Properties of isolated DNA bases, base pairs and nucleosides examined by laser spectroscopy

The vibronic spectra of laser desorbed and jet cooled guanine (G) adenine (A), and cytosine (C) consist of bands from four, two and two major tautomers respectively, as revealed by UV-UV and IR-UV double resonance spectroscopy. The vibronic spectrum of adenine around 277 nm consists of weak nπ^* and strong ππ^* transitions, based on IR-UV and deuteration experiments. Precise ionization potentials of G and A were determined with 2-color, 2-photon ionization. We also measured vibronic and IR spectra of several base pairs. GC exhibits a HNH ^... OH/NH ^... N/C=O ^... HNH bonding similar to the Watson-Crick GC base pair but with C as enol tautomer. One GG isomer exhibits non-symmetric hydrogen bonding with HNH ^... N/NH ^... N/C=O ^... HNH interactions. A second observed GG isomer has a symmetrical hydrogen bond arrangement with C=O ^... NH/NH ^... O=C bonding. Two CC isomers were observed with symmetrical C=O ^... NH/NH ^... O=C bonding and nonsymmetrical C=O ^... HNH/NH ^... N interaction, respectively. Guanosine (Gs), 2-DeoxyGs und 3-DeoxyGs each exhibit only one isomer in the investigated wavelength range around 290 nm with a strong intramolecular sugar(5-OH) ^... enolguanine(3-N) hydrogen bond. Received 16 June 2002 / Received in final form 15 July 2002 Published online 13 September 2002     

Molecular geometries and moments from the maximum overlap criterion

The maximized overlap population, defined as Σ_μ≠νP_μνS_μν, and a related quantity, Σ_μ,νP_μνS²_μν are computed for a series of configurations. The extremum of both approximate molecular geometries, the latter with an accuracy suitable for predictive value. The maximum overlap orbitals predict dipole and quadrupole moments that give reasonable agreement with experimental values.     

Signatures of strong momentum localization via entanglement of translational and internal states

We show that atoms or molecules subject to fields that couple their internal and translational (momentum) states may undergo a crossover from randomization (diffusion) to strong localization (sharpening) of their momentum distribution. The predicted crossover should be manifest by a drastic change of the interference pattern as a function of the coupling fields.     

Geometric doppler effect: spin-split dispersion of thermal radiation

A geometric Doppler effect manifested by a spin-split dispersion relation of thermal radiation is observed. A spin-dependent dispersion splitting was obtained in a structure consisting of a coupled thermal antenna array. The effect is due to a spin-orbit interaction resulting from the dynamics of the surface waves propagating along the structure whose local anisotropy axis is rotated in space. The observation of the spin-symmetry breaking in thermal radiation may be utilized for manipulation of spontaneous or stimulated emission.     

Dynamics and morphology of microvilli driven by actin polymerization

Many different cell types have dynamic protrusions, called microvilli, on their surface. We model these structures as arising from the balance between the force of actin polymerization and the restoring force of the membrane. From this simple model we calculate the distribution function of microvilli heights for several cells. We further describe the phase diagram and the resulting morphology of the microvilli aggregates on the cell surface.     

A Quantitative Balian-Low Theorem

We study functions generating Gabor Riesz bases on the integer lattice. The classical Balian-Low theorem (BLT) restricts the simultaneous time and frequency localization of such functions. We obtain a quantitative estimate on their Zak transform that extends both this result and the more general (p,q) Balian-Low theorem. Moreover, we establish a family of quantitative amalgam-type Balian-Low theorems that contain both the amalgam BLT and the classical BLT as special cases.     

Variational multiple-tensor fitting of fiber-ambiguous diffusion-weighted magnetic resonance imaging voxels

Partial volume effects are often experienced in diffusion-weighted MRI of biologic tissue. This is when the signal attenuation reflects a mixture of diffusion processes, originating from different tissue compartments, residing in the same voxel. Decomposing the mixture requires elaborated models that account for multiple compartments, yet the fitting problem for those models is usually ill posed. We suggest a novel approach for stabilizing the fitting problem of the multiple-tensors model by a variational framework that adds biologically oriented assumption of neighborhood alignments. The framework is designed to address fiber ambiguity caused by a number of neuronal fiber compartments residing in the same voxel. The method requires diffusion data acquired by common, clinically feasible MRI sequences, and is able to derive familiar tensor quantities for each compartment. Neighborhood alignment is performed by adding piece-wise smooth regularization constraints to an energy function. Minimization with the gradient descent method produces a set of diffusion-reaction partial differential equations that describe a tensor-preserving flow towards a best approximation of the data while maintaining the constraints. We analyze fiber compartment separation capabilities on a synthetic model of crossing fibers and on brain areas known to have crossing fibers. We compare the results with diffusion tensor imaging analysis and discuss applications for the framework.     

Automatic locking of a parametrically resonating,base-excited,nonlinear beam

Nonlinear Dynamics - Described is a closed-loop control scheme capable of stabilizing a parametrically excited nonlinear structure in several vibration modes. By setting the relative phase between...     

The role of point defects in melting of solid He

We have recently observed that the resistance to shear of solid ⁴He decreases dramatically near the first-order BCC–HCP transition. In our view, the solid shears via a diffusive counter-flow of atoms and point defects. The mechanism of self diffusion couples point defects with one specific phonon which softens as the transition is approached. Since such a scenario can possibly lead to melting, it is important to understand (a) which type of point defect is associated with the reduction of shear resistance, and (b) can the presence of point defects lead to the softening of phonons. We report here the results of numerical simulations and analytic modeling. Our results indicate that split interstitials are much more effective than vacancies in lowering the resistance to shear. We suggest that these excitations can be generated as a result of a “local mode” excited in the crystal.     

Non-covalent Surface Interactions between Silicone and Biological Macromolecules Yield Bioreactive Substances

While most organometallics enter the en- vironment through their industrial release, silicones are organometallic compounds purposefully introduced in high volume directly into healthy humans. A chemically centered study of the behavior of silicones in the biological environment reveals numerous de- gradative reactions and surface interactions that can produce bioreactive substances. Data from a variety of disciplines suggest that the preponderance of evidence supports the argument that silicone is a toxic organometallic. © 1997 by John Wiley & Sons, Ltd.