Tribute to Biman Bagchi

Photo by S. R. Prasad

Professor Biman Bagchi has made pivotal contributions to thearea of dynamics of chemical and biological systems in an

academic career spanning more than three decades. He has beena great teacher and mentor for a large number of youngtheoretical physical chemists of India and a creative and insightfulcollaborator with leading scientists worldwide. We take greatpride in guest editing this special issue of The Journal of PhysicalChemistry B in honor of Prof. Bagchi who is, arguably, the finesttheoretical physical chemist India has ever produced. Prof.Bagchi has spent about 30 years at the Indian Institute of Science(IISc), Bangalore, during which he has traversed a wide landscapeof physical chemistry. It is impossible to discuss all of his work; it isnot even possible to mention each area he has worked in. In thefollowing, we choose only a few selective topics and give shortcomments on their impact as we understand them. This is howwe chose to pay our tribute to this ever-young, dynamic, andcompletely apolitical scientist who is so dear to all of his students,postdocs, and collaborators and will remain so forever.Professor Bagchi joined IISc, Bangalore, in 1984. His major

research interests during the first ten years at IISc includeddynamics of polar solvation, dielectric relaxation, and dynamicsolvent effects on activated and barrierless chemical reactions. Inthe next ten years, starting from the mid-nineties, he primarilyworked on conductance and viscosity of electrolyte solutions,dynamics of biological water, solvent relaxation near micellarsurfaces, vibrational energy relaxation and dephasing, anddynamics of glassy and supercooled liquids and liquid crystals.In the next ten years, up to the present, he further extended hisstudies of vibrational dynamics to two-dimensional infraredspectroscopy of aqueous systems and continued to work onglassy and supercooled systems and liquid crystals. During thisperiod, he also entered into new areas of research in chemicalbiology such as protein−DNA interactions, enzyme catalysis, andtheory of autoimmune disorders and human immune response.Solvation dynamics was a topic of huge contemporary interest

in the mid-eighties. New results were coming in from the ultrafast

laser spectroscopic groups of one of us (G.R.F.), Paul Barbara,and others. The solvation dynamics was reported to have timesscales faster than dielectric relaxation, which posed a challengeto theorists for proper explanations. Just before joining IISc,Bagchi, in collaboration with G.R.F. and David Oxtoby, haddeveloped a theory of dipolar solvation using a continuummodelof the solvent and a frequency dependent dielectric function.The theory predicted a solvation time that was faster than thedielectric relaxation time of the solvent, thus providing anexplanation of the experimentally observed fast relaxation of thetime dependent solvation energy. The continuum theory wassubsequently generalized by Bagchi and co-workers in manydifferent directions such as incorporation of multi-Debyerelaxation, non-Debye relaxation, inhomogeneity of the mediumaround a solute, etc. Still, being based on continuum models, allthese extensions lacked the molecularity of the solvent. Besides,these theories considered only the rotational motion of solventmolecules because the dynamics came through the frequencydependence of the long wavelength dielectric function. Micro-scopic theories based on molecular solvent models also startedcoming from other groups; however, these microscopic theoriesalso included only the rotational motion of solvent molecules.These rotation-only microscopic theories predicted an averagesolvation time that was longer than the long-wavelengthcontinuum theory predictions due to contributions from finitewavevector processes. Subsequently, a microscopic theory wasdeveloped by the Bagchi group that included translationalcontributions of solvent molecules in addition to their rotationalmotion. The theory showed that the polarization at finitewavevectors can relax at a faster rate in the presence of finitetranslational contributions. This was indeed an important resultbecause, in many dynamical processes, it is the finite wavevectorresponse of the solvent that matters most. Hence, the overallsolvation can occur with a time scale faster than that predicted byrotation-only theories but in better agreement with then availableexperimental results. Subsequently, the theory was furtherextended to incorporate many subtle dynamical aspects such asinertial effects, viscoelastic effects, self-motion of the solute, andthe connections between the so-called microscopic and macro-scopic orientational relaxation times, etc., which lead toquantitative agreements of the theoretical predictions with theexperimental results for a variety of solute−solvent systems.The theories of orientational and dielectric relaxation were

