Soft Report - Dipartimento di Fisica - Sapienza

Dynamics of Hydrated Saccharides and Saccharide GelsCarbohydrates are present in a wide range ofbiological assemblies; the interest on their dynamicalproperties arises not only for basic biophysicalstudies, but also in view of application-orientedinvestigations. Modern approaches to food sciencegive an increasing relevance to dynamic molecularproperties: it has been pointed out that hydrationwater is in dynamically constrained glassy state,rather than in equilibrium thermodynamic phase.Concepts borrowed from synthetic polymer scienceas for instance that of dynamical glass-liketransitions, are now currently applied in ‘‘foodpolymer science’’.Studies on the dynamics of polysaccharide extractedfrom starch (amylose, amylopectin, and theoligomeric building block, glucose) have beenperformed on IN13. They have shown that in the lowhydration regime (up to at least h » 0.6 w/w) waterinduces a kinetic transition, similar to that observedin proteins at similar hydration level. The differentbehaviour of the hydration dependence of the MSD inglucose, amylose, and amylopectine can be tracedback to the specific features of the hydration processWhen hydrated, amylose chains give rise to doublehelical secondary structures. Their regular packinginto hexagonal superstructures, kept together byextra-chain hydrogen bonds (HB), leave a largecentral pore of an average diameter of about 19 Å,into which several water molecules can beaccommodated. During hydration, H 2O moleculesgradually fill the pore up to a saturation point whichcan be located at a water content of about 27%.Our data on the hydration dependence of the meansquare fluctuation support this picture. For amyloseand amylopectin an almost linear increase of theMSD with hydration is observed up to h ~ 0.3, i.e. inthe regime where water fills the pores. Above thishydration value, water molecules that are furtheradded are located outside the pores, and they giveup to a progressive swelling of the sample.Elastic incoherent neutron scattering experimentswere also performed on trehalose-, maltose-,sucrose-H 2O mixtures. The decrease in the elasticintensity above the dynamical ‘glass-like’ transition isless marked in the case of trehalose-water mixturethan for the other disaccharide-water mixtures. Thisresult indicates that trehalose has a larger structuralresistance to temperature changes and a higher“rigidity” in comparison with maltose and sucrose,the latter showing the “softest” dynamic behaviour.The trehalose-H 2O mixture is therefore characterizedby a lower fragility namely by a higher resistance tolocal structural changes when temperature decreasestowards the glass transition value. The “stronger”character of trehalose-H 2O mixture justifies its betterattitude, with respect to maltose and sucrose-H 2Omixtures to encapsulate biostructures in a more rigidmatrix. The effect of the interplay between waterand hydrophilic polymer matrices on the dynamics ofconfined water has also been studied. A detaileddescription of localized diffusive motions in waterand in the polymer moiety is of relevance since, inhydrophilic polymeric networks, they are coupledwith permeation of drug molecules in controlleddelivery functions. A quantitative knowledge of thecorrelation of dynamic parameters with structuralfeatures of the network is necessary for the design ofefficient drug delivery systems. Elastic neutronscattering measurements have been performed onchemically cross-linked polysaccharide matrices(sephadex) at fixed hydration degree, and in atemperature range from 0 to 50 C. These matricesare characterized by a very narrow pore sizedistribution and it is thus possible to study the effectof the pore size on the dynamics of confined solvent.The picture emerging from this investigation is thatof hydration water that behaves differently accordingto the pore size. In the future similar will beextended to more biologically relevant randomnetwork systems.These studies have been carried out as collaborativeprojects between the CRG-IN13 local team (F. Natali,L. Bove, D. Russo, M. Tehei) and several Italianresearch groups.M5004003002001000Sucrose+19H 2OMaltose+19H 2OTrehalose+19HSucrose+19D 2O2OTrehalose+19D 2OSucrose+6H 2OTrehalose+6H 2OTrehalose SucrosePBB 2O 3GlycerolO-terphenylSelenium20 40 60 80 100 120mAuthorsA. Gliozzi, R. Rolandi, Università di Genova, S.Magazù, U. Wanderlingh, Università di Messina, M.Corti, L. Cantù, Univ. Milano, R. Cordone, A.Cupane,Università di Palermo, A. Deriu Università di Parma,G. Onori, A. Paciaroni Università di Perugia, CRS-SOFT, A. Congiu Castellano Università di Roma LaSapienza, A. Filabozzi, G. Paradossi Università diRoma Tor Vergata107SOFT Scientific Report 2004-06