Changes In Atomic Positions Research Articles

A Multifaceted Computational Approach to Understanding the MERS-CoV Main Protease and Brown Algae Compounds' Interaction.

Middle East Respiratory Syndrome (MERS) is a viral respiratory disease caused b a special type of coronavirus called MERS-CoV. In the search for effective substances against the MERS-CoV main protease, we looked into compounds from brown algae, known for their medicinal benefits. From a set of 1212 such compounds, our computer-based screening highlighted four-CMNPD27819, CMNPD1843, CMNPD4184, and CMNPD3156. These showed good potential in how they might attach to the MERS-CoV protease, comparable to a known inhibitor. We confirmed these results with multiple computer tests. Studies on the dynamics and steadiness of these compounds with the MERS-CoV protease were performed using molecular dynamics (MD) simulations. Metrics like RMSD and RMSF showed their stability. We also studied how these compounds and the protease interact in detail. An analysis technique, PCA, showed changes in atomic positions over time. Overall, our computer studies suggest brown algae compounds could be valuable in fighting MERS. However, experimental validation is needed to prove their real-world effectiveness.

The Intrinsic Barrier Width and Its Role in Chemical Reactivity.

Chemical reactions are in virtually all cases understood and explained on the basis of depicting the molecular potential energy landscape, i.e., the change in atomic positions vs the free-energy change. With such landscapes, the features of the reaction barriers solely determine chemical reactivities. The Marcus dissection of the barrier height (activation energy) on such a potential into the thermodynamically independent (intrinsic) and the thermodynamically dependent (Bell-Evans-Polanyi) contributions successfully models the interplay of reaction rate and driving force. This has led to the well-known and ubiquitously used reactivity paradigm of "kinetic versus thermodynamic control". However, an analogous dissection concept regarding the barrier width is absent. Here we define and outline the concept of intrinsic barrier width and the driving force effect on the barrier width and report experimental as well as theoretical studies to demonstrate their distinct roles. We present the idea of changing the barrier widths of conformational isomerizations of some simple aromatic carboxylic acids as models and use quantum mechanical tunneling (QMT) half-lives as a read-out for these changes because QMT is particularly sensitive to barrier widths. We demonstrate the distinct roles of the intrinsic and the thermodynamic contributions of the barrier width on QMT half-lives. This sheds light on resolving conflicting trends in chemical reactivities where barrier widths are relevant and allows us to draw some important conclusions about the general relevance of barrier widths, their qualitative definition, and the consequences for more complete descriptions of chemical reactions.

Doping Effect on the Electronic Structure and Heat-Storage Properties of Ti3O5

It is known that substitution of a small fraction of Ti atoms in Ti3O5 by aliovalent metal atoms alters its heat-storage properties, i.e., β → λ phase-transition enthalpy and temperature, and extends its range of application to, for example, the capture and utilization of waste heat. Heat-storage properties vary depending on the substituting element and its concentration. The exploration of the vast space of possible combinations of these quantities is challenging due to the small energy differences under consideration. Thus, computational approaches which reliably predict heat-storage properties of such defective systems can aid in the screening of potential material candidates for subsequent synthesis. Substituted compounds MxTi3–xO5 with trivalent M = Sc, Al, and Mg have been reported in the literature. Here we present the first thorough study of the doping effect on the electronic structure and the heat-storage properties of Ti3O5 from first principles. Electronic ground states were calculated for all Ti–M substitution positions using the M06 hybrid functional. Doping leads to increased metallic character in both phases, which primarily results from a change in atomic positions and not from the substituting element itself. We applied the r2SCAN-D3 method to the study of heat-storage properties of those materials and found good agreement in phase-transition enthalpies with experimentally recorded data. Calculated phase-transition entropies show larger deviations from experiment. The phase-transition mechanism is studied as a function of the defect concentration by calculating the minimum-energy path. Doping primarily changes the relative energy of both phases and leaves the activation barrier virtually unchanged. Our results suggest that heat-storage systems of this kind are efficient only for M concentrations below 4 atom %.

Manifolds of quasi-constant SOAP and ACSF fingerprints and the resulting failure to machine learn four-body interactions.

