Non-symmetric Arrangement Research Articles

Airflow collision characteristics of double square column attachment ventilation

In the square column attachment ventilation (S-CAV) mode, the introduction of multiple airflows attached to the column into the horizontal diffusion zone inevitably leads to various “airflow collision” phenomena. However, after the floor-attached airflow collides, the indoor air distribution may deviate from the design goal. In this paper, the layout of the axisymmetric and non-axisymmetric arrangement of double columns is summarized. The airflow collision mechanism and flow characteristics for different column layouts are analyzed via experiments and computational fluid dynamics (CFD) simulations, and a new partition form is proposed to more effectively control the indoor environment. A 1:2 scale model was employed for the velocity verification experiments and flow field visualization. The results indicated that the airflow collision area can be divided into impact, transition, and collision zones with axisymmetric double-column layouts. In the range of 1–3 m/s, any variation in the supply air velocity, irrespective of the occurrence of airflow collision, may be considered negligible with respect to its impact on the attenuation trend of the axial velocities. At the same absolute value of the air supply temperature difference, the cooling condition experiences a 3.1 times greater kinetic energy loss due to collision than does the heating condition. In a nonsymmetric arrangement of two columns, the curve formed by the stagnation point rotates, and the direction of rotation is biased toward the side of the column that is farther away. The findings provide a solid theoretical basis for the engineering application and optimal design of CAV modes.

Thermal calculation of the radiation chamber of a hydrocarbon pyrolysis furnace with a non-symmetric arrangement wall-mounted burners

RELEVANCE. The possibility of predicting the operating conditions of individual elements of high-temperature technological units of petrochemical industries even at the design or modernization stage is relevant for many reasons. THE PURPOSE. Carrying out numerical studies of thermal parameters and features of the turbulent movement of flue gases in the radiative part of the hydrocarbon pyrolysis furnace with a central arrangement of coils with an asymmetric arrangement of burners of low thermal power on the side lined walls and on the vault of the chamber. Such furnaces are used to produce lower olefins, which are the primary products for the production of synthetic resins, rubbers, plastics and fibers. METHODS. In the radiant chamber of the furnace, interrelated processes of combustion of gaseous fuel, turbulent flow of combustion products, radiant-convective heat exchange and cracking reactions of hydrocarbons in tubular coils occur with the formation of a mixture of light hydrocarbons rich in olefins. The formation of pyrolysis products of hydrocarbons becomes essential when the temperature of the vapor-gas mixture in tubular reactors is within 800-855 oC in the presence of dilution steam.. The heat required for this will be obtained mainly due to the thermal radiation of the combustion products and the hot lined surfaces of the radiation chamber. The physical processes taking place in the combustion chamber are modeled by two-dimensional equations of the model gorenje hydrocarbons in the air, energy transfer by radiation and equations of motion. The package of applied programs is used, which is based on the numerical solution of the mentioned system of transfer equations. As a result of numerical studies, the velocity and temperature fields of flue gases formed during the combustion of a fuel gas mixture in the furnace chamber of a tube furnace were constructed. In his work, it is assumed that on one side wall of the radiation chamber, wall burners in the amount of 64 pieces are placed in eight horizontal rows, and on the other wall of the chamber, the same burners are installed in seven tiers and one row of burners on the vault of the chamber. The combustion products emanating from these burners form complex velocity and temperature fields in the volume in both halves of the radiation chamber. RESULTS. As a result of numerical calculations, the fields of temperature and flue gas velocities in both parts of the radiation chamber are constructed. The temperatures of the inner surfaces of the lining walls are calculated. The distributions of the surface densities of radiant heat fluxes to the reaction pipes along the height of the pyrolysis furnace of the propane-butane fraction are determined. Comparisons of some of the results obtained were carried out for cases when all burners are installed only on the side walls of the chamber and with the above arrangement of burners. CONCLUSION. Calculations show that the use of a large number of low-power wall burners leads to the emergence of complex velocity and temperature fields in the radiation chambers of tubular furnaces. At the same time, the spread of temperature values in the volume of the furnace chamber is much smaller than for the case when all burners of higher power are installed only on the vault and on the hearth of the furnace. By changing the location of the tiers of burners, it is possible to achieve a relatively uniform supply of heat to the heated product along the length of the pyro coil.

