The study of stabilizer action modes by means of time-resolved techniques: Intermediate radical cations

In organic systems, radiation- and photo-induced electron transfer reactions result in the first step in the formation of radical cations. With pulse radiolysis and laser photolysis experiments radical cations of phenols could be indirectly characterized whereas radical cations of sterically hindered amines (HALS compounds) could be directly observed.     

Peptide synthesis by means of immobilized enzymes

-Chymotrypsin covalently bound to silica, enzacryl AA, and enzacryl AH catalyzes peptide bond formation between N-protected dipeptide methyl esters and H-Leu-NH₂ with results similar to those with the free enzyme. The influence of water-miscible and water-immiscible cosolvents, of the supports, and of the structure of the substrates is shown to be of importance for the ease of the chymotrypsin-medicated coupling reactions. The best yields were obtained using biphasic aqueous-organic solvent mixtures, silica-bound chymotrypsin, and substrates with leucine in the P₂-position. The yields of the syntheses are discussed in terms of the reactivity of substrates with similar structure in enzymatic hydrolyses. All the immobilized chymotrypsin preparations could be re-utilized successfully for further couplings. Abbreviations: IUPAC/IUB rules for peptides are followed, see Eur. J. Biochem.27, 201 (1972); Glt=4-carboxybutyrul (glutaryl),-Nan=4-nitroanilide. All amino acids except glycine are of L-configuration.     

Understanding enzyme catalysis by means of supramolecular artificial enzymes

Enzymes are biomacromolecules responsible for the abundant chemical biotransformations that sustain life. Recently, biochemists have discovered that multiple conformations and numerous parallel paths are involved during the processes catalyzed by enzymes. It is plausible that the entire macromolecular scaffold is involved in catalysis via cooperative motions that result in incredible catalytic efficiency. Moreover, some enzymes can very strongly bind the transition state with an association constant of up to 1024 M-1, suggesting that covalent bond formation is a possible process during the conversion of the transition state in enzyme catalysis, in addition to the concatenation of noncovalent interactions. Supramolecular chemistry provides fundamental knowledge about the relationships between the dynamic structures and functions of organized molecules. By tak-ing advantage of supramolecular concepts, numerous supramolecular enzyme mimics with complex and hierarchical structures have been designed and investigated. Through the study of supramolecular enzyme models, a great deal of information to aid our understanding of the mechanism of catalysis by natural enzymes has been acquired. With the development of supramolec-ular artificial enzymes, it is possible to replicate the features of natural enzymes with regards to their constitutional complexity and cooperative motions, and eventually decipher the conformation-based catalytic mystery of natural enzymes.     

Modeling the influence of a laser pulse on the potential energy surface in optimal molecular control theory

Understanding and modeling the interaction between light and matter is essential to the theory of optical molecular control. While the effect of the electric field on a molecule's electronic structure is often not included in control theory, it can be modeled in an optimal control algorithm by a set or toolkit of potential energy surfaces indexed by discrete values of the electric field strength where the surfaces are generated by Born-Oppenheimer electronic structure calculations that directly include the electric field. Using a new optimal control algorithm with a trigonometric mapping to limit the maximum field strength explicitly, we apply the surface-toolkit method to control the hydrogen fluoride molecule. Potential energy surfaces in the presence and absence of the electric field are created with two-electron reduced-density-matrix techniques. The population dynamics show that adjusting for changes in the electronic structure of the molecule beyond the static dipole approximation can be significant for designing a field that drives a realistic quantum system to its target observable.     

Understanding optimal control results by reducing the complexity

Deciphering control mechanisms from control pulse structures found in closed-loop learning experiments is often complicated due to the complexity of the pulse structure. Simplification of pulse forms is demonstrated by systematically reducing the complexity of the search space, applied on the model-like multi-photon ionization of NaK. Reducing the pulse complexity leads to the exclusion of participating excited states, thereby restricting the involved pathways. The phase function is parameterized by a sinusoidal spectral phase modulation, whose parameters are investigated with respect to the yield and the obtained optimal field. By progressively reducing the number of parameters and thereby the complexity of the phase modulation, control pulses are generated which are more and more reduced to the molecule’s primary dynamical properties. This enables to find optimized control pulses that can be subject to a simple intuitive interpretation.     

Study of the electro-adsorptive behaviour of the herbicide nitralin by means of voltammetric techniques

The electroanalytical behaviour of the herbicide Nitralin has been studied by means of different voltammetric techniques. Nitralin is adsorbed on the mercury electrode and gives 2 reduction waves at -0.6 and -0.7 V versus Ag AgCl reference electrode, for pH values > 10. The electrochemical process is irreversible and strongly influenced by pH solution. A systematic study of the several instrumental and accumulation variables affecting the stripping response was carried out using square wave (SWV; Osteryoung's method) and differential pulse voltammetry (DPV) as redissolution techniques. The limits of detection were 4.4 x 10(-1) mol 1(-1) (AdS-SWV) and 3.5 x 10(-11) mol 1(-1) (AdS-DPV), with variation coefficients of 4.17 and 3.90% respectively, at a concentration level of 8 x 10(-9) mol 1(-1) (n = 10). A method, based on AdS-DPV, for the determination of Nitralin in ground-water is proposed. A detection limit of 8.7 x 10(-11) mol 1(-1) (AdS-DPV) was reached in real samples.     

