Online photolytic optical gating of caged fluorophores in capillary zone electrophoresis utilizing an ultraviolet light‐emitting diode

Photolytic optical gating (POG) facilitates rapid, on‐line and highly sensitive analyses, though POG utilizes UV lasers for sample injection. We present a low‐cost, more portable alternative, employing an ultraviolet light‐emitting diode (UV‐LED) array to inject caged fluorescent dyes via photolysis. Utilizing the UV‐LED array, labeled amino acids were injected with nanomolar limits of detection (270 ± 30 nM and 250 ± 30 nM for arginine and citrulline, respectively). When normalized for the difference in light intensity, the UV‐LED array provides comparable sensitivity to POG utilizing UV lasers. Additionally, the UV‐LED array yielded sufficient beam quality and stability to facilitate coupling with a Hadamard transform, resulting in increased sensitivity. This work shows, for the first time, the use of an UV‐LED for online POG with comparable sensitivity to conventional laser sources but at a lower cost.     

Nanofluidic tuning of photonic crystal circuits

By integrating soft-lithography-based nanofluidics with silicon nanophotonics, we demonstrate dynamic, liquid-based addressing and high deltan/n (approximately 0.1) refractive index modulation of individual features within photonic structures at subwavelength length scales. We show ultracompact tunable spectral filtering through nanofluidic targeting of a single row of holes within a planar photonic crystal. We accomplished this with an optofluidic integration architecture comprising a nanophotonic layer, a nanofluidic delivery structure, and a microfluidic control engine. Variants of this technique could enable dynamic reconfiguration of photonic circuits, selective introduction of optical nonlinearities, or delivery of single molecules into resonant cavities for biodetection.     

A retro-Claisen approach to dolabriferol

[reaction: see text] The protected precursor 30 to dolabriferol was generated by a DBU-induced, ester-forming, retro-Claisen process. The required linear carbon chain present in 22 was synthesized by a stereoselective lithium aldol reaction. The necessary aldehyde and ketone fragments were synthesized using stereocontrolled aldol reactions with the ethyl (S)-lactate derived ketone 13.     

Effect of single-point sequence alterations on the aggregation propensity of a model protein

Sequences of contemporary proteins are believed to have evolved through a process that optimized their overall fitness, including their resistance to deleterious aggregation. Biotechnological processing may expose therapeutic proteins to conditions that are much more conducive to aggregation than those encountered in a cellular environment. An important task of protein engineering is to identify alternative sequences that would protect proteins when processed at high concentrations without altering their native structure associated with specific biological function. Our computational studies exploit parallel tempering simulations of coarse-grained model proteins to demonstrate that isolated amino acid residue substitutions can result in significant changes in the aggregation resistance of the protein in a crowded environment while retaining protein structure in isolation. A thermodynamic analysis of protein clusters subject to competing processes of folding and association shows that moderate mutations can produce effects similar to those caused by changes in system conditions, including temperature, concentration, and solvent composition, that affect the aggregation propensity. The range of conditions where a protein can resist aggregation can therefore be tuned by sequence alterations, although the protein generally may retain its generic ability for aggregation.     

A coherent state approach to semiclassical nonadiabatic dynamics

A semiclassical (SC) approximation to the quantum mechanical propagator for nonadiabatic systems is derived. Our derivation starts with an exact path integral expression that uses canonical coherent states for the nuclear degrees of freedom and spin coherent states for the electronic degrees of freedom. A stationary path approximation (SPA) is then applied to the path integral to obtain the SC approximation. The SPA results in complex classical trajectories of both nuclear and electronic degrees of freedom and a double ended boundary condition. The root search problem is solved using the previously proposed "real trajectory local search" algorithm. The SC approximation is tested on three simple one dimensional two-state systems proposed by Tully [J. Chem. Phys. 93, 1061 (1990)], and the SC results are compared to Ehrenfest and surface hopping predictions. Excellent agreement with quantum results is reached when the SC trajectory is far away from caustics. We discuss the origin of caustics in this SC formalism and the strengths and weaknesses of this approach.     

Reversal of diastereofacial selectivity in hydride reductions of N-tert-butanesulfinyl imines

A variety of N-tert-butanesulfinyl imines were reduced with NaBH4 in THF containing 2% water to provide the corresponding secondary sulfinamides in high yield and diastereoselectivity. By using the same sulfinyl imine starting materials and changing the reductant to L-Selectride, the stereoselectivity could be efficiently reversed to afford the opposite product diastereomer in high yield and selectivity.     

Low temperature properties of the Fermi–Dirac,Boltzmann and Bose–Einstein equations

We investigate low temperature (T  ) properties of three classical quantum statistics models: (I) the Fermi–Dirac equation, (II) the Boltzmann equation, and (III) the Bose–Einstein equation. It is widely assumed that each of these equations is valid for all T>0$T>0$. For each equation we prove that this assumption leads to erroneous predictions as T→0⁺$T\to 0^{+}$. Our approach to correct these errors gives new low temperature predictions which contradict previous theory. We examine a two-state paramagnetism system and show how our new low temperature prediction compares favorably with experimental data.