Stress state analysis and symmetrical optimization of double-row non-symmetrical arrangement of anchorage slot in concrete lining structure

The role of the anchorage slot is to fix the anchorage and tension anchor rope, which is the weakest link of the concrete lining structure. The stress state of the anchorage slot area in the construction and operation stages of the 2# sand discharge of the Xiaolangdi Project was analyzed by modeling in this paper. The results showed that the stress state of the anchorage slot area was the most complex. The tensile stress was generated in the radial direction of the concrete lining during the construction period, and the circumferential pre-compression stress was unevenly distributed along the thickness direction. During the operation period, when the backfill concrete at the anchor slot and the lining jointly bore the internal water pressure, a tensile stress of 0.89 MPa was generated in the inner anchorage slot area of the lining, which was less than the tensile strength of the concrete itself. Finite element calculation results were compared with the field-measured data, and the results were essentially the same. Finite element analysis software was used to optimize the double-row asymmetric arrangement of the anchorage slots into a single-row symmetrical structure. The results showed that the angle of the complex stress area of the lining is reduced by about 75%. The optimized single-row symmetrical anchorage slot layout was safer, and the pre-stressing distribution was more uniform.

Velocity of Flow on Regular Non-Homogeneous Open One-Dimensional Net with Non-Symmetrical Arrangement of Nodes

Velocity of Flow on Regular Non-Homogeneous Open One-Dimensional Net with Non-Symmetrical Arrangement of Nodes

Overlapped joints in Textile Reinforced Concrete with UHPC matrix: An experimental investigation

When producing a Textile Reinforced Concrete structure or element, joining separate textile layers might be a necessity, driven for example by the limited dimensions of commercially available fabrics. A possible way of producing such joints is by overlapping different textile sheets. Overlapped joints, however, need to be cast with particular attention since they might represents weak elements of the structure, leading to premature failure. An experimental campaign was performed, aimed at identifying the effects of a symmetric vs non-symmetric arrangement of the textile fabrics within the overlapping length and tensile characteristics of the matrix on such type of joints. Fifteen specimens, produced using a fully epoxy impregnated carbon textile fabric and an Ultra High Performance Concrete (UHPC) matrix, were tested under tension in a uniaxial setup and measurements were performed using a Digital Image Correlation system. The in-plane and out-of-plane behaviour of each specimen was studied. The results highlight the importance of producing symmetric elements as well as the beneficial effects that the admixture of short dispersed steel fibres to the cementitious matrix provide to such kind of joints

Optimizing a coarse-grained space for approximate normal-mode vibrations of molecular heterodimers.

We applied the method of coarse-graining the intermolecular vibrations to molecular heterodimers assembled by double hydrogen bonding. This method is based on principal component analysis, by which the original atomic displacement vectors are projected onto a lower-dimensional space spanned by a basis set of translations, librations, and intramolecular vibrations of the constituent molecules. Compared with homodimers, the following points are particularly noted: (1) alignment of the constituent molecules in a non-symmetric atomic arrangement of the whole system and (2) the scheme of reordering the bases to construct an optimal coarse-grained space. We tested three schemes for reordering the intramolecular vibration vectors to determine that the best one is equivalent to size reduction based on the singular value decomposition. The coarse-graining analysis affords three parameters, Φintra, Φinter, and Φapp, which are relevant to the mechanical nature of the molecular assembly. The Φintra values account for the internal stiffness of molecules, while the Φinter values are true stiffness constants of the intermolecular force and show a good correlation with the association energies of the dimers. The Φapp values are the apparent intermolecular stiffness smaller than Φinter, as a result of compensation for neglecting intramolecular vibrations. All these values are consistent with each other under the coupled oscillator model, showing that the present coarse-graining analysis is valid for heterodimers as well as homodimers.

Supramolecular Chirality in Azobenzene-Containing Polymer System: Traditional Postpolymerization Self-Assembly Versus In Situ Supramolecular Self-Assembly Strategy.

