Optical parametric interactions in 3-methyl-4-nitropyridine-1-oxide (POM) single crystals

Various configurations of three-wave parametric interactions are reported at the picosecond time scale, in 3-methyl-4-nitropyridine-1-oxide (POM) single crystals. Wavelength dispersion of phase-matched second-harmonic generation orientation is experimentally established within the0.8-2 mum range using as fundamental input the idler and signal emissions of a picosecond LiIO3parametric emitter. Efficient parametric amplification is demonstrated: at degeneracy a gain of 10³is measured for a pump intensity of 130 MW . cm^-2at 0.53 μm and an input signal of 5 kW . cm^-2at 1.06 μm. Single pass parametric emission is achieved, for the first time in an organic crystal, yielding 200 kW . cm^-2at 1.06 μm, using the frequency-doubled output of a passively mode-locked YAG system. Both dispersion and interaction length dependence of the experimental gain are accounted for.     

Solution structure of a Na cation stabilized DNA quadruplex containing G.G.G.G and G.C.G.C tetrads formed by G-G-G-C repeats observed in adeno-associated viral DNA

We have applied NMR and molecular dynamics computations including intensity based refinement to define the structure of the d(G-G-G-C-T4-G-G-G-C) dodecanucleotide in 100 mM NaCl solution. The G-G-G-C sequence is of interest since it has been found as tandem repeats in the DNA sequence of human chromosome 19. The same G-G-G-C sequence is also seen as islands in adeno-associated virus, a human parvovirus, which is unique amongst eukaryotic DNA viruses in its ability to integrate site-specifically into a defined region of human chromosome 19. The d(G-G-G-C-T4-G-G-G-C) sequence forms a quadruplex in Na cation containing solution through head-to-tail dimerization of two symmetry-related stem-hairpin loops with adjacent strands antiparallel to each other around the quadruplex. The connecting T4 loops are of the lateral type, resulting in a quadruplex structure containing two internal G.G.G.G tetrads flanked by G.C.G.C tetrads. The G(anti).G(syn).G(anti).G(syn) tetrads are formed through dimerization associated hydrogen bonding alignments of a pair of Hoogsteen G(anti).G(syn) mismatch pairs, while the G(anti).C(anti).G(anti).C(anti) tetrads are formed through dimerization associated bifurcated hydrogen bonding alignments involving the major groove edges of a pair of Watson-Crick G.C base-pairs. The quadruplex contains two distinct narrow and two symmetric wide grooves with extensive stacking between adjacent tetrad planes. The structure of the quadruplex contains internal cavities that can potentially accommodate Na cations positioned between adjacent tetrad planes. Three such Na cations have been modeled into the structure of the d(G-G-G-C-T4-G-G-G-C) quadruplex. Finally, we speculate on the potential role of quadruplex formation involving G.G.G.G and G.C.G.C tetrads during the integration of the adeno-associated parvovirus into its target on human chromosome 19, both of which involve stretches of G-G-G-C sequence elements.     

Effects of recombination parameters in pn indium phosphide solar cells

This work, using a numerical code PC-1D, describes the effects of surface and bulk recombination on the performance of p⁺n indium phosphide solar cells. It is shown that surface recombination velocity and minority carrier diffusion lengths play a dominant role in controlling the efficiency of p⁺n cells. In order to have an acceptable series resistance, a p⁺n cell must have an emitter that is thicker than a n⁺p cell emitter. Consequently the performance of a p⁺n cell is more sensitive to the front surface recombination velocity. Improved surface and bulk recombination parameters can lead to cell efficiencies in excess of 24% AMO at 25°C.     

Fundamentals of post-combustion in steelmaking vessels

Basic equations for the measurement of post-combustion in oxygen steelmaking are given. A thermodynamic phase diagramm of the system Fe-C-O-H for the post-combustion reactions to CO₂ and H₂O has been set up. According to this diagram both CO₂/(CO + CO₂) and H₂O/(H₂ + H₂O) ratios fall when temperatures exceed the melting point of FeO_n (1377°C). Typical curves for post-combustion as a function of blowing time have been included for both K-OBM and KMS heats. An appropriate index for post-combustion is the specific heat flow in MJ/t · min or m³/t · min of (CO₂ + H₂O).     

Characterization of simulated microstructures in polydisperse particle deposition

The deposition of polydisperse particles under the influence of gravity is examined using computer simulation. A parameter, σ, that represents the standard deviation of particle size is used for studying the effect of the variation in polydispersity on the resulting microstructures. Structural correlations are examined through contact networks, radial and angular distribution functions, and diffraction patterns. The results show that the onset of ordering appears near σ = 0.05 as σ is decreased. The long-range ordering of the structures is not influenced by the introduction of a small amount of polydispersity, which may increase the uniformity of local density distribution in the angular direction. Polydisperse systems with small deviations in size display stronger positional order in some directions and this in turn contributes to the uniformity of overall packing structures.     

Structural polymorphism and dynamism in the DNA segment GATCTTCCCCCCGGAA: NMR investigations of hairpin, dumbbell, nicked duplex, parallel strands, and i-motif

Structure and dynamism in a DNA segment d-GATCTTCCCCCCGGAA have been investigated by nuclear magnetic resonance (NMR) spectroscopy. At neutral pH, the molecule exists largely as a dumbbell, formed by the association of two hairpins with sticky ends. A small percentage of hairpin is also detectable. The stem of the dumbbell, which is 12 base pairs long, has two nicks separated by 4 base pairs. The three-dimensional structures of the dumbbell and also of a 12-mer duplex, the sequence of which is identical to that of the stem of the dumbbell, have been determined by NMR in conjunction with restrained molecular dynamics calculations. It is observed that the presence of nicks causes minor changes in the structure of the duplex. Fraying at the nicks is much less than at the ends of a regular duplex. The loop shows very few nuclear Overhauser effects, which is a reflection on the greater dynamism in its structure. At lower pH, the molecule undergoes a transition to an i-motif type of structure with two parallel stranded duplexes involving C-C+ pairing, interdigitating each other. The structure is highly stable, with a melting temperature >60 degrees C.