Publications (154)358.77 Total impact
 03/2014; 89(4).
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ABSTRACT: We have designed a twodimensional, fractallike lattice and explored, both numerically and analytically, the differences between random walks on this lattice and a regular, squareplanar Euclidean lattice. We study the efficiency of diffusioncontrolled processes for flows from external sites to a centrosymmetric reaction center and, conversely, for flows from a centrosymmetric source to boundary sites. In both cases, we find that analytic expressions derived for the mean walk length on the fractallike lattice have an algebraic dependence on system size, whereas for regular Euclidean lattices the dependence can be transcendental. These expressions are compared with those derived in the continuum limit using classical diffusion theory. Our analysis and the numerical results quantify the extent to which one paradigmatic class of spatial inhomogeneities can compromise the efficiency of adatom diffusion on solid supports and of surfaceassisted selfassembly in metalorganic materials.Physical Review E 03/2014; 89(31):032147. · 2.31 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We present a new approach to visualizing and quantifying the displacement of segments of Pseudomonas aeruginosa azurin in the early stages of denaturation. Our method is based on a geometrical method developed previously by the authors, and elaborated extensively for azurin. In this study, we quantify directional changes in three αhelical regions, two regions having βstrand residues, and three unstructured regions of azurin. Snapshots of these changes as the protein unfolds are displayed and described quantitatively by introducing a scaling diagnostic. In accord with molecular dynamics simulations, we show that the long αhelix in azurin (residues 5467) is displaced from the polypeptide scaffolding and then pivots first in one direction, and then in the opposite direction as the protein continues to unfold. The two βstrand chains remain essentially intact and, except in the earliest stages, move in tandem. We show that unstructured regions 7281 and 8491, hinged by βstrand residues 8283, pivot oppositely. The region comprising residues 7291 (40 % hydrophobic and 16 % of the 128 total residues) forms an effectively stationary region that persists as the protein unfolds. This static behavior is a consequence of a dynamic balance between the competing motion of two segments, residues 7281 and 8491.European Journal of Biochemistry 12/2013; · 3.42 Impact Factor 
Article: Communication: Nonexistence of a critical point within the Kirkwood superposition approximation.
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ABSTRACT: An analytic argument is given to show that the application of the Kirkwood superposition approximation to the description of fluid correlation functions precludes the existence of a critical point. The argument holds irrespective of the dimension of the system and the specific form of the interaction potential and settles a longstanding controversy surrounding the nature of the critical behavior predicted within the approximation.The Journal of Chemical Physics 10/2013; 139(14):141101. · 3.12 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The analytic and numerical methods introduced previously to study the phase behavior of hard sphere fluids starting from the YvonBornGreen (YBG) equation under the Kirkwood superposition approximation (KSA) are adapted to the squarewell fluid. We are able to show conclusively that the YBG equation under the KSA closure when applied to the squarewell fluid: (i) predicts the existence of an absolute stability limit corresponding to freezing where undamped oscillations appear in the longdistance behavior of correlations, (ii) in accordance with earlier studies reveals the existence of a liquidvapor transition by the appearance of a "nearcritical region" where monotonically decaying correlations acquire very long range, although the system never loses stability.The Journal of Chemical Physics 04/2013; 138(16):164506. · 3.12 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We investigate the stability to structural perturbation of Pseudomonas aeruginosa azurin using a previously developed geometric model. Our analysis considers Ru(2,2',6',2″terpyridine)(1,10phenanthroline)(His83)labeled wildtype azurin and five variants with mutations to Culigating residues. We find that in the early stages of unfolding, the βstrands exhibit the most structural stability. The conserved residues comprising the hydrophobic core are dislocated only after nearly complete unfolding of the βbarrel. Attachment of the Rucomplex at His83 does not destabilize the protein fold, despite causing some degree of structural rearrangement. Notably, replacing the Cys112 and/or Met121 Cu ligands does not affect the conformational integrity of the protein. Notably, these results are in accord with experimental evidence, as well as molecular dynamics simulations of the denaturation of azurin.Molecular Physics 04/2013; 111(7):922929. · 1.67 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Supramolecular architectures provide a reproducible template on which surficial processes can be studied. We consider the irreversible reaction A + B → C where B is a stationary reaction center and A is a coreactant diffusing on a finite, discretized d = 2 dimensional surface of a supramolecular assembly. A latticestatistical model is developed to quantify how the reaction efficiency changes when the template is planar, Euler characteristic Ω = 0, or wrapped on the surface of a d = 3 host, Ω = 2. We find that for aperiodic or regular surfaces of finite spatial extent, dispersed Ω = 2 assemblies better optimize surficial reactions than Ω = 0 planar hosts.Chemical Physics Letters 06/2012; 538:86–92. · 2.15 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We discuss here possible models for longrange electron transfer (ET) between a donor (D) and an acceptor (A) along an anharmonic (Morse–Toda) onedimensional (1d)lattice. First, it is shown that the electron may form bound states (solectrons) with externally, mechanically excited solitons in the lattice thus leading to one form of solitonmediated transport. These solectrons