Imine coordinated 2-aminoazole complexes of (CO)5Cr(0) and (CO)5W(0), verification by structural characterisation

The synthesis and structural characterisation (IR, MS, ¹H, ¹³C and ¹⁵N NMR and single crystal X-ray diffraction analysis) of 2-aminoazole complexes of pentacarbonyl Cr(0) and W(0) are described. When provided with endocyclic soft thioether or endocyclic hard amine, endocyclic borderline imine and exocyclic hard amine coordination sites, the softer endocyclic imine coordination site is favoured.     

Quantitative description of the relation between protein net charge and protein adsorption to air-water interfaces

In this study a set of chemically engineered variants of ovalbumin was produced to study the effects of electrostatic charge on the adsorption kinetics and resulting surface pressure at the air-water interface. The modification itself was based on the coupling of succinic anhydride to lysine residues on the protein surface. After purification of the modified proteins, five homogeneous batches were obtained with increasing degrees of modification and zeta-potentials ranging from -19 to -26 mV (-17 mV for native ovalbumin). These batches showed no changes in secondary, tertiary, or quaternary structure compared to the native protein. However, the rate of adsorption as measured with ellipsometry was found to decrease with increasing net charge, even at the initial stages of adsorption. This indicates an energy barrier to adsorption. With the use of a model based on the random sequential adsorption model, the energy barrier for adsorption was calculated and found to increase from 4.7 kT to 6.1 kT when the protein net charge was increased from -12 to -26. A second effect was that the increased electrostatic repulsion resulted in a larger apparent size of the adsorbed proteins, which went from 19 to 31 nm2 (native and highest modification, respectively), corresponding to similar interaction energies at saturation. The interaction energy was found to determine not only the saturation surface load but also the surface pressure as a function of the surface load. This work shows that, in order to describe the functionality of proteins at interfaces, they can be described as hard colloidal particles. Further, it is shown that the build-up of protein surface layers can be described by the coulombic interactions, exposed protein hydrophobicity, and size.     

Nuclear spin dynamics in the quantum regime of a single-molecule magnet

We show that the nuclear spin dynamics in the single-molecule magnet Mn12-ac below 1 K is governed by quantum tunneling fluctuations of the cluster spins, combined with intercluster nuclear spin diffusion. We also obtain the first experimental proof that-surprisingly-even deep in the quantum regime the nuclear spins remain in good thermal contact with the lattice phonons. We propose a simple model for how T-independent tunneling fluctuations can relax the nuclear polarization to the lattice that may serve as a framework for more sophisticated theories.     

Magnetic long-range order induced by quantum relaxation in single-molecule magnets

Can magnetic interactions between single-molecule magnets (SMMs) in a crystal establish long-range magnetic order at low temperatures deep in the quantum regime, where the only electron spin fluctuations are due to incoherent magnetic quantum tunneling (MQT)? Put inversely: can MQT provide the temperature dependent fluctuations needed to destroy the ordered state above some finite T(c), although it should basically itself be a T-independent process? Our experiments on two novel Mn4 SMMs provide a positive answer to the above, showing at the same time that MQT in the SMMs has to involve spin-lattice coupling at a relaxation rate equaling that predicted and observed recently for nuclear-spin-mediated quantum relaxation.     

Long-range ferromagnetic dipolar ordering of high-spin molecular clusters

We report the first example of a transition to long-range magnetic order in a purely dipolarly interacting molecular magnet. For the magnetic cluster compound Mn6O4Br4(Et2dbm)6, the anisotropy experienced by the total spin S = 12 of each cluster is so small that spin-lattice relaxation remains fast down to the lowest temperatures, thus enabling dipolar order to occur within experimental times at T(c) = 0.16 K. In high magnetic fields, the relaxation rate becomes drastically reduced and the interplay between nuclear- and electron-spin lattice relaxation is revealed.     

Laser-induced fluorescence imaging in a 100 kW natural gas flame

A tunable excimer laser at 248 nm (KrF) and 193 nm (ArF) has been used to monitor two-dimensional OH and NO distributions in the turbulent flame of a 100 kW natural gas burner. Spatially resolved fluorescence (spatial resolution better than 1.0 mm) from a 20 cm×20 cm area is collected under single shot conditions. We describe the problems encountered when laser-induced fluorescence imaging techniques are applied to large scale flames. Special experimental arrangements, imposed by the turbulent behavior of the flame we used, are also described.