also used in arriving at microscopic theories of dielectric friction.A self-consistent theory was developed for the dielectric frictionand dielectric relaxation where it was shown that the presence oftranslational contributions can make dielectric relaxation moreDebye-like for cases where only rotational contributions give riseto a highly non-Debye form of dielectric relaxation.The possible roles of solvent dynamics on rates of chemical

reactions constituted another topic of huge interest in theeighties and also in years to follow. The celebrated Grote−Hynes

Special Issue: Biman Bagchi Festschrift

Published: August 27, 2015

Special Issue Preface

pubs.acs.org/JPCB

© 2015 American Chemical Society 10809 DOI: 10.1021/acs.jpcb.5b06606J. Phys. Chem. B 2015, 119, 10809−10812

theory on the effects of frequency dependent friction on activatedbarrier crossing came in 1980. The Bagchi group showed howthe dynamics of ultrafast solvation can affect the rates of outersphere electron transfer reactions in dipolar liquids. Bagchi andco-workers also developed microscopic theories on the effectsof solvent dynamics such as viscosity and diffusion on chemicalprocesses, which proceed without the presence of any significantbarrier along the reaction coordinates. Before Bagchi’s work, theonly existing theoretical description produced unphysical excitedstate decay profiles, and this work stimulated substantial experi-mental and related theoretical efforts in a wide variety of areas.It was known for a long time that the mobility of small ions

does not follow the Stokes−Einstein relation. The concept ofdielectric friction on ions was brought in by Onsager to explainthe discrepancy but still it was not good enough to explain theanomalous diffusion behavior such as the diffusion maximumthat ions exhibit in water on increase of size. Further improve-ments of the dielectric model and early microscopic theoriesfrom different groups were not fully successful in explaining thediffusion anomaly. Bagchi and co-workers developed a micro-scopic theory that self-consistently included the self-motion ofa solute ion and the dynamics of surrounding solvent and wasable to explain the anomalous increase of diffusion for larger ionsin a quantitative manner. A salient feature of their work was toshow how the slow diffusion of ions is controlled by ultrafastdynamics of the surrounding solvent. Subsequently, the groupextended the work to higher concentrations and developeda mode coupling theory of ion conductance that accuratelyexplained the concentration dependence of ionic mobility up to afairly high concentration. Moreover, the theory correctlyreproduced the celebrated Debye−Huckel-Onsager expressionin the limit of low concentration, thus providing a molecularbasis of the limiting law. Subsequently, the theory was extendedto derive microscopic expressions for frequency dependentconductivity and viscosity, which correctly reduced to thewell-known Debye-Falkenhagen and Falkenhagen expressions,respectively, in the limit of low ion concentration. The theoryalso explained correctly the weaker concentration dependenceof ion diffusion than that of conductance which had remaineda puzzling experimental fact for quite some time.Water in contact with biological systems such as proteins,

DNA, reverse micelles, and other macromolecules, which is nowcollectively referred to as biological water, was known to behavein a different manner than normal water. For example, experi-ments revealed that the dielectric and NMR relaxation of suchbiological water can be described by two seemingly unconnectedand very different time scales: One is in the picosecond regimeand the other one is in the regime of nanoseconds with a gap ofabout 3 orders of magnitude between the two. Bagchi and co-workers developed a simple yet pioneering theory of dielectricrelaxation in biological water by describing the dynamics in termsof a dynamic equilibrium between water molecules that are freeand those bound to the biomolecule. The dielectric relaxationwas then determined by the equilibrium constant between thetwo species and the rate of interconversion between the boundand free states. The theory could provide a microscopicexplanation of the experimentally observed dielectric relaxationin biological water and provided a way to study solvation ofbiomolecules in water and other dipolar liquids. The idea of adynamic equilibrium between bound and free states was used inmany of the subsequent investigations of the group to explainmany outstanding problems of macromolecular solutions suchas complex frequency dependence of the dielectric function of

