Toward an ideal integrating nephelometer

The Integrating Sphere Integrating Nephelometer is a novel and unique reciprocal nephelometer that uses an integrating sphere with attached truncation-reduction tubes to contain the sample volume and to integrate the scattered light. Its main advantage compared with current integrating nephelometers is a sevenfold reduction in truncation angle, which reduces errors in measured scattering from large particles. Additional features include improved sampling efficiency for large particles and a well-defined operating wavelength.     

More complex DNA structures

Fibrous polynucleotide duplexes are very polymorphic. Detailed x-ray analysis of the D forms of poly d(AT) ·poly d(AT) and poly d(IC) ·poly d(IC) and of the B form of poly d(GC) ·poly d(GC) show that 5′RpY3′ dinucleoside monophosphates in these molecules have either t,t or t,g conformations at C3′–03′, 03′-P which lead to similar orientations of phosphate groups. On the other hand, 5′YpR3′ fragments commonly have g,t conformations and hence a markedly different orientation of the phosphate group. Studies of the more complex structures of the C form of poly d(ACC) ·poly d(GGT) and of a pleiomeric D form of poly d(AT) ·poly d(AT) where the repeated structural motifs are hexanucleotides reveal further details of the surface “wrinkles” on DNA. In the case of the H form of poly d(A) ·poly d(T), the sugar rings in each strand are puckered differently-C2′-endo in poly d(T) and C3′-endo in poly d(A). Such heteromerous duplexes have unusually pronounced directional properties. Similar structure ma be present in some DNA-RNA hybrids. These surface feature might enable regulatory proteins and enzymes to recognize their host DNA sequences more easily in the blinding process.     

Underwater CCD cameras for PIV flow mapping

Accurate estimation of the forces imposed on offshore structures due to wave and current loading has become more critical due to the introduction of floating production platforms. Work being carried at both Edinburgh and Glasgow Universities aims to measure these effects by force transducer measurements and Particle Image Velocimetry (PIV). A multiple CCD array system is presented which can directly measure accelerations and hence forces acting on offshore structures.     

Surface valence band and plasmon features on clean cleaved silicon

The L₁L_2,3V Auger transition from vacuum cleaved surfaces of silicon has been studied in detail. Features in the distribution which are not due to plasmon losses are shown to correspond with primary features in the valance band density of states. However, effects of transition probability energy dependence apparently smear the structure, which may also be due in part to particular surface region properties. Bulk plasmon losses up to 9 in number are clearly resolved and the relative intensities compared with simple theory. A mean free path for bulk plasmon emission is deduced which is approximately twice the m.f.p. for other loss processes. Background contamination produces a new peak at approximately 4.5 eV.     

Preparation and properties of the system CoPxS2−x

Lattice parameters were determined for members of the system CoP_xS_2?x (0 ≤ x ≥ 1). A cubic region was observed for 0 ≤ x ≤ 0·5 and a tetragonal region for 0·5 < x ≤ 1. Magnetization and susceptibility measurements were made on the sample CoP_xS_2?x (0 ≤ x ≤ 0·5) from 4·2 to 500°K and to 9·5 kOe. The ferromagnetic Curie temperature decreased almost linearly with composition from a value of 122(1)°K for x = 0 to 23(1)°K at x = 0·3. The saturation magnetization passed through a maximum at 41·5(2) e.m.u./g at x = 0·05, increasing from 40·0(2) e.m.u./g for x = 0, and then decreased to below 15 e.m.u./g for x = 0·3. The paramagnetic data gave a P²_eff/CO²⁺ that increased linearly with phosphorus substitution from 4·4(2)μ_B at x = 0 to 5·5(2)μ_B² at x = 0·5. The Weiss constant decreased almost linearly from 150(3)°K for x = 0 to 85(2)°K for x = 0·3. The magnetic properties of this system are compared with those of the system CoAs_xS_2?x and CoSe_xS_2?x. Changes, with composition, in the Curie and Weiss temperatures are almost identical in the three systems for x ≤ 0·25. As with CoAs_xS_2?x the ferromagnetic moment reaches a maximum at x = 0·05 and then decreases with increasing x, although the magnetic interactions remain ferromagnetic for all x ≤ 0·5.     

