In living organisms most chemical reactions take place within the confines of lipid-membrane bound compartments, while confinement within the bounds of a lipid membrane is thought to be a key step in ...abiogenesis. In previous work we demonstrated that confinement in the aqueous cavity of a lipid vesicle affords protection against hydrolysis, a phenomenon that we term here confinement effect (
C
e
) and that we attributed to the interaction with the lipid membrane. Here, we show that both the size and the shape of the cavity of the vesicle modulate the
C
e
. We link this observation to the packing of the lipid following changes in membrane curvature, and formulate a mathematical model that relates the
C
e
to the radius of a spherical vesicle and the packing parameter of the lipids. These results suggest that the shape of the compartment where a molecule is located plays a major role in controlling the chemical reactivity of non-enzymatic reactions. Moreover, the mathematical treatment we propose offers a useful tool for the design of vesicles with predictable reaction rates of the confined molecules,
e.g.
, drug delivery vesicles with confined prodrugs. The results also show that a crude form of signal transduction, devoid of complex biological machinery, can be achieved by any external stimuli that drastically changes the structure of the membrane, like the osmotic shocks used in the present work.
In lipid vesicles, the size and shape modulates the chemistry within.
In living cells, communication requires the action of membrane receptors that are activated following very small environmental changes. A binary all-or-nothing behavior follows, making the organism ...extremely efficient at responding to specific stimuli. Using a minimal system composed of lipid vesicles, chemical models of a membrane receptor and their ligands, we show that bio-mimetic ON/OFF assembly of high avidity, multivalent domains is triggered by small temperature changes. Moreover, the intensity of the ON signal at the onset of the switch is modulated by the presence of small, weakly binding divalent ligands, reminiscent of the action of primary messengers in biological systems. Based on the analysis of spectroscopic data, we develop a mathematical model that rigorously describes the temperature-dependent switching of the membrane receptor assembly and ligand binding. From this we derive an equation that predicts the intensity of the modulation of the ON signal by the ligand-messenger as a function of the pairwise binding parameters, the number of binding sites that it features and the concentration. The behavior of our system, and the model derived, highlight the usefulness of weakly binding ligands in the regulation of membrane receptors and the pitfalls inherent to their binding promiscuity, such as non-specific binding to the membrane. Our model, and the equations derived from it, offer a valuable tool for the study of membrane receptors in both biological and biomimetic settings. The latter can be exploited to program membrane receptor avidity on sensing vesicles, create hierarchical protocell tissues or develop highly specific drug delivery vehicles.
In lipid vesicles near their membrane phase-transition temperature, the presence of a small, weakly binding ligand tips the balance for the assembly of multivalent receptor domains. We recapitulate this behaviour using a global binding-clustering model.
Understanding self-assembly in confined spaces is essential to fully understand molecular processes in confined cell compartments and will offer clues on the behaviour of simple confined systems, ...such as protocells and lipid-vesicle based devices. Using a model system composed of lipid vesicles, a membrane impermeable receptor and a membrane-permeable ligand, we have studied in detail how compartmentalization modulates the interaction between the confined receptor and its ligand. We demonstrate that confinement of one of the building blocks stabilizes complex self-assembled structures to the extent that dilution leads, counterintuitively, to the formation of long range assemblies. The behaviour of the system can be explained by considering a confinement factor that is analogous, although not identical, to the effective molarity for intramolecular binding events. The confinement effect renders complex self-assembled species robust and persistent under conditions where they do not form in bulk solution. Moreover, we show that the formation of stable complex assemblies in systems compartmentalized by semi-permeable membranes does not require the prior confinement of all components, but only that of key membrane impermeable building blocks. To use a macroscopic analogy, lipid vesicles are like ship-in-a bottle constructs that are capable of directing the assembly of the confined ship following the confinement of a few key wooden planks. Therefore, we believe that the confinement effect described here would have played an important role in shaping the increase of chemical complexity within protocells during the first stages of abiogenesis. Additionally, we argue that this effect can be exploited to design increasingly efficient functional devices based on comparatively simple vesicles for applications in biosensing, nanoreactors and drug delivery vehicles.
Polymeric vesicles, also called polymersomes, are highly efficient biomimetic systems. They can generate compartmentalized volumes at the nanoscale supported by synthetic amphiphilic membranes that ...closely mimic their biological counterparts. Membrane permeability and the ability to separate extreme pH gradients is a crucial condition a successful biomimetic system must meet. We show that polymersomes formed by non‐ionic polybutadiene‐b‐polyethylene oxide (PBd‐b‐PEO) amphiphilic block copolymers engineer robust and stable membranes that are able to sustain pH gradients of 10 for a minimum of eight days. The cells′ endo‐lysomal compartments separate gradients between three and one, while we generated a pH gradient of threefold as great. This feature clearly is of great importance for applications as nanoreactors and drug‐delivery systems where separating different aqueous volumes at the nanoscale level is an essential requirement.
