Specific and nonspecific collapse in protein folding funnels

Experiments with fast folding proteins are beginning to address the relationship between collapse and folding. We investigate how different scenarios for folding can arise depending on whether the folding and collapse transitions are concurrent or whether a nonspecific collapse precedes folding. Many earlier studies have focused on the limit in which collapse is fast compared to the folding time; in this work we focus on the opposite limit where, at the folding temperature, collapse and folding occur simultaneously. Real proteins exist in both of these limits. The folding mechanism varies substantially in these two regimes. In the regime of concurrent folding and collapse, nonspecific collapse now occurs at a temperature below the folding temperature (but slightly above the glass transition temperature).     

Nonequilibrium raftlike membrane domains under continuous recycling

We present a model for the kinetics of spontaneous membrane domain (raft) assembly that includes the effect of membrane recycling ubiquitous in living cells. We show that domains can have a broad power-law distribution with an average radius that scales with the 1/4 power of the domain lifetime when the line tension at the domain edges is large. For biologically reasonable recycling and diffusion rates, the average domain radius is in the tens of nm range, consistent with observations. This represents one possible link between signaling (involving rafts) and traffic (recycling) in cells. Finally, we present evidence that suggests that the average raft size may be the same for all scale-free recycling schemes.     

Molecular dynamics simulations of a poly(p-phenylene) oligomer

We have studied the conformation and coefficient of thermal expansion in the poly(p-phenylene) oligomer p-sexiphenyl (C₃₆H₂₆) by molecular dynamics simulations. Studies of the backbone phenyl–phenyl torsion angle in a simulated p-sexiphenyl crystal at room temperature indicate the presence of torsional librations of approximately ±20°. Further analysis of the phenyl–phenyl backbone torsion angle in less closely packed regions of the simulated crystal (crystal ends) indicate the presence of 180° phenyl ring flips, in agreement with solid-state deuterium NMR data on poly(p-phenylene oligomers). The linear coefficient of thermal expansion was also calculated and found to be negative, in qualitative agreement with experimental data on rigid-rod compounds. © 1993 John Wiley & Sons, Inc.     

含油岩心显微荧光成像光谱研究

发展了一种显微荧光光谱成像技术,并将其应用于天然岩心进行显微荧光成像光谱研究。利用这种技术同时采集含油岩心表面的荧光光谱信息和空间信息．并对获得的光谱图像给予光谱学和地质学解释。结果表明,不但能显示岩心形貌和组分的大致趋势,而且能揭示其精细细节．为石油地质研究提供了一种新方法,为今后的石油勘探开发工作提供了一种先进的指导手段。     

Multistage high-order microring-resonator add-drop filters

We propose and demonstrate a multistage design for microphotonic add-drop filters that provides reduced drop-port loss and relaxed tolerances for achieving high in-band extinction. As a result, the first microring-resonator filters with a rectangular notch stopband in the through port (to our knowledge) are shown, with extinctions exceeding 50 dB. Reaching 30 dB beyond previous results, without postfabrication trimming, such extinction levels open the door to microphotonic notch circuits for spectroscopy, wavelength conversion, and quantum cryptography applications. Combined with a low-loss, high-index-contrast electromagnetic design in SiN and frequency-matched microring resonators, this approach led to the first demonstration of flattop microphotonic filters meeting the stringent criteria for high-spectral-efficiency integrated add-drop multiplexers. The 40 GHz wide filters show a 20 nm free spectral range, 2 dB drop loss, and suppression of adjacent channels by over 30 dB.     

Analysis of a shielded TE011 mode composite dielectric resonator for stable frequency reference

Analysis of a TE₀₁₁ mode composite sapphire-rutile dielectric resonator has been carried out to study the temperature variation of resonance frequency, close to the Cs atomic clock hyperfine frequency of 9.192 GHz. The complementary behavior of dielectric permittivity with temperature of the composite has been exploited to obtain the desired turning point in the resonant frequency. The frequency of the composite structure is found to be independent of the shield diameter beyond four times the puck diameter.     

Molecular dynamics and X-ray scattering simulations of cyclic siloxane-based liquid crystal mesogens

Molecular dynamics simulations of cyclic siloxane-based liquid crystals offer new insights into the conformational flexibility of these materials. Interdigitation between the cholesteryl-4'-allyloxybenzoate and biphenyl-4'-allyloxybenzoate mesogens pendant on the cyclic siloxane ring is observed in the simulated structures. All molecular models considered viz. disc, cone, and cylinder, display a large conformational flexibility, which is important regarding the liquid crystalline phase behavior. The disc molecular model exhibits the largest flexibility as indicated by mean dihedral angles and their range for certain principal torsions, evaluated from the molecular dynamics simulations. Results from the dynamics simulations of cylinder molecular pairs indicate a significant amount of conformational flexibility in the siloxane rings. The degree of interdigitation between mesogens is dependent on the flexibility of the siloxane rings, as shown by calculations for a fixed ring system resulting in less interdigitation, also reflected in calculated X-ray scattering sections along the starting molecular direction. Weaker molecular transforms were observed for the non-fixed system due to a lack of boundary conditions. In general, the qualitative agreement of the starting structure's reflections and those shown by the experimental data is encouraging.     

Static and dynamic molecular mechanics modelling and X-ray scattering calculations for a cyclic siloxane macromolecule

A liquid crystalline system consisting of cyclic penta(methylsiloxane) cores with biphenylyl 4-propoxybenzoate mesogens is studied with respect to its molecular conformation and the intermolecular ordering of pendant groups by employing molecular mechaincs calculations, while a molecular dynamics simulation indicates the degree of conformational flexibility. The calculated X-ray scattering patterns for these structures provide insight into the origin of the experimentally observed results. A good agreement is found between the calculated and experimental reflections data, supporting the ‘cylinder’ global topology as the basis for forming a ‘columnar’ structure. Further, a comparison of the calculated and experimental meriodional sections shows a strong similarity of both the meridional intensities and d-spacings.     

Long-range soliton interactions in dispersion-managed links

A numerical analysis of soliton propagation in a dispersion-compensated transmission system reveals the presence of a small dispersive-wave contribution to the propagating soliton that increases dramatically whenever the input soliton energy is detuned from its optimal value. A straightforward consequence of the dispersive-wave emission is an unexpectedly strong contribution to the long-range soliton-soliton interaction. We show that the combination of long- and short-range interactions induces soliton timing jitter that grows larger than that which arises from the Gordon-Haus effect.