Recently, the design of novel supramolecular chiral materials has received a great deal of attention due to rapid developments in the fields of supramolecular chemistry and molecular self-assembly. Supramolecular chirality has been widely introduced to polymers containing photoresponsive azobenzene groups. On the one hand, supramolecular chiral structures of azobenzene-containing polymers (Azo-polymers) can be produced by nonsymmetric arrangement of Azo units through noncovalent interactions. On the other hand, the reversibility of the photoisomerization also allows for the control of the supramolecular organization of the Azo moieties within polymer structures. The construction of supramolecular chirality in Azo-polymeric self-assembled system is highly important for further developments in this field from both academic and practical points of view. The postpolymerization self-assembly strategy is one of the traditional strategies for mainly constructing supramolecular chirality in Azo-polymers. The in situ supramolecular self-assembly mediated by polymerization-induced self-assembly (PISA) is a facile one-pot approach for the construction of well-defined supramolecular chirality during polymerization process. In this review, we focus on a discussion of supramolecular chirality of Azo-polymer systems constructed by traditional postpolymerization self-assembly and PISA-mediated in situ supramolecular self-assembly. Furthermore, we will also summarize the basic concepts, seminal studies, recent trends, and perspectives in the constructions and applications of supramolecular chirality based on Azo-polymers with the hope to advance the development of supramolecular chirality in chemistry.

The synthesis and photophysical properties of tris-coumarins.

A structurally unique cyclic tris-coumarin possessing three identical coumarin units bridged by amide linkers as well as two linear analogs has been synthesized. There is a remarkable agreement between crystallographic data, 1H NMR and results of calculations for the cyclic tris-coumarin, showing in all cases a non-symmetric arrangement of identical coumarin moieties. Weak polarization of the coumarin subunits, resulting from the presence of only CONH- groups as electron-donors, results in a hypsochromic shift of both absorption and emission in this dye. We have proven that in non-cyclic, head-to-tail linked tris-coumarins, the photophysics is controlled not only by the substituents but also by the conformation of the molecule, which in turn depends on the nature of the linker's interactions. These can be controlled by the presence/absence of an amide-type hydrogen atom responsible for the formation of intramolecular hydrogen bonds. The presence of a hydrogen bond favors a stretched trans conformation of the dye, while in its absence, folding of the molecule occurs leading to a more compact conformation. Although, the increased number of covalently linked coumarin units does not drastically change the preferred conformation, the fluorescence quantum yields of tris-coumarins are significantly lower than for analogous bis-coumarins composed of the same units.

Features of tensoresistance depending on the crystallographic orientation of γ-irradiated (60Co) germanium and silicon single crystals

The features of the longitudinal tensoresistance of γ-irradiated (60Co) n-Ge and n-Si crystals, as well as γ-irradiated n-Ge crystals after n→p conversion, at fixed temperatures depending on the direction (X→∥J→∥ [111,110], [100]) of application of the mechanical compressive stress 0≤X≤1.2GPa were investigated. The charge carrier concentrations and the Hall mobility values before and after γ-irradiation were controlled by measurements of the Hall effect. It was established that under conditions of the nonsymmetrical arrangement of deformation axis relative to the isoenergetic ellipsoids, the dependences of tensoresistance in the γ-irradiated n-Ge and n-Si crystals pass through a maximum. With a symmetrical placement of the deformation axis such maximum is not observed. In the converted n-Ge crystals under applying of mechanical stress the presence of the region of the increasing resistivity in the initial area of deformation was found, which is explained by increase of the energy gap between the deep level and the top of the valence band with increasing pressure.

Physicochemical behaviour of a dinuclear uranyl complex formed with an octaphosphinoylated para-tert-butylcalix[8]arene. Spectroscopic studies in solution and in the solid state

Spectrophotometric titrations of an octaphosphinoylated para-tert-butylcalix[8]arene (B8bL8) by uranyl nitrate and vice versa in anhydrous ethanol indicate that the species with 2:1 (uranyl:calixarene) stoichiometry is the major complex in solution. Based on these results, a synthesis route was designed to isolate this complex. The latter is an orange, non-hygroscopic polycrystalline powder, with chemical formula [(UO2)2(NO3)4(B8bL8)·2H2O]2(H2O). (Compd. 1), as ascertained by elemental analysis. Spectroscopic characterization of Compd. 1 in the solid and liquid states suggests that a neutral dinuclear uranyl calixarene complex was formed. MIR and FIR spectra indicate that, four phosphinoyl arms of the calixarene and two monodentate nitrates are bound to each 6-coordinate uranyl ion in the complex because no vibrational frequencies from un-coordinated OP groups or from ionic nitrates are present; in addition the spectra reveal that water molecules form intramolecular hydrogen bonding with monodentate nitrates. The de-convoluted luminescence and XPS spectra obtained in the solid state point to a similar chemical environment around each uranyl ion, as confirmed by the mono-exponential decay of the luminescence. The more rigid conformation acquired by the calixarene in the complex and the non-symmetrical arrangement of the coordinated nitrates result in a particular feature of the emission spectra at 77K. No evidence of cation-cation interaction was found. A rough approach to the molecular structure of the complex by molecular modelling based on the experimental findings yielded a molecule that was useful for understanding the physicochemical behaviour of Compd. 1.