Atomic fingerprints are commonly used for the characterization of local environments of atoms in machine learning and other contexts. In this work, we study the behavior of two widely used fingerprints, namely, the smooth overlap of atomic positions (SOAP) and the atom-centered symmetry functions (ACSFs), under finite changes of atomic positions and demonstrate the existence of manifolds of quasi-constant fingerprints. These manifolds are found numerically by following eigenvectors of the sensitivity matrix with quasi-zero eigenvalues. The existence of such manifolds in ACSF and SOAP causes a failure to machine learn four-body interactions, such as torsional energies that are part of standard force fields. No such manifolds can be found for the overlap matrix (OM) fingerprint due to its intrinsic many-body character.

Cold and hot uranium in DFT calculations: Investigation by the GTH pseudopotential, PAW, and APW + lo methods

The comparison of the three DFT methods: pseudopotential of the Goedecker, Teter, and Hutter form, projected augmented waves, and augmented plane waves with localized orbitals, is presented. Uranium as one of the actinides is considered. One of the main aims of this work is to check a new uranium pseudopotential in the form proposed by Goedecker, Teter, and Hutter. Another significant point is an estimation of the correctness of PAW approach in following number of tasks: structures, energies, and pressure of four uranium phases up to threefold compression, phase transitions, electron densities at different atomic volumes, and thermal contributions (caused by changes of atomic positions) in free energy and forces of bcc uranium at twofold compression and temperatures not higher than 5000 K.

Mapping Atomic Motions with Electrons: Toward the Quantum Limit to Imaging Chemistry

Recent advances in ultrafast electron and X-ray diffraction have pushed imaging of structural dynamics into the femtosecond time domain, that is, the fundamental time scale of atomic motion. New physics can be reached beyond the scope of traditional diffraction or reciprocal space imaging. By exploiting the high time resolution, it has been possible to directly observe the collapse of nearly innumerable possible nuclear motions to a few key reaction modes that direct chemistry. It is this reduction in dimensionality in the transition state region that makes chemistry a transferable concept, with the same class of reactions being applicable to synthetic strategies to nearly arbitrary levels of complexity. The ability to image the underlying key reaction modes has been achieved with resolution to relative changes in atomic positions to better than 0.01 Å, that is, comparable to thermal motions. We have effectively reached the fundamental space-time limit with respect to the reaction energetics and imaging the acting forces. In the process of ensemble measured structural changes, we have missed the quantum aspects of chemistry. This perspective reviews the current state of the art in imaging chemistry in action and poses the challenge to access quantum information on the dynamics. There is the possibility with the present ultrabright electron and X-ray sources, at least in principle, to do tomographic reconstruction of quantum states in the form of a Wigner function and density matrix for the vibrational, rotational, and electronic degrees of freedom. Accessing this quantum information constitutes the ultimate demand on the spatial and temporal resolution of reciprocal space imaging of chemistry. Given the much shorter wavelength and corresponding intrinsically higher spatial resolution of current electron sources over X-rays, this Perspective will focus on electrons to provide an overview of the challenge on both the theory and the experimental fronts to extract the quantum aspects of molecular dynamics.

Average and local strain fields in nanocrystals

This article presents a rigorous and self-consistent comparison of lattice distortion and deformation fields existing in energy-optimized pseudo-spherical gold nanoparticles obtained from real-space and powder diffraction strain analysis techniques. The changes in atomic positions resulting from energy optimization (relaxation) of ideally perfect gold nanoparticles were obtained using molecular dynamics modeling. The relaxed atomic coordinates were then used to compute the displacement, rotation and strain components in all unit cells within the energy-optimized (relaxed) particles. It was seen that all of these terms were distributed heterogeneously along the radial and tangential directions within the nanospheroids. The heterogeneity was largest in the first few atomic shells adjacent to the nanoparticle surface, where the continuity of crystal lattice vectors originating from the interior layers was broken because of local lattice rotations. These layers also exhibited maximum shear and normal strains. These (real-space) strain values were then compared with the average lattice strains obtained by refining the computed diffraction patterns of such particles. The results show that (i) relying solely on full-pattern refinement techniques for lattice strain analysis might lead to erroneous conclusions about the dimensionality and symmetry of deformation within relaxed nanoparticles; (ii) the lattice strains within such relaxed particles should be considered `eigenstrains' (`inherent strains') as defined by Mura [Micromechanics of Defects in Solids, (1991), 2nd ed., Springer]; and (iii) the stress/strain state within relaxed nanoparticles cannot be analyzed rigorously using the constitutive equations of linear elasticity.

