Soft Report - Dipartimento di Fisica - Sapienza

Molecular Clustering by High Resolution NMRAmong all self-aggregation processes, the selfaggregationof lipid molecules to form bilayermembranes is a process fundamental to theorganization of life. Thanks to their spontaneoustendency to aggregate, some lipid moleculesconstitute local phases of highly concentratedorganic compounds, embedded in the membrane andtherefore could enhance concentration dependentchemical interactions.temperature dependence has given us usefulinformation on clusters formation, making itsdependence on temperature and gangliosideconcentration more clear.References[1] D’Emiliano D., Casieri C., Paci M. and De Luca F.2006. Detection of Ganglioside Clustering in DOPCBilayers by 1H-NMR Spectroscopy. PhysicaA (2006),doi:10.1016/j.physa.2006.07.021.AuthorsD. D’Emiliano (a,b), C. Casieri (a,c) and F. De Luca(a,b).(a) CRS SOFT-INFM-CNR, Roma, Italy(b) Dip. di Fisica, Univ. di Roma La Sapienza, Roma,I(c) Dip. di Fisica, Università di L’Aquila, L’Aquila, IFig. 1: The appearance of the resonance shift onCH=CH 1H-DOPC line in function of GM1concentration.1H-NMR spectroscopy has been used [1] toinvestigate the formation of ganglioside GM1 clustersin DOPC liposome bilayer. The 1H-DOPC NMR spectrashow that the presence of GM1 modifies someresonance lines (fig. 1). These modifications areimputable to the interaction of DOPC with GM1.Above a certain GM1 concentration the trend of suchdistortions suggests the formation of GM1aggregates. Our work provides a new and simpleNMR proof of the GM1 clustering in DOPC liposomesand therefore on model cell membranes; it allows toestimate the mean size of GM1 aggregates and theGM1 concentration at which clustering starts. Inaddition, NMR-PFG self-diffusion coefficientmeasurements have enable us to confirm and extendthe chemical shift results. Thanks to our approachthe influence of cluster formation on the generalorganization of the DOPC bilayer has been shown. Inparticular its local disordering effect on DOPC-DOPCinteractions has been proven. More over, cluster radiiTwo binary aqueous mixtures which contain thesmall amphiphilic molecules TMAO (trimethylamine-N-oxide) and TBA (tert-butyl alcohol) have beeninvestigated by molecular dynamics simulations andNMR chemical shift and self-diffusion measurements.TMAO is an osmolyte, while TBA is a monohydratealcohol. Both possess bulky hydrophobic groups andpolar heads, namely, NO in TMAO and OH in TBA.The hydrophilic/hydrophobic content of theseisosteric molecules strongly modulates the structureand dynamics of the hydration shell, which isthought to be responsible for the effects observed onproteins and phospholipids. Simulation results,especially on hydrogen-bond networking, spatialcorrelations, and self-diffusivity, are consistent withNMR data and agree well with previous numericalstudies on similar solutions. The methods employedallow the elucidation of the microscopic features ofthe solutions. For TBA solutions, the hydration shellis found to have a low density and a large spatialspread, and thus, above the molar fractionof 0.03,reduction of hydrophobic hydration drives selfaggregationof the solute. This effect does not takeplace in TMAO solutions, where the hydration shell ismore compact and stable, maintaining its structureover a wider range of solute concentrations.References[1] Sinibaldi R., Casieri C., Melchionna S., Onori G.,Segre A.L., Viel S., Mannina L. and De Luca F., J.Phys. Chem. B, 110 (2006) 8885-8892.Authors:R. Sinibaldi (b), C. Casieri (a,c) and F. De Luca (a,b).(a) CRS SOFT-INFM-CNR, Roma, Italy(b) Dip. di Fisica, Univ. di Roma La Sapienza, Roma,I(c) Dip. di Fisica, Università di L’Aquila, L’Aquila, I97SOFT Scientific Report 2004-06