generally move with supersonic velocity. Then, in a thermally excited lattice, it is shown that solitons can also trap electrons, forming similar solectron bound states; here, we find that ET based on hopping can be modeled as a diffusionlike process involving not just one but several solitons. It is shown that either of these two solitonassisted modes of transport can facilitate ET over quite long distances.International Journal of Bifurcation and Chaos 05/2012; 20(01). · 0.92 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We present a comparative analysis of sequential versus hierarchical mechanisms of selfassembly in supramolecular architectures. The analysis is multifaceted, drawing on and interrelating insights from a kinetic meanfield analysis and one based on the theory of finite Markov processes, complemented by Monte Carlo calculations. We give explicit results for two reaction pathways that are likely to dominate in early stages of selfassembly, and draw attention to experimental studies to which our results pertain: crystallization of zeolites from the bulk phase and aggregation of surfacesupported supramolecular structures. Among the several conclusions that can be drawn from the theory and the simulations is a crossover from one mechanism to another, depending on the values of system parameters.Molecular Physics 01/2012; · 1.67 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: A comparative study of the early stages of unfolding of five proteins: cyt c, cb562, cyt c′, azurin, and lysozyme is reported. From crystallographic data, helical regions and intervening nonhelical (or ‘turning’) regions are identified in each. Exploiting a previously introduced geometrical model, the paper describes quantitatively the stepwise extension of a polypeptide chain subject to the geometrical constraint that the spatial relationship among the residues of each triplet is fixed by nativestate crystallographic data. Despite differences among the abovecited proteins, remarkable universality of behavior is found in the early stages of unfolding. At the very earliest stages, internal residues in each helical region have a common unfolding history; the terminal residues, however, are extraordinarily sensitive to structural perturbations. Residues in nonhelical sections of the polypeptide unfold after residues in the internal helical regions, but with increasing steric perturbation playing a dominant role in advancing denaturation.Molecular Physics 01/2012; · 1.67 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We explore the consequences of metrically decomposing a finite phase space, modeled as a ddimensional lattice, into disjoint subspaces (lattices). Ergodic flows of a test particle undergoing an unbiased random walk are characterized by implementing the theory of finite Markov processes. Insights drawn from number theory are used to design the sublattices, the roles of lattice symmetry and system dimensionality are separately considered, and new lattice invariance relations are derived to corroborate the numerical accuracy of the calculated results. We find that the reaction efficiency in a finite system is strongly dependent not only on whether the system is compartmentalized, but also on whether the overall reaction space of the microreactor is further partitioned into separable reactors. We find that the reaction efficiency in a finite system is strongly dependent not only on whether the system is compartmentalized, but also on whether the overall reaction space of the microreactor is further partitioned into separable reactors. The sensitivity of kinetic processes in nanoassemblies to the dimensionality of compartmentalized reaction spaces is quantified.ISRN Computational Mathematics. 10/2011; 2012.  [Show abstract] [Hide abstract]
ABSTRACT: We use an analytic criterion for vanishing of exponential damping of correlations developed previously [J. Piasecki et al., J. Chem. Phys. 133, 164507 (2010)] to determine the threshold volume fractions for structural transitions in hard sphere systems in dimensions D = 3, 4, 5, and 6, proceeding from the YvonBornGreen hierarchy and using the Kirkwood superposition approximation. We conclude that the theory does predict phase transitions in qualitative agreement with numerical studies. We also derive, within the superposition approximation, the asymptotic form of the analytic condition for occurrence of a structural transition in the D → ∞ limit.The Journal of Chemical Physics 08/2011; 135(8):084509. · 3.12 Impact Factor  The Journal of Chemical Physics 02/2011; 134(5):059902. · 3.12 Impact Factor
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ABSTRACT: We study the early stages of selfassembly of elementary building blocks of nanophase materials, considering explicitly their structure and the symmetry and the dimensionality of the reaction space. Previous work [Kozak et al., J. Chem. Phys. 134, 154701 (2007)] focused on characterizing selfassembly on small squareplanar templates. Here we consider larger lattices of squareplanar symmetry having N = 255 sites, and both hexagonal and triangular lattices of N = 256 sites. Furthermore, to assess the consequences of a depletion zone above a basal layer (λ = 1), we study selfassembly on an augmented diffusion space defined by λ = 2 and λ = 5 stacked layers having the same characteristics as the basal plane. The effective decrease in the efficiency of selfassembly of individual nanophase units when the diffusion space is expanded, by increasing the template size and/or by enlarging the depletion zone, is then quantified. The results obtained reinforce our earlier conclusion that the most significant factor influencing the kinetics of formation of a final selfassembled unit is the number of reaction pathways from one or more precursor states. We draw attention to the relevance of these results to zeolite synthesis and reactions within pillared clays.The Journal of Chemical Physics 02/2011; 134(6):064701. · 3.12 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We have developed a geometrical model to study the unfolding of iso1 cytochrome c. The model draws on the crystallographic data reported for this protein. These data were used to calculate the distance between specific residues in the folded state, and in