Dynamic Nuclear Polarization in the solid state: a transition between the cross effect and the solid effect

Proton Dynamic Nuclear Polarization (DNP) experiments were conducted on a 3.4 T homebuilt hybrid pulsed-EPR-NMR spectrometer, on static samples containing 10 mM or 40 mM TEMPOL in frozen glassy solutions of DMSO/water. During DNP experiments proton-NMR signals are enhanced with the help of microwave (MW) irradiation on or close to the Electron Paramagnetic Resonance (EPR) spectrum of the free radicals in the sample, transferring polarization from the free electrons to the nuclei. In the solid state a distinction is made between three DNP enhancement mechanisms: the Solid Effect (SE), the Cross Effect (CE) and Thermal Mixing (TM). In an effort to determine the dominant DNP mechanisms responsible for the enhancement of the nuclear signals, electron and nuclear spin-lattice relaxation rates, enhancement buildup times and microwave (MW) swept DNP spectra were measured as a function of temperature and MW irradiation strength. We observed lineshape variations of the DNP spectra that indicated changes in the relative contributions of SE-DNP and CE-DNP with temperature and MW power. Using a theoretical model describing the SE-DNP and CE-DNP the DNP spectra could be analyzed without involving the TM-DNP mechanism and the relative SE-DNP and CE-DNP contributions to the nuclear enhancement could be determined. From this analysis it follows that lowering the temperature beyond 20 K increases the SE-DNP and decreases the CE-DNP contributions. Possible explanations for this behavior are suggested.     

Slow Unfolded-State Structuring in Acyl-CoA Binding Protein Folding Revealed by Simulation and Experiment

Protein folding is a fundamental process in biology, key to understanding many human diseases. Experimentally, proteins often appear to fold via simple two- or three-state mechanisms involving mainly native-state interactions, yet recent network models built from atomistic simulations of small proteins suggest the existence of many possible metastable states and folding pathways. We reconcile these two pictures in a combined experimental and simulation study of acyl-coenzyme A binding protein (ACBP), a two-state folder (folding time ～10 ms) exhibiting residual unfolded-state structure, and a putative early folding intermediate. Using single-molecule FRET in conjunction with side-chain mutagenesis, we first demonstrate that the denatured state of ACBP at near-zero denaturant is unusually compact and enriched in long-range structure that can be perturbed by discrete hydrophobic core mutations. We then employ ultrafast laminar-flow mixing experiments to study the folding kinetics of ACBP on the microsecond time scale. These studies, along with Trp-Cys quenching measurements of unfolded-state dynamics, suggest that unfolded-state structure forms on a surprisingly slow (～100 μs) time scale, and that sequence mutations strikingly perturb both time-resolved and equilibrium smFRET measurements in a similar way. A Markov state model (MSM) of the ACBP folding reaction, constructed from over 30 ms of molecular dynamics trajectory data, predicts a complex network of metastable stables, residual unfolded-state structure, and kinetics consistent with experiment but no well-defined intermediate preceding the main folding barrier. Taken together, these experimental and simulation results suggest that the previously characterized fast kinetic phase is not due to formation of a barrier-limited intermediate but rather to a more heterogeneous and slow acquisition of unfolded-state structure.     

Reactivity of long conjugated systems: selectivity of Diels-Alder cycloaddition in oligofurans

Taking advantage of the synthetic availability and solubility of long oligofurans, their reactivity toward dienophiles was studied as a model for the rarely investigated reactivity of long conjugated systems. Unlike oligoacenes, the reactivity of oligofurans decreases or remains constant with increasing chain length. Terminal ring cycloadducts of oligofurans are kinetically and thermodynamically favored, whereas central ring cycloadducts are preferred in oligoacenes, because of the different driving forces in the two reactions: π-conjugation in oligofurans and aromatization/dearomatization in oligoacenes.     

