Publications (177)457.9 Total impact

Physical Review E 03/2014; 89(4). DOI:10.1103/PhysRevE.89.049903 · 2.33 Impact Factor

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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. DOI:10.1103/PhysRevE.89.032147 · 2.33 Impact Factor 
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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; 19(45). DOI:10.1007/s0077501310772 · 3.16 Impact Factor 
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ABSTRACT: We present exact, analytic results for the mean time to trapping of a random walker on the class of deterministic Sierpinski graphs embedded in d≥2 Euclidean dimensions, when both nearestneighbor (NN) and nextnearestneighbor (NNN) jumps are included. Mean firstpassage times are shown to be modified significantly as a consequence of the fact that NNN transitions connect fractals of two consecutive generations.Physical Review E 11/2013; 88(51):052139. DOI:10.1103/PhysRevE.88.052139 · 2.33 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. DOI:10.1063/1.4824388 · 3.12 Impact Factor 
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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. DOI:10.1063/1.4801329 · 3.12 Impact Factor 
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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. DOI:10.1080/00268976.2012.758324 · 1.64 Impact Factor 
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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. DOI:10.1016/j.cplett.2012.04.032 · 1.99 Impact Factor 
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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). DOI:10.1142/S0218127410025508 · 1.02 Impact Factor 
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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 04/2012; 110(7):111. DOI:10.1080/00268976.2011.651168 · 1.64 Impact Factor 
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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 04/2012; 110(7):18. DOI:10.1080/00268976.2011.648966 · 1.64 Impact Factor 
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ABSTRACT: We consider an unbiased random walk on a finite, nth generation Sierpinski gasket (or "tower") in d = 3 Euclidean dimensions, in the presence of a trap at one vertex. The mean walk length (or mean number of time steps to absorption) is given by the exact formula The generalization of this formula to the case of a tower embedded in an arbitrary number d of Euclidean dimensions is also found, and is given by This also establishes the leading largen behavior that may be expected on general grounds, where Nn is the number of sites on the nth generation tower and is the spectral dimension of the fractal.International Journal of Bifurcation and Chaos 11/2011; 12(11). DOI:10.1142/S0218127402006138 · 1.02 Impact Factor 
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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.10/2011; 2012. DOI:10.5402/2012/981501 
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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 09/2011; 514(1):8893. DOI:10.1016/j.cplett.2011.08.019 · 1.99 Impact Factor 
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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. DOI:10.1063/1.3622597 · 3.12 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 03/2011; 109(6):905916. DOI:10.1080/00268976.2011.558855 · 1.64 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. DOI:10.1063/1.3541822 · 3.12 Impact Factor 
The Journal of Chemical Physics 02/2011; 134(5):059902. DOI:10.1063/1.3552177 · 3.12 Impact Factor

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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. DOI:10.1080/00268976.2010.521202 · 1.64 Impact Factor 
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ABSTRACT: We report lattice Monte Carlo calculations to study the efficiency of diffusioncontrolled reactive processes involving geminate radical pairs. Whereas our earlier study [J.J. Kozak, C. Nicolis, G. Nicolis, N.J. Turro, J. Phys. Chem. 105 (2001) 10949] focused on factors affecting the reaction efficiency when a pair of coreactants moved synchronously on a surface modeled as a d = 2 dimensional square planar lattice subject to periodic boundary conditions, we document here differences in reaction efficiency when a pair of coreactants can move synchronously or asynchronously on surfaces which are topologically different but characterized by the same number N of sites. As before, the first surface is taken to be a d = 2 square planar lattice; the second surface considered is a Cartesian shell, the bounded surface of a cube. When studied as a function of system size, we find that synchronous dynamics are more efficient than asynchronous dynamics in optimizing diffusion–reaction processes; and, reactions on planar, periodic surfaces are more efficient than on cubic shells. The relevance of these conclusions to experimental studies on two radiationinduced, radical decay reactions [the one cited above and A.J. Frank, M. Grätzel, J.J. Kozak, J. Am. Chem. Soc. 98 (1976) 3317] is noted and discussed.Physica A: Statistical Mechanics and its Applications 10/2010; 389:40614070. DOI:10.1016/j.physa.2010.06.009 · 1.72 Impact Factor
Publication Stats
2k  Citations  
457.90  Total Impact Points  
Top Journals
Institutions

2005–2014

DePaul University
 Department of Chemistry
Chicago, Illinois, United States


2010

Cray
Seattle, Washington, United States


1968–2008

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


2007

University Hospital Brussels
Bruxelles, Brussels Capital Region, Belgium


2006–2007

California Institute of Technology
 Beckman Institute
Pasadena, California, United States 
University of Illinois, UrbanaChampaign
Urbana, Illinois, United States


1969–2007

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


1994–2003

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


2002

Australian National University
Canberra, Australian Capital Territory, Australia 
Massachusetts General Hospital
Boston, Massachusetts, United States


1989–2002

University of Georgia
 Department of Chemistry
Атина, Georgia, United States


1978

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