a sequence of extended states defined by n= 3, 5, 7, 9, 11, 13, and 15 residue units. Exact calculations carried out for each of the 103 residues in the polypeptide chain demonstrate that different regions of the chain have different unfolding histories. Regions where there is a persistence of compact structures can be identified, and this geometrical characterization is fully consistent with analyses of timeresolved fluorescence energytransfer (TrFET) data using dansylderivatized cysteine sidechain probes at positions 39, 50, 66, 85, and 99. Our calculations were carried out assuming that different regions of the polypeptide chain unfold synchronously. To test this assumption, we performed lattice Monte Carlo simulations to study systematically the possible importance of asynchronicity. Our calculations show that small departures from synchronous dynamics can arise if displacements of residues in the main body of the chain are much more sluggish than nearterminal residues.Molecular Physics 01/2011; 109(2):301313. · 1.67 Impact Factor 
Article: Unfolding fourhelix bundles
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ABSTRACT: A geometrical model has been developed to describe the early stages of unfolding of cytochromes c′ and cb562. Calculations are based on a stepwise extension of the polypeptide chain subject to the constraint that the spatial relationship among the residues of each triplet is fixed by the nativestate crystallographic data. The response of each protein to these structural perturbations allows the evolution of each of the four helices in these two proteins to be differentiated. It is found that the two external helices in c′ unfold before its two internal helices, whereas exactly the opposite behaviour is demonstrated by cb562. Each of these cytochromes has an extended, internal, nonhelical (‘turning’) region that initially lags behind the most labile helix but then, at a certain stage (identified for each cytochrome), unravels before any of the four helices present in the native structure. It is believed that these predictions will be useful in guiding future experimental studies on the unfolding of these two cytochromes.Molecular Physics 01/2011; 109(6):905916. · 1.67 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Quantum–mechanical studies have predicted, and experimental studies on SrCu2O3 have confirmed, that the propagation of bound pairs on evenchain copper oxide ladders is possible, but not on ladders with an odd number of legs. To study whether this quantum–mechanical lattice parity effect has a classical analog, and to document the consequences of assuming different coupling scenarios between the ladder and adjacent sublattices, we develop a classical Markovian latticestatistical model to monitor the efficiency of migration on a composite lattice. Regions of parameter space where significant departures from results obtained via a symmetrical random walk are identified.Chemical Physics Letters 01/2011; 514(1):8893. · 2.15 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Starting from the second equilibrium equation in the BBGKY hierarchy under the Kirkwood superposition closure, we implement a new method for studying the asymptotic decay of correlations in the hard disk fluid in the high density regime. From our analysis and complementary numerical studies, we find that exponentially damped oscillations can occur only up to a packing fraction η(∗)∼0.718, a value that is in substantial agreement with the packing fraction, η∼0.723, believed to characterize the transition from the ordered solid phase to a dense fluid phase, as inferred from Mak's Monte Carlo simulations [Phys. Rev. E 73, 065104 (2006)]. Next, we show that the same method of analysis predicts that the exponential damping of oscillations in the hard sphere fluid becomes impossible when λ=4nπσ(3)[1+H(1)]≥34.81, where H(1) is the contact value of the correlation function, n is the number density, and σ is the sphere diameter in exact agreement with the condition, λ≥34.8, which is first reported in a numerical study of the Kirkwood equation by Kirkwood et al. [J. Chem. Phys. 18, 1040 (1950)]. Finally, we show that our method confirms the absence of any structural transition in hard rods for the entire range of densities below close packing.The Journal of Chemical Physics 10/2010; 133(16):164507. · 3.12 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We continue the study of a particle (atom, molecule) undergoing an unbiased random walk on the Sierpinski gasket, and obtain for the gasket and tower the eigenvalue spectrum of the associated stochastic master equation. Analytic expressions for recurrence relations among the eigenvalues are derived. The recurrence relations obtained are compared with those determined for two Euclidean lattices, the closed chain with an absorbing site and a finite chain with an absorbing site at one end. We check and confirm the internal consistency between the smallest eigenvalue and the mean walklength in each of the cases studied. Attention is drawn to the relevance of the results obtained to a problem of electron transfer in proteins.Physical Review E 07/2010; 82(1 Pt 1):011137. · 2.31 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Recent observations of the growth of protein crystals have identified two different growth regimes. At low supersaturation, the surface of the crystal is smooth and increasing in size due to the nucleation of steps at defects and the subsequent growth of the steps. At high supersaturation, nucleation occurs at many places simultaneously, the crystal surface becomes rough, and the growth velocity increases more rapidly with increasing supersaturation than in the smooth regime. Kinetic roughening transitions are typically assumed to be due to the vanishing of the barrier for twodimension nucleation on the surface of the crystal. We show here, by means of both analytic meanfield models and kinetic Monte Carlo simulations, that a transition between different growth modes reminiscent of kinetic roughening can also arise as a kinetic effect occurring at finite nucleation barriers.The Journal of Chemical Physics 01/2010; 132(3):035102. · 3.12 Impact Factor
Publication Stats
681  Citations  
358.77  Total Impact Points  
Top Journals
Institutions