The behaviour of water droplets on the silicone rubber surface in an electric field

This paper describes the influence of a water droplet placed on flat samples of silicone rubber for enhancement the local electric field and generate electrical discharges. Studies have shown a significant influence of the droplet geometry on the electric strength of the samples. For non-symmetrical arrangement of the three droplets in the inter-electrode space electrohydrodynamic phenomena was observed: a stable change in the droplets shape placed near the electrodes and stretching and tearing down of the water droplets placed far from the electrodes. Captured photos and films of the water droplets behavior placed on the surface of the samples provided data to perform the simulation of the distribution of electric field and an estimate the value of the electric field, which was followed by the development of electric surface discharges.

Elastic strain energy induced split during precipitation in alloys

The elastic strain energy induced split during the precipitation of the isostructural transformation in the modeled A–B alloys is simulated by the phase field method. The simulation implies that for the single precipitating particle in the modeded alloy, the particle with initial scale in the range from 80l to 320l (l=12.18 Å) is to split during aging. For the modeled alloy, the γ′ particle is to split at about 5.0×103τ (τ=4.45 s) after the split incubation period. At the end of split the total interfacial energy obtains a maxima and the total elastic energy holds a minima at about 2.4×104τ. The requirement of the four-blocks split may be given by the relationship between the elastic strain energy coefficient, the positive gradient energy coefficient and the solute concentration inhomogeneity. In multi-particles systems, the particles split into other appearances due to the additional interaction energy induced by the non-symmetrical arrangement of the particles, The increment of the interaction elastic energy has little effect on the interfacial energy during precipitation in multi-particles systems.

Tetrahydrothiophene and Tetrahydrofuran, Computational and X-ray Studies in the Crystalline Phase

Calculations at various levels of theory with different methods and respective evaluations confirm that the twist conformation (C2) is preferred for tetrahydrothiophene (THT) in the gas phase. In the crystalline phase, achieved by a laser assisted crystallization device, THT has C1 symmetry (slightly distorted C2 symmetry) in the chiral space group P212121. This is obviously a packing effect caused by the nonsymmetrical arrangement of neighboring molecules. The distortion from C2 symmetry costs very little energy as confirmed by computational methods in the gas phase. Only one enantiomer of the chiral THT is found in the cell which requires spontaneous crystallization, which results in a racemic mixture of crystals, or a racemization occurs prior to/during nucleation or in the “embryonic” state. The racemization happens by a mechanism that can be described as a partial pseudo rotation within a five-membered mono-heterocycle with a C2–CS–C2′ transition (C2 and C2′ are enantiomers) maintaining the heteroatom residing within the symmetry elements. While THT has the molecular symmetry of the gas phase almost also in the crystalline phase, THF has an envelope conformation (CS). This was also established by calculations at various levels of theory which agrees well with the previously experimentally found conformation by electron diffraction. However, in the X-ray crystal structure, previously determined by Luger & Buschmann, THF has C2 symmetry in the centrosymmetric space group C2/c with the oxygen atom situated on the crystallographic C2 polar axis, requesting a racemic crystal for the twisted conformers of the enantiomers. No solid-state phase transitions were detected within the experimental ranges for THT and THF. Following the stabilization by molecular clustering, and ending at the crystal lattice, we stepwise increased the number of molecules by calculation of the respective monomers, dimers, trimers, and tetramers for THF and THT. The starting point was taken from the arrangements as found in the respective crystal structures. Both conformational enantiomers are equal in energy. In such cases, a crystal may contain either a racemate of conformers or one of the conformational enantiomers only. The first case is observed in THF, the latter one in THT. It is quite likely that the selection of one enantiomeric conformer of THT from an equilibrium of conformers at the early stage of nucleation (“embryonic” stage) is responsible for the spontaneous crystallization. In order to check if THF could form a polymorph with the molecular packing of THT and vice versa, we first calculated THF and THT in their respective crystal lattices as determined by X-ray diffraction. Exchanging the compounds in the THT and THF crystal lattices (i.e., replacing O against S and vice versa) results in significantly worse lattice energies indicating that such a polymorph is not a probable option.

