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Pseudo‐Contact NMR Shifts over the Paramagnetic Metalloprotein CoMMP‐12 from First Principles

Long‐range pseudo‐contact NMR shifts (PCSs) provide important restraints for the structure refinement of proteins when a paramagnetic metal center is present, either naturally or introduced artificially. Here we show that ab initio quantum‐chemical methods and a modern version of the Kurland–McGarvey approach for paramagnetic NMR (pNMR) shifts in the presence of zero‐field splitting (ZFS) together provide accurate predictions of all PCSs in a metalloprotein (high‐spin cobalt‐substituted MMP‐12 as a test case). Computations of 314 ¹³C PCSs using g‐ and ZFS tensors based on multi‐reference methods provide a reliable bridge between EPR‐parameter‐ and susceptibility‐based pNMR formalisms. Due to the high sensitivity of PCSs to even small structural differences, local structures based either on X‐ray diffraction or on various DFT optimizations could be evaluated critically by comparing computed and experimental PCSs. Many DFT functionals provide insufficiently accurate structures. We also found the available 1RMZ PDB X‐ray structure to exhibit deficiencies related to binding of a hydroxamate inhibitor. This has led to a newly refined PDB structure for MMP‐12 (5LAB) that provides a more accurate coordination arrangement and PCSs.     

Development of minimal fermentation media supplementation for ethanol production using two Saccharomyces cerevisiae strains

Ethanol production by fermentation is strongly dependent on media composition. Specific nutrients, such as trace elements, vitamins and nitrogen will affect the physiological state and, consequently, the fermentation performance of the micro-organism employed. The purpose of this study has been to assess the highest ethanol production by a minimal medium, instead of the more complex nutrients supplementation used during alcoholic fermentation. All fermentation tests were carried out using a microwell plate reader to monitor the processes. Two Saccharomyces cerevisiae strains (NCYC 2826 and NCYC 3445) were tested using three nitrogen sources, supplied with different vitamin and salts. The results show that solutions made of urea phosphate, KCl, MgSO₄·7H₂O, Ca-panthothenate, biotin allowed an ethanol yield of 22.9 and 23.4 g/L for strain NCYC 2826 and NCYC 3445, respectively, representing 90 and 92% of the theoretical yield. All tests were carried out using glucose as common reference carbon source.     

Post-buckling behaviour of graphite/epoxy stiffened panels with initial imperfections subjected to eccentric biaxial compression loading

The post-buckling behaviour of anisotropic stiffened panels with initial imperfections is investigated. Since buckling of the skin between the stiffeners often occurs first, a non-linear analysis is developed for symmetric panels under biaxial compression in order to obtain the out-of-plane panel deflection in the post-buckling range. The non-linear differential equations are expressed in terms of the out-of-plane displacement and the Airy function. They are solved with the Galerkin method for various boundary conditions by imposing an edge displacement control. The theoretical and experimental results obtained by the present analysis show that the transverse load can greatly influence the buckling loads and halfwave number. Since no experimental results have been found in the literature, several tests have been carried out on graphite/epoxy blade stiffened panels 900 mm long and 620 mm wide applying simultaneously biaxial compression loads with several combined ratios. An eccentricity results between longitudinal and transverse load, because the longitudinal compression is applied along the centroidal axes of the stiffened section while the transverse compression is applied to the skin panel. The correlation between the experimental and analytical results has been quite good; the experimental results demonstrate the influence of eccentricity of the transverse load on panel deflection in the pre- and post-buckling range.     

Microcavity‐Stabilized Quantum Cascade Laser

Narrow‐linewidth lasers are key elements in optical metrology and spectroscopy. Spectral purity of these lasers determines accuracy of the measurements and quality of collected data. Solid state and fiber lasers are stabilized to relatively large and complex external optical cavities or narrow atomic and molecular transitions to improve their spectral purity. While this stabilization technique is rather generic, its complexity increases tremendously moving to longer wavelenghts, to the infrared (IR) range. Inherent increase of losses of optical materials at longer wavelengths hinders realization of compact, room temperature, high finesse IR cavities suitable for laser stabilization. In this paper, we report on demonstration of quantum cascade lasers stabilized to high‐Q crystalline mid‐IR microcavities. The lasers operating at room temperature in the 4.3‐4.6 μm region have a linewidth approaching 10 kHz and are promising for on‐chip mid‐IR and IR spectrometers.