Microstructure, indentation and first principles study of AlCuFeMn alloy

Equi-atomic AlCuFeMn alloy with yield strength of ∼1000 MPa and elastic modulus of ∼175 GPa was synthesized using arc-melting. The cast alloy possesses a cellular microstructure with cells exhibiting segregation into Copper and Iron rich regions. Room temperature characterization reveals that the principal phase in Copper rich region is a disordered Body Centered Cubic phase (Im3¯m) of threaded morphology in a disordered body centered cubic matrix. The phase morphology is typical of spinodal decomposition with additional ordered face centered cubic nano-precipitates observed. Conversely, the Iron rich region consists of a partially ordered oval Face Centered Cubic phase(Fm3¯m) in a disordered Body Centered Cubic matrix. The distribution is typical of first order chemical ordering with negligible amount of disordered Face Centered Cubic phase observed. The formation energy of the Fm3¯m phase is lower than that of Im3¯m phase and varies with change in atomic position. The overall valence electron concentration of the alloy is 7.25 which results in a combination of body and face centered cubic phases. Such multiple phase formation is an outcome of significant electronic contribution to entropy of mixing for the two principal phases as is reflected from their density of states distribution.

Effect of transition metal ion substitution on structural and magnetic properties of Eu2O3 sesquioxide system

The effect of trivalent transition metal ion (Fe3+) substitution in Eu2O3 system has been carried out. The ceramic compounds of Eu2−xFexO3 (where x = 0–0.5) are prepared by solid-state reaction technique. The crystal symmetries of synthesized materials have been calculated from powder X-ray diffraction patterns through Rietveld refinement process. The Rietveld refinement confirms that the pure Eu2O3 compound exhibits multiphase structure (monoclinic and cubic) of space group (c 1 2/m 1) and (I a -3). The incorporation of Fe3+ ion in the rare earth site (Eu3+) leads with change of cubic crystal symmetry to orthorhombic multiphase (monoclinic and orthorhombic) of space group (c 1 2/m 1) and (p b n m). The field emission scanning electron microscopic images of the samples reveals the substitutional effect of transition metal ion with the increasing grain growth and grain boundaries. The energy dispersive analysis confirms the increasing concentration of Fe3+ ion in to the host Eu2O3 system. From the Raman spectra, it is observed that most of intense Raman resonance peaks are dully assigned with cubic phase Eu2O3 compound. Further, it is noticed that the major intensity Raman signal of Eu2O3 compound at 109 cm−1 assigned with Fg mode is goes on decreasing with respect to increasing concentration of Fe which evident the change of crystal structure from cubic phase to Orthoferrite phase in Eu2−xFexO3 compounds The diffused reflectance study confirms the semiconducting nature of all the prepared compounds. Magnetization analysis reveals that due to substitution of transition metal ion in rare earth site leads with decreasing trend of magnetic moment which is due to the fact of substituting lower ionic radii element (Fe3+) in non-magnetic Eu3+ ion leads with change in position of atoms in their crystal structure by variation in bond length and bond angles between RE–O and induces co-linearity in magnetic exchange interactions. Hence even the partial substitution of ferromagnetic Fe3+ ions in Eu3+ lattice site induces change in magnetic sub-lattices due to the variation in crystallographic position of atoms and results in paramagnetic ordering with lower magnetic moment than the parent compound.

Molecular Dynamics Simulation of Anatase TiO<SUB>2</SUB> Nanoparticles

Nanoparticles, which are desirable to a wide range of industries, have been shown to exhibit enhanced properties compared to their bulk counterparts. This enhancement has mostly been attributed to their large surface area-to-volume ratio and has attracted huge research interest in recent years. In this work, molecular dynamics simulations have been performed on anatase TiO2 nanoparticles with sizes ranging between 2 and 6 nm and at different temperatures. Thermodynamic and structural properties such as total system energies and radial distribution functions are reported for the different nanoparticle sizes and their dependence on temperature, revealed. At high temperatures, the structures are seen to transform from a highly crystalline to liquid form. Studies conducted on the change of final structure (after simulation) with respect to the initial structure (before simulation) revealed that after simulation, structural disordering (i.e., change in atom position) is more visible at the surface layer compared to the bulk of the final structure. Surface energies for the different particle sizes are also reported and it is observed that surface energy at 300 K increases to a maximum (optimum value) as the particle size increases after which no further significant increase is observed. A study on the sphericity of the nanoparticles showed that the particles became less spherical with increase in temperature.