Multi-physics computational grains (MPCGs) for direct numerical simulation (DNS) of piezoelectric composite/porous materials and structures

Conceptually simple and computationally most efficient polygonal computational grains with voids/inclusions are proposed for the direct numerical simulation of the micromechanics of piezoelectric composite/porous materials with non-symmetrical arrangement of voids/inclusions. These are named "Multi-Physics Computational Grains" (MPCGs) because each "mathematical grain" is geometrically similar to the irregular shapes of the physical grains of the material in the micro-scale. So each MPCG element represents a grain of the matrix of the composite and can include a pore or an inclusion. MPCG is based on assuming independent displacements and electric-potentials in each cell. The trial solutions in each MPCG do not need to satisfy the governing differential equations, however, they are still complete, and can efficiently model concentration of electric and mechanical fields. MPCG can be used to model any generally anisotropic material as well as nonlinear problems. The essential idea can also be easily applied to accurately solve other multi-physical problems, such as complex thermal-electro-magnetic-mechanical materials modeling. Several examples are presented to show the capabilities of the proposed MPCGs and their accuracy.

NMR, DFT and luminescence studies of the complexation of Al(iii) with 8-hydroxyquinoline-5-sulfonate

Multinuclear ((1)H, (13)C and (27)Al) magnetic resonance spectroscopy (1D and 2D), DFT calculations and fluorescence have been used to study the complexation of 8-hydroxyquinoline-5-sulfonate (8-HQS) with Al(III). The study combines the high sensitivity of luminescence techniques, the selectivity of multinuclear NMR spectroscopy with the structural details accessible through DFT calculations, and aims to provide a detailed understanding of the complexation between the Al(3+) ion and 8-HQS. A full speciation study has been performed and over the concentration region studied, the Al(3+) ion forms complexes with 8-HQS in an aqueous solution in the pH range 2-6. At higher pH, the extensive hydrolysis of the metal limits complexation. Using Job's method, three complexes were detected, with 1 : 1, 1 : 2 and 1 : 3 (metal : ligand) stoichiometries. These results are in agreement with those previously reported using potentiometric and electrochemical techniques. The geometries of the complexes are proposed based on the combination of NMR results with optimized DFT calculations. All the complexes in aqueous solutions at 25 °C are mononuclear species, and have an approximately octahedral geometry with the metal coordinated to one molecule of 8-HQS and four molecules of water (1 : 1 complex), two molecules of 8-HQS and two molecules of water mutually cis (1 : 2 complex), and to three molecules of 8-HQS in non-symmetrical arrangement (mer-isomer), for the 1 : 3 (metal : ligand) complex. On binding to Al(III), 8-HQS shows a more marked fluorescence than the weakly fluorescent free ligand. In addition, as previously noted, there are marked changes in the absorption spectra, which support the use of 8-HQS as a sensitive optical sensor to detect Al(3+) metal ions in surface waters and biological fluids. These complexes also show potential for applications in organic light emitting diodes (OLEDs).

Phase transition at 320 K in a new layered organic metal conductor (BEDT-TTF)4CoBr4(C6H4Cl2)

A new layered organic conductor θ-(BEDT-TTF)4CoBr4(1,2-C6H4Cl2) with different resistivity behaviour that has metallic conducting layers and is semiconducting perpendicular to the layers has been prepared. A phase transition near 320 K was found in this salt by temperature variable resistivity and X-ray diffraction measurements. Precise crystal structure data, below transition (triclinic phase) and above transition (tetragonal phase), was determined. The crystal structures of both phases are composed of θ-type radical cation layers alternating with layers involving [CoBr4]2− anions and 1,2-dichlorobenzene molecules. The lower symmetry in the crystal structure of the triclinic phase is due to a non-symmetrical arrangement of the solvent molecules within the anion layers. In each structure, the neighbouring conducting layers of the unit cell differ in stacking directions. In the tetragonal phase, all conducting layers are similar in structure, and in the low-temperature triclinic phase, the neighbouring conducting layers differ in periods along the stacks (4.703 A and 4.993 A) and in dihedral angles θ between the BEDT-TTF radical cations of the adjacent stacks. The finding of compounds of the θ-(BEDT-TTF)4CoBr4(1,2-C6H4Cl2) type with phase transitions above room temperature provides a wide possibility of investigating magnetoresistance, quantum oscillations, and the mechanism of interlayer transport in layered conductors.