Determination of gallium in tumour-affected tissues by means of spectroscopic techniques

A comparative investigation was carried out on the suitability of atomic absorption spectrometry and of emission spectrography with arc and hollow-cathode excitation sources for determination of gallium (μg g^-1) in biological samples. The three methods give reliable results. Hollow-cathode emission spectrography was found to be influenced to a lesser extent by matrix effects and to be more precise than the other two techniques.     

Quantum control by means of hamiltonian structure manipulation

A traditional quantum optimal control experiment begins with a specific physical system and seeks an optimal time-dependent field to steer the evolution towards a target observable value. In a more general framework, the Hamiltonian structure may also be manipulated when the material or molecular 'stockroom' is accessible as a part of the controls. The current work takes a step in this direction by considering the converse of the normal perspective to now start with a specific fixed field and employ the system's time-independent Hamiltonian structure as the control to identify an optimal form. The Hamiltonian structure control variables are taken as the system energies and transition dipole matrix elements. An analysis is presented of the Hamiltonian structure control landscape, defined by the observable as a function of the Hamiltonian structure. A proof of system controllability is provided, showing the existence of a Hamiltonian structure that yields an arbitrary unitary transformation when working with virtually any field. The landscape analysis shows that there are no suboptimal traps (i.e., local extrema) for controllable quantum systems when unconstrained structural controls are utilized to optimize a state-to-state transition probability. This analysis is corroborated by numerical simulations on model multilevel systems. The search effort to reach the top of the Hamiltonian structure landscape is found to be nearly invariant to system dimension. A control mechanism analysis is performed, showing a wide variety of behavior for different systems at the top of the Hamiltonian structure landscape. It is also shown that reducing the number of available Hamiltonian structure controls, thus constraining the system, does not always prevent reaching the landscape top. The results from this work lay a foundation for considering the laboratory implementation of optimal Hamiltonian structure manipulation for seeking the best control performance, especially with limited electromagnetic resources.     

The assessment of radiochemical separation procedures by means of computational techniques

The behaviour of 66 radio-ions towards 11 exchangers and 11 solutions under standardized experimental conditions has been evaluated and classified in order to have the possibility of a rapid assessment of radiochemical separation procedures. Two systems of utilizing the bulk of data obtained have been devised and employed. The first is based on the use of a deck of cards similar to those used for fast retrieval of bibliographic information, in which the behaviour of ions is coded by means of punched holes; the second is based on the treatment of the data by means of an electronic computer. The operational modes of the two systems and examples of their practical applications are presented.     

Emerging drugs: mechanism of action,mass spectrometry and doping control analysis

The number of compounds and doping methods in sports is in a state of constant flux. In addition to ‘traditional’ doping agents, such as anabolic androgenic steroids or erythropoietin, new therapeutics and emerging drugs have considerable potential for misuse in elite sport. Such compounds are commonly based on new chemical structures, and the mechanisms underlying their modes of action represent new therapeutic approaches arising from recent advances in medical research; therefore, sports drug testing procedures need to be continuously modified and complementary methods developed, preferably based on mass spectrometry, to enable comprehensive doping controls. This tutorial not only discusses emerging drugs that can be categorized as anabolic agents (selective androgen receptor modulators, SARMs), gene doping [hypoxia‐inducible factor stabilizers, peroxisome‐proliferator‐activated receptor (PPAR)δ‐agonists] and erythropoietin‐mimetics (Hematide) but also compounds with potentially performance‐enhancing properties that are not classified in the current list of the World Anti‐Doping Agency. Compounds such as ryanodine‐calstabin‐complex modulators (benzothiazepines) are included, their mass spectrometric properties discussed, and current approaches in sports drug testing outlined. Copyright © 2009 John Wiley & Sons, Ltd.     

On-line measurement of extracellular enzymes during fermentation by using membrane techniques

The measurement of enzymes during fermentation is of general interest for process control in industrial biotechnology. Studies are presented to develop an on-line measurement for extracellular hydrolases by using membrane separations as the main unit operation, first for achieving continuous sampling from the bioreactor prior to analysis, and secondly for measuring hydrolytic activities, by separating the cleavage products during reaction of enzymes with high-molecular-weight substrates by an ultrafiltration membrane. The evaluation of batchwise and continuous flow analysis for an alkaline protease and pullulanase are described, and the feasibility of on-line measurement during fermentation of Bacillus licheniformis and Klebsiella pneumoniae on a 30- and 70-l pilot scale is demonstrated, including aseptic continuous sampling.     