protein solutions and anomalous diffusive behavior of a solutelike the cross over from subdiffusive to superdiffusive dynamicsnear biological surfaces. The dynamic equilibrium between thefree and bound states was further quantified in terms of inter-conversion rates through computer simulations of model proteinsin water. An important finding of the simulations was the rolesof protein−water and water−water hydrogen bond fluctuationsin determining such dynamic equilibrium and their possiblecorrelations with the biological activity of proteins.Beginning in early nineties, experiments from different groups

(one of the pioneers being Prof. K. Bhattacharyya, a long-timefriend of Prof. Bagchi since his college days) showed thatsolvation dynamics in restricted environments such as in acyclodextrin cavity, at the surface of micelles or inside reversemicelles involves time scales which are 2−3 orders of magnitudeslower than that of bulk water. The initial experiments on thesesystems of so-called organized assemblies used nanosecondpulses. Later experiments involving pico- and femtosecondpulses revealed the presence of a faster component with timescales similar to that of bulk water in addition to the slowercomponent extending to nanoseconds. These systems are verycomplex containing not only macromolecules, which themselvesare in motion, but also moving water in a variety of environmentssuch as in bound and free states. Bagchi and co-workers carriedout, for the first time, a set of pioneering dynamical simulationsto study solvation dynamics at the surface of micelles. Theirsimulations did show a time scale in the nanosecond regime forthe solvation dynamics at a micellar surface. In addition, theirstudies revealed many molecular details of the dynamics thtawere not known before for these systems. For example, theirstudies showed that the slow part of the solvation dynamicsmainly originates from dynamical interactions of the solute ionwith polar head groups of the surfactants. Thus, slow movementof the micelle contributes significantly to the observed slowdynamics of solvation. The slow orientational motion of watermolecules on the micellar surface also contributes but was foundto play more of a secondary role in the overall dynamics.Studies of vibrational dynamics has constituted another area of

active research of the Bagchi group since the late nineties. Initialwork in the area involved formulation of a mode coupling theoryfor vibrational energy relaxation and providing a molecular basisfor the similarity between the vibrational energy relaxation andnonpolar solvation dynamics observed in experiments. Subsequentstudies dealt with vibrational dephasing in liquid nitrogen andwater and also extension of these studies to time dependentinfrared spectroscopy of aqueous systems.The dynamics of supercooled liquids are known to showmany

fascinating properties such as nonexponential relaxation of stress,density, polarization, and orientational correlations. Bagchi andco-workers showed that in a deeply supercooled liquid, the stressrelaxation is locally anisotropic, which can provide the necessarydriving force for hopping on the free energy surface. Their worksuggested a decoupling of diffusion from viscosity even at thelocal level, thus providing a deep understanding of the stressrelaxation, hopping, and diffusion in deeply supercooled liquids.A subsequent study from the group established a correlationbetween the breakdown of the well-known Debye model ofrotational diffusion and themanner of exploration of the underlyingenergy landscape. Their study also helped in understanding thecorrelations of decoupling between rotational and translationalmotionwith the energy landscape. The energy landscape viewwasfurther extended to understand antiplasticization and growth ofdynamic heterogeneity in supercooled polydisperse liquids.