     

Gelation of waxy crude oils by ultrasonic and dynamic mechanical analysis

A novel technique, based on low intensity ultrasonic wave propagation, has been applied to investigate the gelation of a waxy crude oil, caused by the crystallization of paraffin fractions as the temperature reduces below a threshold value, called WAT (wax appearance temperature). Because this phenomenon significantly affects the rheological behavior of crude oils, the knowledge of the conditions under which it occurs, during oil storage and/or transportation, is a topical issue in the oil industry. In this work, an ultrasonic equipment has been set up, able to propagate longitudinal waves in the MHz range and to display in real time the behavior of ultrasonic velocity and attenuation when the crude oil sample is subjected to heating and cooling cycles. When the ultrasonic probes alternatively rotate as parallel plates of a conventional rheometer, low intensity longitudinal waves (in the megahertz range), and shear oscillations (in the hertz range) are simultaneously applied on the sample, thus widely broadening the frequency range of investigation. On cooling, the crystallization of paraffin fractions and the consequent formation of a network structure in the oil matrix are responsible of the development and growth of the crude oil elastic response, which becomes dominant over the viscous response. This process can be reliably detected by dynamic mechanical analysis and by ultrasonic analysis through the increase of the storage modulus G′ and longitudinal velocity, respectively. The growth and further association between wax crystals causes a dissipation of acoustic energy, which is indicated by the increase of the wave attenuation. The combination of rheological and ultrasonic methods has provided a better insight both on the gel transition of crude oils and the viscoelastic behavior of gelled samples. The ultrasonic wave propagation has demonstrated to be a powerful tool for monitoring the sol–gel transition in waxy crude oils. Finally, the effect of ultrasonic waves with different intensity on the gel build-up has been also evaluated. A reduction of the gel strength with increasing wave intensity has been observed and the recovery of elastic response after removing ultrasonic irradiation has been monitored. This paper was presented at Annual European Rheology Conference (AERC) held in Hersonisos, Crete, Greece, April 27–29, 2006.     

Drift Estimation of Multiscale Diffusions Based on Filtered Data

We study the problem of drift estimation for two-scale continuous time series. We set ourselves in the framework of overdamped Langevin equations, for which a single-scale surrogate homogenized equation exists. In this setting, estimating the drift coefficient of the homogenized equation requires pre-processing of the data, often in the form of subsampling; this is because the two-scale equation and the homogenized single-scale equation are incompatible at small scales, generating mutually singular measures on the path space. We avoid subsampling and work instead with filtered data, found by application of an appropriate kernel function, and compute maximum likelihood estimators based on the filtered process. We show that the estimators we propose are asymptotically unbiased and demonstrate numerically the advantages of our method with respect to subsampling. Finally, we show how our filtered data methodology can be combined with Bayesian techniques and provide a full uncertainty quantification of the inference procedure.

     

Optical co-ordinates in general relativity 1 — An axiomatic approach

Summary An optical observer in the space-time manifoldV ₄ is defined as a time-like world linea carrying an orthonormal tetrad of vector fields e_(i) (i=0, 1, 2, 3) with e₍₀₎ always tangent toa. To the world linea we associate the congruence of null geodesics outgoing from points ofa and directed towards the past. The resulting geometrical structure is analysed in detail.

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Sommario Un osservatore ottico nella varietà spaziotempoV ₄ è definito come linea di universoa, dotata di una tetrade ortonormale di campi vettoriali e_(i) (i=0, 1, 2, 3), con e₍₀₎ sempre tangente ada Alla linea di universoa è associata la congruenza di geodetiche nulle uscenti da punti dia e dirette verso il passato. La struttura geometrica che ne risulta è esaminata in dettaglio.

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Work performed under the auspices of the «Gruppo Nazionale per la Fisica Matematica» (C.N.R.).