We discuss several interesting phenomena in the dynamics of strong field-triggered torsional wavepackets, which carry implications for the problem of torsional alignment in nonrigid molecules. Our results point to the origin and consequences of the fundamental differences between rotational and torsional coherences. In addition, we provide design guidelines for torsional control experiments by illustrating the role played by the laser intensity, pulse width, temperature, and molecular parameters. Specifically, as an example of several classes of molecules expected to make suitable candidates for laboratory experiments, we explore the torsional control of 9-[2-(anthracen-9-yl)ethynyl]anthracene and contrast it with that of biphenyl. Finally, we propose several potential applications for coherent torsional control in chemistry, physics, and material science.

Surface morphology and phase transitions in mixed NaCl/MgSO4 aerosol particles.

Probe molecule spectroscopy characterizes the surface environment of mixed NaCl/MgSO(4) (0.01-50 wt % MgSO(4)) aerosol particles as a model for marine aerosol. Two complementary measurements, the probe's excited state spectroscopy and photoionization efficiency, measure the electronic properties of the particle surface and monitor phase changes that are driven by changes in relative humidity (RH). The results illustrate that over a wide range of composition, these particles have a layered structure with NaCl in the core and primarily hydrated MgSO(4) at the surface. Modeling the spectroscopic data reveals that the surface layer is not a uniform shell and that the coumarin 314 probe molecules partition selectively to the MgSO(4) domains. The surface layer has a pi* value of 1.7, indicative of a very high interfacial polarity. In cases where MgSO(4) is a minor component (< or = 10 wt %), the NaCl component crystallizes at 44% RH, consistent with the single salt NaCl result. Deliquescence-mode experiments with these particles show that the MgSO(4) component forms a solution at 42% RH, prior to the full deliquescence of the particle. For mixed particles with 50 wt % MgSO(4), the crystallization of NaCl occurs at 35% RH, and the predeliquescence of MgSO(4) occurs at 38% RH owing to the contribution of MgCl(2) in the surface layer. A model surfactant, SDS, slightly lowers the RH of the NaCl formation to approximately 42% and leads to the formation of a thin soap film that persists to low values of RH.