Chemical activity of the nitrogenase FeMo cofactor with a central nitrogen ligand: density functional study

We investigate the chemical consequences of a central ligand in the nitrogenase FeMo cofactor using density functional calculations. Several studies have shown that the central ligand most probably is a nitrogen atom, but the consequences for the chemical reactivity of the cofactor are unknown. We investigate several possible routes for insertion of the central nitrogen ligand and conclude that all routes involve barriers and intermediate states, which are inaccessible at ambient conditions. On this basis we suggest that the central nitrogen ligand is present at all times during the reaction. Furthermore, we investigate how the FeMoco with the central ligand can interact with N(2) and reduce it.     

Complexation of polyacrylates by Ca2+ ions. Time-resolved studies using attenuated total reflectance Fourier transform infrared dialysis spectroscopy

The attenuated total reflectance Fourier transform infrared dialysis technique is introduced for the time-resolved investigation of the binding processes of Ca2+ to polyacrylates dissolved in water. We observed transient formation of intermediates in water with various types of coordination of the carboxylate group to Ca2+ throughout the complexation steps. Time-resolved changes in the spectra were analyzed with principal component analysis, from which the spectral species were obtained as well as their formation kinetics. We propose a model for the mechanisms of Ca2+ coordination to polyacrylates. The polymer chain length plays an important role in Ca2+ binding.     

Colon cancer risk factors from nutrition

Nutrition and health are closely connected and malnutrition can seriously endanger health. The consequences are higher risks of developing diseases. Of these, cancers are of special importance. The most frequent nutrition-associated type of cancer is colon cancer. This review summarises the predisposing factors for the development of colon cancer, and molecular mechanisms responsible for sporadic colon cancer. Moreover, it is pointed out that individual genetic predisposition such as polymorphisms of biotransformation systems might affect susceptibility to cancer risk factors. Selected findings of epidemiological research and nutritional habits, foods, and metabolites that could increase cancer risk are discussed. Furthermore, toxicological assessment of food-related risk factors, e.g. heterocyclic amines, and potential protective effects of food ingredients, e.g. the induction of phase II enzymes or removal of already degenerated cells from tissue by apoptosis, inhibition of proliferation, and cell differentiation are discussed. Risks are not necessarily reduced by avoiding certain substances but by healthy and well-balanced nutrition. Knowledge of individual susceptibility will also make it possible to recognise risks and design more specific nutritional recommendations.     

Leading edge design process using a commercial flow solver

A design process of an asymmetric leading edge, for laminar stability measurements in a flat plate boundary layer, is reported. The purpose is to minimize the leading edge pressure gradient region, which affects the stability characteristics of the flow. Finally, the design success is verified by wind tunnel testing.     

Localization in Hermitian K-Theory of Rings

Localization and dévissage theorems are proved for thehermitian K-theory of rings that are analogous to well-knowntheorems in algebraic K-theory. The proofs rely on, among otherthings, a study of derived categories, a generalization of atheorem of Pedersen and Weibel to the hermitian setting, anda cofinality result for triangular Witt groups. Applicationsinclude a proof of a conjecture of Karoubi and algebraic Bottperiodicity.     

Measurement of Cell Volume Changes by Fluorescence Self-Quenching

At high concentrations, certain fluorophores undergo self-quenching, i.e., fluorescence intensity decreases with increasing fluorophore concentration. Accordingly, the self-quenching properties can be used for measuring water volume changes in lipid vesicles. In cells, quantitative determination of water transport using fluorescence self-quenching has been complicated by the requirement of relatively high (mM) and often toxic loading concentrations. Here we report a simple method that uses low (M) loading concentrations of calcein-acetoxymethyl ester (calcein-AM) to obtain intracellular concentrations of the fluorophore calcein suitable for measurement of changes in cell water volume by self-quenching. The relationship between calcein fluorescence intensity, when excited at 490 nm (its excitation maximum), and calcein concentration was investigated in vitro and in various cultured cell types. The relationship was bell-shaped, with the negative slope in the concentration range where the fluorophore undergoes fluorescence self-quenching. In cultured retinal pigment epithelial cells, calcein fluorescence and extracellular osmolarity were linearly related. A 25-mOsm hypertonic challenge corresponded to a decrease in calcein fluorescence with high signal-to-noise ratio (>15). Similar results were obtained with the fluorophore BCECF when excited at its isosbestic wavelength (436 nm). The present results demonstrate the usefulness of fluorescence self-quenching to measure rapid changes in cell water volume.     

Fourth-order magnetic anisotropy and tunnel splittings in Mn(12) from spin-orbit-vibron interactions

From density-functional-theory based methods, we calculate the vibrational spectrum of the Mn(12)O(12)(COOH)(16)(H(2)O)(4) molecular magnet. Calculated infrared intensities are in accord with experimental studies. There have been no ab initio attempts at determining which interactions account for the fourth-order anisotropy. We show that vibrationally induced distortions of the molecule contribute to the fourth-order anisotropy Hamiltonian and that the magnitude and sign of the effect (-6.2 K) is in good agreement with all experiments. Vibrationally induced tunnel splittings in isotopically pure and natural samples are predicted.     

A general method for determining the electron self-exchange rates of blue copper proteins by longitudinal NMR relaxation

A general NMR method is presented that allows a precise determination of the second-order rate constant, k(ese), for the electron self-exchange in blue copper proteins, from the longitudinal relaxation rates of the nuclei in the protein. The method relies on the use of partly oxidized (paramagnetic) samples of the protein. In contrast to previous NMR approaches for the determination of electron self-exchange rates, the applicability of the method extends beyond the slow-exchange limit, k(ese)c < R(ip), i = 1, 2, where c is the protein concentration, and R(ip) is the paramagnetic relaxation enhancement of the observed nuclei.