Investigation of the electronic and geometric structures of the (110) surfaces of arsenopyrite (FeAsS) and enargite (Cu3AsS4)

AbstractThe (110) surfaces of arsenopyrite (FeAsS) and enargite (Cu3AsS4) have been modelled using a Density Functional Theory (DFT) plane-wave pseudopotential method (CASTEP) in order to better understand aspects of the geometric and electronic structures of these minerals, which have important implications for the release of arsenic in acid mine drainage environments. In this study, bulk calculations of these minerals have been conducted to give consistent geometries for surface models and to establish reference states for changes at the surfaces in these models. Surface structure experimental data for enargite were collected using low-energy electron diffraction which confirmed an unreconstructed 161 (110) lattice, with a and b values of 6.38±0.3 Å and 9.92±0.5 Å, respectively. Surface calculations demonstrate geometric and electronic relaxation of both arsenopyrite and enargite (110) surfaces. Changes in atomic positions, interatomic distances, Mulliken charges and electronic configurations are reported. Enargite has a surface energy of ∼0.02 eV/Å2 compared with arsenopyrite which has a surface energy of ∼0.11 eV/Å2, indicating that the enargite (110) surface is more energetically stable than that of arsenopyrite. The most stable surfaces are those which relax to restore the surface coordination and partial charge balance. For both minerals this is achieved by the formation of covalent bonds. Arsenic has the most positive Mulliken charge of all the surface atoms and is, therefore, predicted to be the most reactive atom at the arsenopyrite and enargite (110) surfaces. This implies that, according to these calculations, arsenic is most likely to react with oxidative species such as O2 and H2O in environments such as those associated with acid mine drainage, potentially releasing oxides and acids of arsenic into the environment.

Growth of Nd-doped YAG powder by sol spray process

Yttrium aluminum garnet (YAG) and neodymium-doped yttrium aluminum garnet (Nd-YAG) nano-crystalline powders were successfully grown using cost effective sol spray process without the addition of any chelating agent or organic templates. Thermal decomposition behavior was studied by thermogravimetry (TG) and differential thermal analysis (DTA). Results revealed that crystallization of YAG started around 920 °C. The shrinkage/expansion behavior of synthesized powder was examined by dilatometer and revealing that sintering kinetics of these materials can be related to the evaporation of binder and formation of crystalline phases. Nano-crystallinity and garnet structure of YAG and Nd-YAG specimens were analyzed by Raman, fourier transform infra red (FTIR), and X-ray diffraction (XRD) techniques. XRD patterns were indexed using Rietveld refinement method. Smaller lattice parameter and a small change in atomic position of oxygen were found in Nd-YAG when compared with YAG structure. Scanning electron microscope (SEM) results indicated that particle size of Nd-YAG was <150 nm. The morphology of Nd-YAG nanosized powder was rounded in shape.

Asymmetric Graphene Model Applied to Graphite-like Carbon-based Ferromagnetism

Several experiments have recently found room-temperature ferromagnetism in graphite-like carbon based materials. This paper offers a model explaining such ferromagnetism by using an asymmetric nano-graphene. Our first typical model is C48H24 graphene molecule, which has three dihydrogenated (-CH2) zigzag edges. There are several multiple spin states competing for stable minimum energy in the same atomic topology. Both molecular orbital and density function theory methods indicate that the quartet state(S=3/2) is more stable than that of doublet (S=1/2), which means that larger saturation magnetization will be achieved. We also enhanced this molecule to an infinite length ribbon having many (-CH2) edges. Similar results were obtained where the highest spin state was more stable than lower spin state. In contrast, a nitrogen substituted (-NH) molecule C45N3H21 demonstrated opposite results. that is, the lowest spin state(S=1/2) is more stable than that of highest one(S=3/2), which arises from the slight change in atom position.

