An Improved Method for Calculating Zeta-Potentials from Measurements of the Electrokinetic Sonic Amplitude

The surface electric properties of the commercially available silica, Monospher 1000 (Fa. Merck), have been studied by conductivity and ESA (electrokinetic sonic amplitude) experiments. It could be shown that accounting for the contribution of the stagnant layer to surface conductivity is indispensable in the interpretation of electrokinetic data at low ionic strength. A general method has been put forward which allows to take into account the total, experimentally accessible surface conductivity in the evaluation of ESA data of moderately concentrated suspensions. This includes additional conductivity measurements which serve for the independent estimation of the total relative surface conductivity. The resulting zeta-potentials are clearly higher than those obtained after neglecting the contribution of the stagnant layer to surface conductivity. In addition, the ionic mobilities of potassium and magnesium in the hydrodynamically stagnant layer have been investigated in some detail. It has been found that the ionic mobility of potassium is of the same order of magnitude as in the bulk solution while the mobility of magnesium is significantly reduced. Copyright 2000 Academic Press.     

Rapid hydrolytic cleavage of the mRNA model compound HPNP by glycine based macrocyclic lanthanide ribonuclease mimics

The lanthanide ion based macrocyclic complexes 1.Ln mimic the hydrophobic nature of ribonucleases, where the lanthanide ions induce the formation of a hydrophobic cavity for 1, giving rise to a large order of magnitude enhancement in the hydrolytic cleavage of HPNP.     

High temperature Mössbauer spectroscopy of titanomagnetite and maghemite in basalts

Mössbauer spectroscopy of basalt lava samples, exhibiting reversible thermal magnetization (J _s -T) curves with Curie temperatures of about 840 K, has revealed considerable amounts of maghemite (-Fe₂O₃) in many samples. In view of the expected instability of maghemite at temperatures above 620 K, this reversibility came as a surprise. For further studies of the magnetization-temperature relationship of these minerals, we have constructed an elliptical radiation-heated furnace in which Mössbauer spectra can be acquired at temperatures between 300 and 900 K. Measurements at different temperatures have been obtained for two types of basalts, one in which the magnetic minerals are nearly pure magnetite and the other where the room temperature spectrum indicates a mixture of maghemite and magnetite. The two series show different features of the collapse of the internal magnetic hyperfine field, and the composition of minerals in the samples changes during the treatment, showing maghemite.     

Anion recognition and anion-mediated self-assembly with thiourea-functionalised fused [3]polynorbornyl frameworks

Three conformationally preorganised host molecules based on the [3]polynorbornyl framework and incorporating di-urea receptors were synthesised and their interaction with a series of anions investigated by (1)H NMR spectroscopy. A high affinity of each host molecule for dihydrogenphosphate (H(2)PO(4)(-)) and dihydrogenpyrophosphate (H(2)P(2)O(7)(2-)) was identified. In addition to binding to the urea receptors of the host molecules, evidence for an interaction involving the non-polar C-H groups within the binding cavity of the framework and guest anions was also discovered. Furthermore, an unusual 2 : 1 host-to-anion stoichiometry was indicated when binding H(2)P(2)O(7)(2-), and a model for the anion-mediated self-assembly of this complex species is proposed.     

Reversal of a Single Base‐Pair Step Controls Guanine Photo‐Oxidation by an Intercalating Ruthenium(II) Dipyridophenazine Complex

Small changes in DNA sequence can often have major biological effects. Here the rates and yields of guanine photo‐oxidation by Λ‐[Ru(TAP)₂(dppz)]²⁺ have been compared in 5′‐{CCGG AT CCGG}₂ and 5′‐{CCGG TA CCGG}₂ using pico/nanosecond transient visible and time‐resolved IR (TRIR) spectroscopy. The inefficiency of electron transfer in the TA sequence is consistent with the 5′‐TA‐3′ versus 5′‐AT‐3′ binding preference predicted by X‐ray crystallography. The TRIR spectra also reveal the differences in binding sites in the two oligonucleotides.     

The application of chiroptical spectroscopy (circular dichroism) in quantifying binding events in lanthanide directed synthesis of chiral luminescent self-assembly structures

The binding of asymmetrical and optically pure tridentate ligands (L = 1(S) and 1(R)) containing one carboxylic group and 2-naphthyl as an antenna to lanthanide ions (M = La(iii) and Eu(iii)) was studied in CH₃CN, showing the successive formation of M:L, M:L₂ and M:L₃ stoichiometric species in solution. The europium complexes EuL₃ were also synthesised, structurally characterised and their photophysical properties probed in CH₃OH and CH₃CN. The changes in the chiroptical properties of both 1(S) and 1(R) were used (by circular dichroism (CD) spectroscopy) to monitor the formation of these chiral self-assemblies in solution. While circularly polarised luminescence (CPL) showed the formation of Eu(1(S))₃ and Eu(1(R))₃ as enantiomers, with high luminescence dissymmetry factors (g _lum), fitting the CD changes allowed for binding constants to be determined that were comparable to those seen in the analyses of absorbance and luminescence changes.     

Identification of substitutional and interstitial Fe in 6H-SiC

Iron impurities on interstitial (Fe_i) and substitutional sites (Fe_S) in SiC have been detected by ⁵⁷Fe emission Mössbauer spectroscopy following implantation of radioactive ⁵⁷Mn⁺ parent ions. At temperatures <900 K two Fe_i species are found, assigned to quasi-tetrahedral interstitial sites surrounded by, respectively, four C (Fe_i,C) or Si atoms (Fe_i,Si). Above 900 K, the Fe_i,Si site is proposed to “transform” into the Fe_i,C site by a single Fe_i jump during the lifetime of the Mössbauer state (T _1/2?=?100 ns). Fe_i,C and substitutional Fe_S sites are stable up to >1,070 K.     

Simulations of the magnetic properties experiment on Mars Exploration Rovers

We present some of the main findings from simulation studies of the Magnetic Properties Experiment on the Mars Exploration Rovers. The results suggest that the dust has formed via mechanical breakdown of surface rocks through the geological history of the planet, and that liquid water need not have played any significant role in the dust formation processes.