The Journal of Physical Chemistry B Special Issue Preface

DOI: 10.1021/acs.jpcb.5b06606J. Phys. Chem. B 2015, 119, 10809−10812

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Thermotropic liquid crystals form another area of majorinterest of the Bagchi group because of their rich phase behaviorand fascinating dynamics on variation of temperature. The twomost important phases of thermotropic liquid crystals are thenematic and smectic phases. While the former phase isorientationally ordered but translationally disordered, the latterphase possesses partial translational order in addition to anincreased orientational order. Bagchi and co-workers inves-tigated the potential energy landscapes of a family of model liquidcrystals to understand the interplay between orientational andtranslational orders in different phases that liquid crystalstypically exhibit upon cooling. It was shown that the depth ofthe potential energy minimum explored by the system growsthrough the nematic phase upon cooling. The onset of thegrowth of the orientational order in the nematic phase inducesa translational order leading to smectic-like layers. Subsequentstudies from the group looked at orientational dynamics ofthermotropic liquid crystals across the isotropic−nematic phasetransition and provided molecular explanations for manyinteresting dynamical properties such as power law behaviorat intermediate times. Interestingly, a critical analysis of theirdynamical results of thermotropic liquid crystals and supercooledmolecular liquids revealed an array of analogous features in thesetwo important types of soft matter systems.Studies of protein folding constituted another area of active

interest of the Bagchi group, beginning with a seminal paperin 1991 written in collaboration with Zwanzig and Szabo inwhich the authors showed an elegant biased search resolutionof Leventhal’s paradoxthe notion that protein folding via arandom search would take enormously long times. Under-standing the mechanism or the pathway followed by an unfoldedprotein to its final folded state, the so-called native state, hasremained a challenging task in chemical biology. Bagchi andco-workers looked at the correlations between the energylandscape and topology in the folding of a model protein(chicken villin headpiece or HP-36) by using a minimalisticmodel that incorporated the effects of water through a hydro-pathy scale and the role of helical propensity of amino acidsthrough a nonlocal harmonic potential. The dynamics of foldingwas found to involve multistage decay in energy and otherrelevant quantities such as the radius of gyration, relative contactorder, etc. There was a fast initial decay due to hydrophobiccollapse, which was then followed by two distinct slow stages.The very slow last stage of folding, which could be identified as therate-determining step of folding, was found to be accompaniedby a significant increase in the contact order parameter but arelatively small decrease in energy. These are important findingswhich seemed to suggest that the slow late stage of the foldingprocess arises due to long-range contact formation and also thatthe free energy barrier of the rate-determining step of folding isentropic in origin. The process of unfolding of the same proteinfrom its native to an open state has also been studied by the group,which again confirmed the presence of multistage decay involvingmultiple intermediates. Subsequent studies on effects ofdenaturant cosolvents such as dimethyl sulfoxide (DMSO) onthe unfolding mechanism revealed that the mixed solvents canprofoundly transform the nature of the energy landscape, therebychange the route accessed by the unfolding process.Interparticle interactions leading to complexation and associa-

tion of macromolecules are ubiquitous in nature. Examplesinclude DNA−dendrimer and drug−DNA complexation,protein−DNA interaction and protein association, macroion−polyelectrolyte complexation, etc. With an aim to achieve a

molecular-level understanding of the pathway and mechanism ofsome of these complexation processes, Bagchi and co-workersconsidered the cases of DNA−dendrimer and DNA−drugcomplexation through atomistic simulations. These are complexprocesses because of the involvement of many degrees offreedom of not only the reactants but also of the surroundingwater and counterions. The group determined the free energylandscape along the complexation pathway and discovered theexistence of a stable bound state between the DNA and thedendrimer that was separated from a metastable state by a freeenergy barrier. Their study also showed that water moleculesalong the DNA backbone move away as the DNA bends toembrace the dendrimer, thus revealing the role of water in suchbinding processes for the first time. Their work also elucidatedsome of the finer details of macroion−polyelectrolyte complex-ation at a molecular level. Subsequently, Bagchi and co-workersinvestigated the binding of a drug (Daunomycin which is ananticancer drug) with DNA and constructed the free energylandscape, which allowed a molecular-level understanding of thethermodynamics, DNA structural changes, and kinetic pathwaysfor the intercalation process. Their work revealed a mechanismin which the drug first binds to the minor groove and thenintercalates into the DNA in an activated process, thus providinga molecular picture of the experimental kinetic results.Understanding the molecular details of enzyme catalysis of