Diffractive Optics Based Four-Wave, Six-Wave, ..., ν-Wave Nonlinear Spectroscopy

A detailed understanding of chemical processes requires information about both structure and dynamics. By definition, a reaction involves nonstationary states and is a dynamic process. Structure describes the atomic positions at global minima in the nuclear potential energy surface. Dynamics are related to the anharmonicities in this potential that couple different minima and lead to changes in atomic positions (reactions) and correlations. Studies of molecular dynamics can be configured to directly access information on the anharmonic interactions that lead to chemical reactions and are as central to chemistry as structural information. In this regard, nonlinear spectroscopies have distinct advantages over more conventional linear spectroscopies. Because of this potential, nonlinear spectroscopies could eventually attain a comparable level of importance for studying dynamics on the relevant time scales to barrier crossings and reactive processes as NMR has for determining structure. Despite this potential, nonlinear spectroscopy has not attained the same degree of utility as linear spectroscopy largely because nonlinear studies are more technically challenging. For example, unlike the linear spectrometers that exist in almost all chemistry departments, there are no "black box" four-wave mixing spectrometers. This Account describes recent advances in the application of diffractive optics (DOs) to nonlinear spectroscopy, which reduces the complexity level of this technology to be closer to that of linear spectroscopy. The combination of recent advances in femtosecond laser technology and this single optic approach could bring this form of spectroscopy out of the exclusive realm of specialists and into the general user community. However, the real driving force for this research is the pursuit of higher sensitivity limits, which would enable new forms of nonlinear spectroscopy. This Account chronicles the research that has now extended nonlinear spectroscopy to six-wave processes and to a completely generalized "nu-wave" mixing form to fully control state preparation and coherences. For example, direct observation of global protein motions and energetics has led to the collective mode coupling model to understand structure-function correlations in biological systems. Direct studies of the hydrogen bond network of liquid H(2)O have recently shown that both intramolecular and intermolecular degrees of freedom are strongly coupled so that the primary excitations of water have an excitonic-like character. This fundamentally different view of liquid water has now resolved a 100-year-old problem of homogeneous versus inhomogeneous broadening of the vibrational line shapes. By adding programmable pulse shaping, we can access new information about the many-body interactions directly relevant to chemical reaction dynamics. We can also steer the course of the reaction along multidimensional surfaces to provide information about fluctuations far from the equilibrium, which are most relevant to chemical reactivity.

Atomic Structure of Biogenic Aragonite

Atomic positions in geological aragonite and biogenic aragonite, obtained from several mollusk shells, were precisely measured by high-resolution neutron diffraction, which is more sensitive than X-ray diffraction to the positions of light atoms, C and O, in the CaCO3 structure. Comparable analysis of atomic positions revealed, for the first time, the changes in some bond lengths and atomic configurations in biogenic specimens with respect to geological ones. Most pronounced are modifications in the aplanarity of the carbonate groups. These modifications correlate with the observed shifts of the ν2 frequency in the IR absorption spectra. The changes in atomic positions as well as the changes in lattice parameters detected by high-resolution X-ray diffraction are attributed to the interaction between organic macromolecules and growing crystallites during biomineralization. The obtained results allow us deeper understanding of the specific routes used by nature for growing bio-composites with superior chara...

Virtual Synthesis and Optoelectronic Properties of Prismatic Artificial Molecules of In-N and Zn-O: Comparative Studies

ABSTRACTThe Hartree-Fock (HF), restricted open shell HF (ROHF), configuration interaction (CI), complete active space (ICASCF), and multiconfiguration self-consistent field (MCSCF) methods provide sophisticated fundamental theory-based, computational tools to study structure, composition,chemistry and electronic properties of small artificial molecules composed of semiconductor compound atoms. These tools are used to synthesize virtually several prismatic In-N and Zn-O artificial molecules whose structure is derived from that of the symmetry elements of the respective wurtzite bulk lattices. Applications of spatial constraints to the atomic coordinates allow modeling molecular synthesis in quantum confinement, to obtain pre-designed molecules with tunable electronic properties. Relaxation of these constraints, or optimization, leads to the corresponding molecules synthesized in “vacuum”. The development of computational templates of the studied artificial molecules synthesized in confinement reflects effects of quantum confinement on the electronic level structure, bonding, the direct optical transition energy, and charge and spin density distributions of the molecules. Comparison of the structure and properties of these molecules to those of their vacuum counterparts leads to a conclusion that a small changes in atomic positions in otherwise structurally similar molecules cause a significant change in their electronic properties. Thus, the electronic properties of artificial molecules can be tuned by changing their synthesis conditions that are defined by atomistic details of quantum confinement where the molecules are synthesized.

Structure-property correlation over five phases and four transitions in Pb5Al3F19.

