128 research outputs found
Cluster phases of membrane proteins
A physical scenario accounting for the existence of size-limited
submicrometric domains in cell membranes is proposed. It is based on the
numerical investigation of the counterpart, in lipidic membranes where proteins
are diffusing, of the recently discovered cluster phases in colloidal
suspensions. I demonstrate that the interactions between proteins, namely
short-range attraction and longer-range repulsion, make possible the existence
of stable small clusters. The consequences are explored in terms of membrane
organization and diffusion properties. The connection with lipid rafts is
discussed and the apparent protein diffusion coefficient as a function of their
concentration is analyzed.Comment: 5 pages - enhanced versio
Thermodynamics of nano-cluster phases: a unifying theory
We propose a unifying, analytical theory accounting for the self-organization
of colloidal systems in nano- or micro-cluster phases. We predict the
distribution of cluter sizes with respect to interaction parameters and colloid
concentration. In particular, we anticipate a proportionality regime where the
mean cluster size grows proportionally to the concentration, as observed in
several experiments. We emphasize the interest of a predictive theory in soft
matter, nano-technologies and biophysics.Comment: 4 pages, 1 figur
Correlations between Structure and Spectroscopic Properties in Membrane Model System. Fluorescence and Circular Dichroism of the Cytochrome c-Cardiolipin System
STRUCTURE DES PHASES PRĂSENTES DANS LES SYSTĂMES LIPIDE-EAU
L'Ă©tude par diffraction des rayons X de nombreux systĂšmes lipide-eau a mis en Ă©vidence un grand nombre de phases, dont la structure a pu ĂȘtre Ă©tablie. Le domaine d'existence de ces phases dĂ©pend de la nature du lipide (dont le choix s'Ă©tend de composĂ©s chimiques purs, tels les savons saturĂ©s, jusqu'aux mĂ©langes trĂšs hĂ©tĂ©rogĂšnes extraits d'organites cellulaires), de la teneur en eau et de la tempĂ©rature. On peut examiner diffĂ©rents aspects de la structure de ces phases : a) Conformation Ă courte Ă©chelle. Les chaĂźnes paraffiniques peuvent adopter diffĂ©rentes conformations : totalement ordonnĂ©e, comme dans les cristaux de savon ; trĂšs dĂ©sordonnĂ©e, analogue Ă celle du caoutchouc partiellement Ă©tirĂ© ; partiellement ordonnĂ©e, avec les chaĂźnes soit Ă©tirĂ©es soit enroulĂ©es en hĂ©lice, organisĂ©es selon des rĂ©seaux Ă deux dimensions avec dĂ©sordre rotationnel. b) Organisation Ă grande Ă©chelle. L'organisation peut ĂȘtre pĂ©riodique Ă zĂ©ro, une, deux ou trois dimensions. Le premier cas correspond Ă l'Ă©tat liquide ; le second aux stases smectiques, Ă structure lamellaire ; le quatriĂšme Ă l'Ă©tat cristallin. Le troisiĂšme cas (pĂ©riodique Ă deux dimensions) satisfait la dĂ©finition de Friedel d'organisation mĂ©somorphe, mais prĂ©sente un degrĂ© d'ordre plus Ă©levĂ© que celui des stases smectique et nĂ©matique. c) Forme des Ă©lĂ©ments de structure. Les Ă©lĂ©ments de structure, c'est-Ă -dire les rĂ©gions occupĂ©es respectivement par les parties paraffiniques et par les parties polaires, adoptent des formes djffĂ©rentes, qui peuvent ĂȘtre groupĂ©es en trois classes : 1° Lamelles, qui consistent soit de couches planes indĂ©finies, soit de rubans de largeur finie et de longueur infinie, soit de disques de taille finie. 2° BĂątonnets, soit tiges rigides de longueur infinie, soit bĂątonnets de longueur finie, tous identiques, joints trois Ă trois ou quatre Ă quatre et formant des filets Ă deux ou trois dimensions. 3° SphĂšres. d) Distribution des rĂ©gions polaires et parafiniques. Dans tous les cas oĂč la structure se prĂȘte Ă une distinction topologique entre rĂ©gions "intĂ©rieure" et "extĂ©rieure" deux types de distributions peuvent exister, selon que les parties paraffiniques occupent les rĂ©gions "intĂ©rieures" et les parties polaires les rĂ©gions "extĂ©rieures", ou vice-versa. Les deux types de distributions sont observĂ©s couramment. L'ensemble des donnĂ©es expĂ©rimentales se prĂȘte Ă des considĂ©rations d'ordre chimique, si on envisage la corrĂ©lation entre la structure des phases et la nature chimique des lipides, d'ordre cristallographique, si on examine le rĂŽle des Ă©lĂ©ments de symĂ©trie et l'aspect " cristaux-liquides " de ces structures, et