A study of radiative properties of fractal soot aggregates using the superposition T-matrix method

We employ the numerically exact superposition T-matrix method to perform extensive computations of scattering and absorption properties of soot aggregates with varying state of compactness and size. The fractal dimension, D_f, is used to quantify the geometrical mass dispersion of the clusters. The optical properties of soot aggregates for a given fractal dimension are complex functions of the refractive index of the material m, the number of monomers N_S, and the monomer radius a. It is shown that for smaller values of a, the absorption cross section tends to be relatively constant when D_f<2 but increases rapidly when D_f>2. However, a systematic reduction in light absorption with D_f is observed for clusters with sufficiently large N_S, m, and a. The scattering cross section and single-scattering albedo increase monotonically as fractals evolve from chain-like to more densely packed morphologies, which is a strong manifestation of the increasing importance of scattering interaction among spherules. Overall, the results for soot fractals differ profoundly from those calculated for the respective volume-equivalent soot spheres as well as for the respective external mixtures of soot monomers under the assumption that there are no electromagnetic interactions between the monomers. The climate-research implications of our results are discussed.     

Remote stereocontrol using allylstannanes: reversal in stereoselectivity using indium(III) and bismuth(III) halides as promoters

Allyl-indium(III) and -bismuth(III) dihalides, generated by transmetallation of 5-benzyloxy-4-methylpent-2-enyl(tributyl)stannane 1, react with aldehydes with useful levels of 1,5-stereocontrol, a 93:7 ratio of 1,5-epimers in favour of the 1,5-anti-(E)-stereoisomers 7 and 11 typically being obtained using bismuth(III) iodide. The 4-benzyloxypent-2-enylstannane 4 similarly gives the 1,5-syn-(E)-hex-3-enols 13 also with ca. 93:7, stereoselectivity.     

Photon tunneling contributions to extinction for laboratory grown hexagonal columns

A new treatment predicting the extinction and absorption properties of ice particles is evaluated in this study using laboratory measurements of the extinction efficiency, Q_ext. In this treatment, the degree of ‘photon tunneling’ for ice crystals is unspecified, and laboratory measurements of Q_ext were used in conjunction with this scheme to quantify the significance of this process by determining a tunneling factor, denoted t_f. The term tunneling here refers to the interaction of a particle with radiation outside its area cross-section. A t_f of 1.0 corresponds to tunneling exhibited by ice spheres as predicted by Mie theory, while a t_f of 0 indicates no tunneling.

The laboratory work entailed Fourier transform infrared spectroscopy (FTIR) for optical depth measurements in an ice cloud grown in a chamber, over a wavelength range of 2–18 μm. From these measurements, the extinction efficiency Q_ext as a function of wavelength was determined. Ice particle size spectra were measured in the cloud chamber, and were used to predict Q_ext using the radiation scheme noted above and also using a new implementation of T-matrix, which is based on the exact geometry of a ‘pristine’ hexagonal ice crystal, without approximating the crystal as a spheroid.

Results show that t_f values determined from the laboratory measurements and the new radiation scheme are qualitatively in agreement with t_f values based on fundamental theory. Mean Q_ext errors (relative to measured Q_ext) over all wavelengths sampled were 3.0% when using a constant optimized t_f in the radiation scheme, and 2.3% when using a t_f scheme based on complex angular momentum theory. Moreover, Q_ext as predicted from T-matrix over the wavelength interval 8–12 μm is also in excellent agreement with the measured Q_ext. A single wavelength calculation at 14 μm was performed using the finite difference time domain (FDTD) and T-matrix methods, both of which agreed precisely with the measured Q_ext value. This validates the integrity of T-matrix, FDTD, the new radiation scheme, and the laboratory measurements for the corresponding range of wavelengths and size parameters. Collectively, these results indicate the tunneling contributions predicted for solid hexagonal columns are realistic.     

Comparison of glycosaminoglycan structures induced by different monovalent cations as determined by X-ray fiber diffraction

Glycosaminoglycans are prominent components of mammalian connective tissue and possess important ion-binding properties. X-ray fiber diffraction studies have demonstrated that glycosaminoglycans show different conformational responses to mono-and divalent cations.