The calorimetric and dielectric properties of Pb(5)Al(3)F(19) in the five phases stable under ambient pressure are correlated with structure for fuller characterization of each phase. The first-order transition between ferroelectric phase V and antiferroelectric phase IV at T(V,IV) = 260 (5) K exhibits a thermal hysteresis of 135 (5) K on heating, with a maximum atomic displacement Delta(xyz)(max) = 1.21 (6) A; the transition from phase IV to ferroelastic phase III at 315 (5) K is also first order but with a thermal hysteresis of 10 (5) K and Delta(xyz)(max) = 0.92 (7) A; that from phase III to paraelastic phase II at 360 (5) K is second order without hysteresis and has Delta(xyz)(max) = 0.69 (4) A; and the transition from phase II to paraelectric phase I at 670 (5) K is second or higher order, with Delta(xyz)(max) = 0.7 (4) A. The measured entropy change DeltaS at T(V,IV) agrees well with DeltaS as derived from the increased configurational energy by Stirling's approximation. For all other phase transitions, 0.5 > or = DeltaS > 0 J mol(-1) K(-1) is consistent with an entropy change caused primarily by the changes in the vibrational energy. The structure of phase III is determined both by group theoretical/normal mode analysis and by consideration of the structures of phases II, IV and V reported previously; refinement is by simultaneous Rietveld analysis of the X-ray and neutron diffraction powder profiles. The structure of prototypic phase I is predicted on the basis of the atomic arrangement in phases II, III, IV and V. The introduction of 3d electrons into the Pb(5)Al(3)F(19) lattice disturbs the structural equilibrium, the addition of 0.04% Cr(3+) causing significant changes in atomic positions and increasing T(IV,III) by approximately 15 K. Substitution of Al(3+) by 20% or more Cr(3+) eliminates the potential minima that otherwise stabilize phases IV, III and II.

First-Principles Structural Optimization of Cubic Approximant Cd6Ca

ABSTRACTFirst-principles structural optimization is carried out for cubic Cd6Ca crystal with 168 atoms in a unit cell. The unit cell of Cd6Ca involves two four-layered icosahedral atomic clusters and 36 glue Cd atoms. Calculations are performed to determine energy cost owing to different orientations of the Cd tetrahedron, which is the innermost shell of the icosahedral cluster. Energetically favorable ordering of central Cd tetrahedra is such that the nearest neighboring tetrahedra is oriented in an anti-parallel fashion. As a result of the structural optimization, significant changes of atomic positions are observed in the first and second shells. The optimal nearest neighbor interatomic distance between the first and second shells is found to be close to the nearest neighbor distance of pure Cd. It is found that the pseudogap in the total density of states is enhanced as a result of the structural relaxation.

FAST SWITCH X-RAY

A NEW TECHNIQUE CAN CONtrol beams of high-energy X-rays on an ultrafast timescale. The procedure may enable scientists to study subtle changes in atomic positions and other phenomena in chemical reaction dynamics with very high time resolution. By shining intense laser light on a thin germanium crystal while the crystal is transmitting X-rays, University of Michigan physicists Matthew F. DeCamp, David A. Reis, and Philip H. Bucksbaum and colleagues can control X-ray transmission [ Nature, 413 , 825 (2001)]. In the absence of the laser pulse, an X-ray beam passes through the crystal and emerges as two diffracted beams with roughly equal intensity. But when the laser light irradiates the crystal, the lattice spacings are altered rapidly, allowing the team to switch the X-ray beams on and offon a subpicosecond timescale. A number of other advances in ultrafast methods for probing reaction dynamics using light have been reported in the past several years, but they have all had shortcomings. ...

Engineered Bacillus lentus subtilisins having altered flexibility

The three-dimensional structures of engineered variants of Bacillus lentus subtilisin having increased enzymatic activity, K27R/N87S/V104Y/N123S/T274A (RSYSA) and N76D/N87S/S103A/V104I (DSAI), were determined by X-ray crystallography. In addition to identifying changes in atomic position we report a method that identifies protein segments having altered flexibility. The method utilizes a statistical analysis of variance to delineate main-chain temperature factors that represent significant departures from the overall variance between equivalent regions seen throughout the structure. This method reveals changes in main-chain mobility in both variants. Residues 125-127 have increased mobility in the RSYSA variant while residues 100-104 have decreased mobility in the DSAI variant. These segments are located at the substrate-binding site and changes in their mobility are believed to relate to the observed changes in proteolytic activity. The effect of altered crystal lattice contacts on segment flexibility becomes apparent when identical variants, determined in two crystal forms, are compared with the native enzyme.