d'ordre thermodynamique, si on s'intĂ©resse Ă la nature des forces responsables de la stabilitĂ© des diffĂ©rentes structures et du polymorphisme.X-ray scattering data on lipid water systems display a number of phases ; the structure of these phases has been established. The phase diagram depends on the nature of the lipid (pure chemical compounds such as the soaps, or heterogeneous mixtures extracted from cellular organites), on the water content and on temperature. The following features are reviewed : a) small scale conformation of the paraffine chains : this may be completely ordered (e. g. pure soap crystals), completely disordered (rubber like), or partially ordered (stretched chains or helical chains in a two-dimensional organisation with rotational disorder) ; b) large scale organization : this corresponds to periodicity in zero, one, two, or three dimensions. The first case is found with liquids, the-second with lamellar phases (smectics), the fourth with crystals. The third case (periodicity in 2 dimensions) is mesomorphic in the Friedel sense, but is more highly ordered than smectics or nematics. c) shape of the structural units, i. e. of the paraffinic and polar regions. One finds three classes : 1) lamellae (indefinite plane layers), ribbons (of indefinite length), discs ; 2) rods of infinite length, or jointed rods associated in two-dimensional (or three-dimensional) networks ; 3) spheres ; d) distribution of the polar and paraffinic regions. In some phases the topology allows for an "inside" region, and an "outside" region. This may occur either as "oil in water" or as "water in oil". These data are discussed from three points of view : 1) chemistry (correlation between phase diagram and chemical composition) ; 2) crystallography (symmetries and relations with liquid crystals) ; 3) thermodynamics (nature of the forces stabilizing the various structures)
PROPRIĂTĂS OPTIQUES ROTATOIRES DE QUELQUES SYSTĂMES A STRUCTURE SMECTIQUE FORMES DE PROTĂINES, LIPIDES ET EAU
Nous avons étudié par dichroïsme circulaire plusieurs préparations à organisation smectique dont la structure a été établie par ailleurs par diffraction des rayons X. On discute quelques problÚmes que pose une telle étude et on analyse les corrélations entre les propriétés rotatoires et l'organisation des chromophores, en insistant sur les effets d'orientation et d'interactions.We have studied the circular dichroism of smectic layers made of lipids + protein + water ; the structure of these layers has been established independently by X-ray measurements. We discuss the correlations between the optical properties and the spatial organization of the chromophores, with particular stress on the effects of orientation and interaction
IMAGE ANALYSIS OF FREEZE-FRACTURED LIPID-WATER CUBIC PHASES OF SPACE GROUP Ia3d
L'analyse d'images de microscopie électronique par cryodécapage de phases cubiques Q230 (groupe d'espace Ia3d) de type I ("huile-dans-l'eau") et de type II ("eau-dans-l'huile") nous a permis de visualiser, des aspects locaux de ces structures qui sont en excellent accord avec les profils de densité électronique correspondants, obtenus à partir d'analyses par diffraction de rayons X. Nos résultats indiquent que les principales surfaces de fracture passent, dans ces phases, entre les éléments de structure constitués de tiges relativement rigides, selon des directions bien déterminées.Image analysis by real-space cross correlation averaging of X-ray controlled freeze-fractured cubic Q230 (space group Ia3d) type I ("oil-in-water") and type II ("water-in-oil") phases allowed us to visualize, the local aspects of these structures which are in excellent agreement with the corresponding sections of the computed electron density profiles obtained from X-ray diffraction analysis. Our results indicate that the main fracture surfaces propagate, in these cubic phases, between the relatively rigid rod-like structure elements, following well-defined directions
Structure of the body-centered cubic phase of lipid systems does not consist of indefinitely long straight rods
Electron microscopic study of supramolecular liquid crystalline polymers formed by molecular-recognition-directed self-assembly from complementary chiral components.
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