biochemical reactions has remained a long-standing challenge inchemistry and biology. It is known that the same biochemicalreactions would still occur in water without the enzyme butat a hugely slower rate. The presence of the enzyme alters thereaction pathway so as to provide a more favorable path witha much lower activation energy. Although the role of water incontrolling the enzyme catalysis and reaction pathway had oftenbeen discussed in the literature, the molecular details of howwater molecules assist an enzyme in altering the reaction pathwayand its free energetics were still unclear. Bagchi and co-workersinvestigated the catalytic conversion of adenosine triphosphate(ATP) and adenosine monophosphate (AMP) to adenosinediphosphate (ADP) by the enzyme adenylate kinase (ADK) inthe presence of explicit water interactions. The process involvedbinding of one ATP molecule and an AMP molecule to twodifferent domains of the enzyme followed by a phosphate transferand subsequent release of two ADP molecules. They calculateda novel two-dimensional free energy surface that revealed thepresence of a half-open half-closed intermediate state of theenzyme. Cycling the enzyme through such an intermediate statewas found to greatly reduce the conformational free energybarrier of the reaction. The intermediate state of the enzyme wasstabilized largely by enhanced attractive interactions of the polaramino acid side chains with surrounding water, thus showing theimportance of explicit involvement of water in providing analternate reaction pathway through enzyme catalysis. Subsequentstudies revealed the presence of interesting cross correlationsbetween the two domains of the enzyme that bind the ATPand AMP molecules, thus providing further insights into the keyfactors that can influence an enzymatic reaction.Prof. Bagchi continues to move into new areas and generate

new insights, which is typified by his recent work on the humanimmune system.Prof. Bagchi has contributed immensely to our understanding

of the dynamics of complex chemical and biological systemsover his long academic career. When Prof. Bagchi returned toIndia from the USA about 30 years ago, theoretical chemistryin India essentially meant quantum chemistry. Prof. Bagchi

The Journal of Physical Chemistry B Special Issue Preface

DOI: 10.1021/acs.jpcb.5b06606J. Phys. Chem. B 2015, 119, 10809−10812

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single-handedly changed the scenario by his intellectualleadership and tireless commitment. Over the past three decades,he has guided a large number of Ph.D. students and postdoctoralscholars many of whom are now pursuing independent researchin the area of statistical mechanics and molecular simulations.In any major science university or institute in India today, onecan easily find a faculty member who is a student of Prof. Bagchior a student of a student of Prof. Bagchi. For the youngergeneration, the biggest contribution of Prof. Bagchi is that hehas changed the landscape of theoretical chemistry research inIndiaa nontrivial feat!Prof. Bagchi has always remained an internationally renowned

physical chemist. This Festschrift issue of The Journal of PhysicalChemistry B is a fitting way to honor his huge contributions in thefield of chemical dynamics and impact on the physical chemistrycommunity in general. To our knowledge, this is only the secondtime a JPC Festschrift has been dedicated to an Indian. On behalfof all students, postdoctoral scholars, and collaborators ofProf. Bagchi, we thank the Editor-in-Chief of The Journal ofPhysical Chemistry for bringing out this special issue. Prof. Bagchiturned 60 in 2014. We wish him many more years of creative,productive, and enjoyable life in the deep mist of physicalchemistry.

Amalendu Chandra, Guest EditorDepartment of Chemistry, Indian Institute of TechnologyKanpur, India 208016Ranjit Biswas, Guest EditorS.N. Bose National Centre for Basic Sciences, Kolkata, India700 098Graham R. Fleming, Guest EditorDepartment of Chemistry, University of California,Berkeley, CA 94720

The Journal of Physical Chemistry B Special Issue Preface

DOI: 10.1021/acs.jpcb.5b06606J. Phys. Chem. B 2015, 119, 10809−10812

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