2005–2014

DePaul University
 Department of Chemistry
Chicago, Illinois, United States


2010–2013

University of Warsaw
 Institute of Theoretical Physics
Warsaw, Masovian Voivodeship, Poland 
Cray
Seattle, Washington, United States


2006–2013

California Institute of Technology
 Beckman Institute
Pasadena, CA, United States


1999–2008

Pomona College
 Department of Chemistry
Claremont, California, United States


1968–2008

University of Chicago
 • James Franck Institute
 • Department of Chemistry
Chicago, IL, United States


2007

University of Texas at Austin
Austin, Texas, United States 
Free University of Brussels
Bruxelles, Brussels Capital Region, Belgium


1969–2007

University of Notre Dame
 • Department of Chemistry and Biochemistry
 • Department of Physics
Indiana, PA, United States


1994–2006

Iowa State University
 Department of Chemistry
Ames, IA, United States


2003

Université Libre de Bruxelles
 Faculty of Sciences
Bruxelles, Brussels Capital Region, Belgium


1990

University of Georgia
 Department of Chemistry
Athens, GA, United States


1988

University of Oxford
 Physcial and Theoretical Chemistry Laboratory
Oxford, ENG, United Kingdom


1978

Queen's University
 Department of Economics
Kingston, Ontario, Canada