Different techniques for determining the coating weight of phosphate layers on galvanized steel by means of FT-IR-spectrometry

The determination of the weight per unit area (“coating weight”) of phosphate layers on galvanized steel by means of FT-IR-spectrometry has been performed by three different techniques: (i) principal component regression (PCR) of absorption data, (ii) utilizing the thickness dependent wavenumber shift of a band due to a special optical surface effect observed with parallel-polarized radiation and (iii) of an interference band observed with perpendicular-polarized radiation.     

Characterization of immobilized enzymes by flow-injection techniques with variable forward flow

The use of variable forward flow with small packed enzyme reactors is shown to be valuable for improving the efficiency of enzymatic conversion. Designs with stopped flow, oscillating flow and variable (fast/slow/fast) flow are compared for the spectrophotometric determination of glucose with glucose oxidase and horseradish peroxidase immobilized on controlled-pore glass in the same reactor. Variable forward flow increased the sensitivity considerably without excessive time consumption. The technique is also useful for characterizing the activity of immobilized enzyme reactors, e.g., peroxidase reactors for hydrogen peroxide determinations.     

Investigation of various SiC materials by means of EPMA/WDS techniques

The microstructure and compositions of SiC materials from different sources and processing routes were investigated by means of EPMA/WDS and image analysis techniques. The influence of various sources of errors like carbon contamination and spectrometers defocusing on the analysis has been assessed.The presence of dissolved sintering aids, or impurities, and their distributions were investigated by EPMA/WDS. In addition, inhomogeneities, porosity agglomerations and heterogeneous inclusions were found in almost all the SiC materials, which are known to influence the corrosion and mechanical behavior of the material.Quantification of secondary phase contents was performed by means of image analysis, EPMA and, when possible, by density measurement. All methods are affected by errors of difficult assessment. In particular, the EPMA/WDS technique has to handle the problem of non homogeneous volumes of analysis. Two quantitative approaches were attempted, both based on the averaging of many points. In the first, the beam was highly focused. In the second approach, large areas (from 10 to 50 m in diameter) were illuminated. The errors and limits of these methods are discussed and the results compared.     

Analysis of antiepileptic drugs in biological fluids by means of electrokinetic chromatography

An overview of the electrokinetic chromatographic methods for the analysis of antiepileptic drug levels in biological samples is presented. In particular, micellar electrokinetic capillary chromatography is a very suitable method for the determination of these drugs, because it allows a rapid, selective, and accurate analysis. In addition to the electrokinetic chromatographic studies on the determination of antiepileptic drugs, some information regarding sample pretreatment will also be reported: this is a critical step when the analysis of biological fluids is concerned. The electrokinetic chromatographic methods for the determination of recent antiepileptic drugs (e.g., lamotrigine, levetiracetam) and classical anticonvulsants (e.g., carbamazepine, phenytoin, ethosuximide, valproic acid) will be discussed in depth, and their pharmacological profiles will be briefly described as well.     

Improving solid-state NMR dipolar recoupling by optimal control

We present the first solid-state NMR experiments developed using optimal control theory. Taking heteronuclear dipolar recoupling in magic-angle-spinning NMR as an example, it proves possible to significantly improve the efficiency of the experiments while introducing robustness toward instrumental imperfections such as radio frequency inhomogeneity. The improvements are demonstrated by numerical simulations as well as practical experiments on a 13Calpha,15N-labeled powder of glycine. The experiments demonstrate a gain of 53% in the efficiency for 15N to 13Calpha coherence transfer relative to the typically double-cross-polarization experiments.     

Thiamine models and perspectives on the mechanism of action of thiamine-dependent enzymes

Thiamine dependent enzymes catalyze ligase and lyase reactions near a carbonyl moiety. Chemical models for these reactions serve as useful tools to substantiate a detailed mechanism of action. This tutorial review covers all such studies performed thus far, emphasizing the role of each part around the active site and the conformation of the cofactor during catalysis.     

Modeling boiling points of cycloalkanes by means of iterated line graph sequences

A class of models for predicting boiling points of cycloalkanes is put forward, based on iterated line graphs L(i), i = 1, 2,., of the molecular graph G = L(0). Let m(i) be the number of edges of L(i), i = 0, 1, 2,. The models analyzed are of the form a(0)m(i)()(0) + a(1)m(i)(1) + a(2)m(i)(2) +. + a(k)m(ik) + b. Our optimal QSPR formulas contain m(0), m(1), m(2), m(3), and/or m(4) but never m(5) and m(6). Their precision is as good as or better than the approximations recently reported by Rücker and Rücker (J. Chem. Inf. Comput. Sci. 1999, 39, 788-802).