Optical characteristics and plasma parameters of a blue-green exciplex lamp

The optical characteristics and plasma parameters of an exciplex lamp radiating in the blue-green spectral range are studied. A plasma is generated by an atmospheric-pressure barrier discharge initiated in a quaternary mixture including mercury dibromide, sulfur hexafluoride, nitrogen, and helium. It is shown that the exciplex lamp can radiate at an elevated repetition rate of pump pulses (1–12 kHz) under the conditions of mixture self-heating. A tradeoff between the radiation power and nitrogen partial pressure is found. The mean specific radiation power in the blue-green range at a level of 480 mW/cm³ is achieved at partial vapor pressures of mercury dibromide, sulfur hexafluoride, nitrogen, and helium of 0.70, 0.07, 4.00, and 117.20 kPa, respectively. The plasma parameters, namely, the electron energy distribution function; concentration, temperature, and mean energy of electrons; transport properties; and rate constants of elastic and inelastic electron scattering by the working mixture components are determined as functions of ratio E/N (where E is the electric field strength and N is the total concentration of mercury dibromide, sulfur hexafluoride, and nitrogen molecules and helium atoms). It is found that mercury monobromide molecules and also excited and higher energy states take part in the population of exciplex molecules HgBr* (B²Σ₁^2/+ states) in the course of quenching these exciplexes by sulfur hexafluoride and nitrogen molecules.     

Optimization of Adiabatic Selective Pulses

Adiabatic RF pulses play an important role in spin inversion due to their robust behavior in presence of inhomogeneous RF fields. These pulses are characterized by the trajectory swept by the tip of theB_effvector and the rate of motion upon it. In this paper, a method is described for optimizing adiabatic inversion pulses to achieve a frequency-selective magnetization inversion over a given bandwidth in a shorter time and to improve slice profile. An efficient adiabatic pulse is used as an initial condition. This pulse allows for flexibility in choosing its parameters; in particular, the transition sharpness may be traded off against the inverted bandwidth. The considerations for selecting the parameters of the pulse according to the requirements of the design are discussed. The optimization process then improves the slice profile by optimizing the rate of motion along the trajectory of the pulse while preserving the trajectory itself. The adiabatic behavior of the optimized pulses is fully preserved over a twofold range of variation in the RF amplitude which is sufficient for imaging applications in commercial high-field MRI machines. Design examples demonstrate the superiority of the optimized pulses over the conventional sech/tanh pulse.     

Interactions between inorganic salts and polyacrylamide in aqueous solutions and gels

The swelling of poly(acrylamide) (PAAm) gels and the osmotic pressure of linear PAAm in aqueous solutions were predominantly affected by anion type and increased according to the lyotropic series ranking of sodium halide anions: F^? < (H₂O) < Cl^? < Br^? < I^?. The osmotic pressure of PAAm in all examined salt solutions followed the scaling theory, with an exponent of 2.3 ± 0.1. In solutions of a sodium halide series, the value of the pre‐exponential factor seemed to depend on salt concentration, anion radius, and the apparent “anionic‐portion radius” of the water molecule. This radius, extracted from the literature data, marks a transition point of the anion radius effect. Larger anions increase the osmotic pressure of PAAm more significantly as their concentration increases and vice versa. The effects of the anions on the osmotic pressure of PAAm are related to their preferential interactions with the polymer. Iodide, which increased the osmotic pressure of PAAm with respect to its value in pure water, seemed to preferentially adsorb onto the polymer with a binding constant of K_b = 9.7 ± 2.0 M^?1 determined by isothermal titration microcalorimetry. However, fluoride, which decreased the osmotic pressure, was preferentially repulsed. The mechanisms of attraction and repulsion were attributed to ion‐water‐polymer interactions and the solvent quality of the hydrated ions. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 508–519, 2003     

SL(2,C) Gauge Theory of Gravitation and the Quantization of the Gravitational Field

A new approach to quantize the gavitationalfield is presented. It is based on the observation thatthe quantum character of matter becomes more significantas one gets closer to the big bang. As the metric loses its meaning, it makes sense to considerSchrodinger's three generic types of manifolds —unconnected differentiable, affinely connected, andmetrically connected — as a temporal sequencefollowing the big bang. Hence one should quantize thegravitational field on general differentiable manifoldsor on affinely connected manifolds. The SL(2,C) gaugetheory of gravitation is employed to explore thispossibility. Within this framework, the quantization itselfmay well be canonical.