Vibrational ground state properties of H5+ and its isotopomers from diffusion Monte Carlo calculations

Diffusion Monte Carlo computations, with and without importance sampling, of the zero-point properties of H(5)(+) and its isotopomers using a recent high accuracy global potential energy surface are presented. The global minimum of the potential possesses C(2v) symmetry, but the calculations predict a D(2d) geometry for zero-point averaged structure of H(5)(+) with one H atom "in the middle" between two HH diatoms. The predicted zero-point geometries of the deuterated forms have H in the middle preferred over D in the middle and for a nonsymmetric arrangement of D atoms the preferred arrangement is one which maximizes the number of D as the triatomic ion. We speculate on the consequences of these preferences in scattering of H(2)+H(3)(+) and isotopomers at low energies, such as those in the interstellar medium.

Electronic transport properties of molecular bipyridine junctions: Effects of isomer and contact structures

The effects of contact structures and isomers on the current-voltage characteristics of molecular bipyridine junctions have been investigated by means of a generalized quantum chemical approach based on the elastic scattering Green's function method. For the four isomers $4,{4}^{\ensuremath{'}}$-bipyridine, $2,{2}^{\ensuremath{'}}$-bipyridine, $2,{4}^{\ensuremath{'}}$-bipyridine, and $2,{6}^{\ensuremath{'}}$-bipyridine under investigation, the $2,{4}^{\ensuremath{'}}$-bipyridine junction is found to be the poorest conductor owing to its nonsymmetrical arrangement. Numerical simulations of the $4,{4}^{\ensuremath{'}}$-bipyridine junction show that the contact structures between molecule and metallic electrodes have a noticeable effect on the electron transport characteristics of molecular junctions. The shortening of the distance between two metallic electrodes results in a stronger coupling and lower potential barrier between them, leading to larger conductance. The external electric fields cause the charge redistribution within molecule and lead to the presence of resistivity dipoles at the molecule-metal interface, and the bias voltages are mainly dropped there. The nonlinear charge transport effect on the current and conductance of $4,{4}^{\ensuremath{'}}$-bipyridine molecular junctions at the lower-bias region is presented. The experimental measurement has been reproduced by the theoretical calculations for a well-defined structure.

Some considerations for 3D EFP computations

This article identifies some issues, options and results associated with 3D Explosively Formed Penetrator (EFP) computations. The three broad categories of interest (for Lagrangian computations) are the sliding/contact interfaces, the type and arrangement of elements, and the generation of the grid. The sliding/contact algorithm uses a virtual particle approach that is very robust, and it is described in detail. Tetrahedral elements are considered for both symmetric and non-symmetric arrangement, each with some advantages and disadvantages. A mixed element algorithm is also considered for the non-symmetric arrangement, in an effort to eliminate some of the locking that occurs for the non-symmetric arrangement. Hexahedral (brick) elements are not considered herein, but some of the effects (for tetrahedral elements) also apply to hexahedral elements. For axisymmetric EFPs there are also some trade-offs (symmetric grid vs. equally sized elements) associated with the generation of the grid. Computational results are provided to illustrate each of the issues and effects. The virtual particle algorithm is also well suited for high-velocity impact computations, and an example is included to demonstrate this capability for a complex problem.

Kinetic studies and molecular modelling attribute a crucial role in the specificity and stereoselectivity of penicillin acylase to the pair ArgA145-ArgB263.

Kinetic experiments with a substrate series of phenylacetyl-arylamides reveal that at least one polar group in the amine moiety is required for the proper orientation of the substrate in the large nucleophile-binding subsite of penicillin acylase of Escherichia coli. Quantum mechanical molecular modelling of enzyme-substrate interactions in the enzyme active site shows that in the case of substrates lacking local symmetry, the productive binding implies two nonsymmetrical arrangements with respect to the two positively charged guanidinium residues of ArgA145 and ArgB263. This indicates a crucial role of the specified arginine pair in the substrate- and stereoselectivity of penicillin acylase.