Soft Report - Dipartimento di Fisica - Sapienza

Role of water around biomolecules and surfactantsIn solution the mutual interactions between solutesand solvents define all the conformation anddynamical properties, as well as the deviation of thesolvent from its pure state. Our work is devoted tothe analysis of these interactions, both withexperiments and Molecular Dynamics simulations, inaqueous solutions of biopolymers and surfactants.Network of vicinal water around C 12E 6 surfactant.We analysed the solute-water interactions in proteinsbelonging to organisms living at ambienttemperature, named mesophiles, or extremetemperatures, even exceeding the boilingtemperature of water, named thermophiles. Up tonow a satisfactory explanation on the origins ofadaptation at extreme conditions is still lacking. Therelated scenario depends on many independentfactors, the amino acidic primary sequence, thetypes of amino acids exposed to the solvent, i.e. thesecondary and tertiary structure, the morphologyand energetics of the interfacial region. Wesimulated the thermal response of two proteinshaving a high homology in the primary structure, theG-domain of the Elongation Factor of Escherichia coli(mesophilic) and Thermus thermophilus (moderatelythermophilic) with denaturation temperaturesdiffering by about 15 K. Our studies showed thatunfolding in the mesophile is preceded by anenhanced exchange of interfacial water moleculeswith the bulk, accompanied by concomitantabsorption of heat by hydrophilic aminoacids [1].Pertaining the comparative study of mesophilic andthermophilic counterparts, and notwithstanding thesame hydrophobic/hydrophilic composition, the moststriking result regards the high efficiency of thethermophile in adsorbing water molecules, inparticular at high temperature and the largerhydrophilic character of the thermophile [2]. Thedata support the notion that water is an ambivalentelement, destabilizing proteins due to its efficiency inhydrogen bonding and acting as a bio-protectantwith respect to thermal or mechanical stress. Asuggestive interpretation is that the strategydeveloped by thermophilic species to defend againsttemperature and, indirectly, water flooding theprotein core, is to allow for a small, but significantportion of water molecules to partially penetrate theprotein-water interface.Solutions of non-ionic surfactants were investigatedby looking at the molar volumetry andcompressibility as a function of temperature andconcentration. The results allowed the determinationof the hydration state and its temperaturedependence [3]. The visco-elastic response wasrevealed at high concentration and an extracontribution to the compressibility at lowconcentration, probably due to water exchange [4].To understand the microscopic origin of suchbehaviour we performed Molecular DynamicSimulation on C 12E 6 spherical micelle. The resultsindicated that: 1) dehydration occurs mainly on theoil core exposed surface [5]; 2) water moleculesaround the oil core form H-bound clusters whose sizeand distribution change with temperature; 3) in thisregion the water cluster have the largest size [6].The diffusion of water close to biological materialsdoes not grow linearly in time, as expected by thebrownian motion. The onset of spatial and temporalcorrelations, arising from the heterogeneity of theexposed, is usually invoked to explain thisphenomenon. Our simulations of water confined by amodel membrane showed that the dynamics can befairly well described by a simple brownian model withan effective diffusion coefficient [7]. Longitudinaldiffusion follows a standard behavior whereas theeffect of confinement manifests itself in mixing thelongitudinal and transversal dynamics. The essenceof the process has been captured by our simplifiedapproach which covers a considerably large timeinterval.References[1] S. Melchionna, G. Briganti, P. Londei, P.Cammarano, Phys.Rev.Lett., 92, 158101 (2004).[2] S.Melchionna, R.Sinibaldi, G.Briganti, Biophys. J.90, 4204 (2004).[3] G. Briganti, M. Maccarini, G. D’Arrigo Journal ofPhysical Chemistry B,108, 4039 (2004).[4] G. D’Arrigo, G. Briganti, M. Maccarini, Journal ofPhysical Chemistry B,110, 4612 (2006).[5] F. Sterpone, C. Pierleoni, G. Briganti, M. Marchi,Langmuir 20, 4311 (2004).[6] F. Sterpone, C. Pierleoni, G. Briganti, M. Marchi,Journal of Physical Chemistry B,110, 18254 (2006).[7] M. Sega, R. Vallauri, S. Melchionna, Phys. Rev. E72, 041201 (2005).Authors:S. Melchionna (a), G. Briganti (a), P. Londei (b), P.Cammarano (b), R.Sinibaldi (c), M. Sega (d), R.Vallauri (d), C. Pierleoni (e), F. Sterpone (f), M.Marchi (g), M. Maccarini (h), G. D’Arrigo (i).(a) CRS-SOFT, Department of Physics, University “LaSapienza” (Rome, Italy), (b) Department of MedicalBiochemical and Biological Medicine, University “LaSapienza” (Rome, Italy), (c) Department of AppliedScience, “Politecnico delle Marche” (Ancona, Italy),(d) Department of Physics, University of Trento(Italy), (e) Department of Physics, University ofL’Aquila (Italy), (f) Department of Chemistry,University of Texas (USA) (g) Centre d’etudes Saclay(France), (h) Institut fϋr physikalische chemie(Heidelberg, Germany), (i) Department of Energetic,University “La Sapienza” (Rome, Italy)91SOFT Scientific Report 2004-06