Polymeric vesicles, also called polymersomes, are highly efficient biomimetic systems. The authors show that polymersomes formed by polybutadiene‐b‐polyethylene oxide (PBd‐b‐PEO) amphiphilic block copolymers can engineer robust and stable membranes that are able to sustain pH gradients of 10 pH units for at least eight days.
All-or-nothing molecular assembly events, essential for the efficient regulation of living systems at the molecular level, are emerging properties of complex chemical systems that are largely ...attributed to the cooperativity of weak interactions. The link between the self-assembly and the interactions responsible for the assembly is however often poorly defined. In this work we demonstrate how the chelate effect (multivalence cooperativity) can play a central role in the regulation of the all-or-nothing assembly of structures (supramolecular polymers here), even if the building blocks are not multivalent. We have studied the formation of double-stranded supramolecular polymers formed from Co-metalloporphyrin and bi-pyridine building blocks. Their cooperative nucleation-elongation assembly can be summarized as a thermodynamic cycle, where the monomer weakly oligomerizes linearly or weakly dimerizes laterally. But thanks to the chelate effect, the lateral dimer readily oligomerizes linearly and the oligomer readily dimerizes laterally, leading to long double stranded polymers. A model based on this simple thermodynamic cycle can be applied to the assembly of polymers with any number of strands, and allows for the determination of the length of the polymer and the all-or-nothing switching concentration from the pairwise binding constants. The model, which is consistent with the behaviour of supramolecular polymers such as microtubules and gelators, clearly shows that all-or-nothing assembly is triggered by a change in the mode of assembly, from non-multivalent to multivalent, when a critical concentration is reached. We believe this model is applicable to many molecular assembly processes, ranging from the formation of cell-cell focal adhesion points to crystallization.
Linear oligomeric supramolecular assemblies of defined length have been generated using the Vernier principle. Two molecules, containing a different number (n and m) of mutually complementary binding ...sites, separated by the same distance, interact with each other to form an assembly of length (n x m). The assembly grows in the same way as simple supramolecular polymers, but at a molecular stop signal, when the binding sites come into register, the assembly terminates giving an oligomer of defined length. This strategy has been realized using tin and zinc porphyrin oligomers as the molecular building blocks. In the presence of isonicotinic acid, a zinc porphyrin trimer and a tin porphyrin dimer form a 3:4 triple stranded Vernier assembly six porphyrins long. The triple strand Vernier architecture introduced here adds an additional level of cooperativity, yielding a stability and selectivity that cannot be achieved via a simple Vernier approach. The assembly properties of the system were characterized using fluorescence titrations and size-exclusion chromatography (SEC). Assembly of the Vernier complex is efficient at micromolar concentrations in nonpolar solvents, and under more competitive conditions, a variety of fragmentation assemblies can be detected, allowing determination of the stability constants for this system and detailed speciation profiles to be constructed.
Cooperativity in the Self-Assembly of Porphyrin Ladders Camara-Campos, Amaya; Hunter, Christopher A.; Tomas, Salvador
Proceedings of the National Academy of Sciences - PNAS,
02/2006, Letnik:
103, Številka:
9
Journal Article
Recenzirano
Odprti dostop
Cooperativity is a general feature of intermolecular interactions in biomolecular systems, but there are many different facets of the phenomenon that are not well understood. Positive cooperativity ...stabilizes a system as progressively more interactions are added, and the origin of the beneficial free energy may be entropic or enthalpic in origin. An "enthalpic chelate effect" has been proposed to operate through structural tightening that improves all of the functional group interactions in a complex, when it is more strongly bound. Here, we present direct calorimetric evidence that no such enthalpic effects exist in the cooperative assembly of supramolecular ladder complexes composed of metalloporphyrin oligomers coordinated to bipyridine ligands. The enthalpic contributions of the individual coordination interactions are 35 kJ.$mol^{-1}$and constant over a range of free energies of self-assembly of -35 to -111 kJ.$mol^{-1}$. In rigid well defined systems of this type, the enthalpies of individual interactions are additive, and no enthalpic cooperative effects are apparent. The implication is that in more flexible, less well defined systems such as biomolecular assemblies, the enthalpy contributions available from specific functional group interactions are well defined and constant parameters.
Using a combination of NMR and fluorescence measurements, we have investigated the structure and dynamics of the complexes formed between calcium-loaded calmodulin (CaM) and the potent breast cancer ...inhibitor idoxifene, a derivative of tamoxifen. High-affinity binding (
nM) saturates with a
complex. The complex is an ensemble where each idoxifene molecule is predominantly in the vicinity of one of the two hydrophobic patches of CaM but, in contrast with the lower-affinity antagonists TFP, J-8, and W-7, does not substantially occupy the hydrophobic pocket. At least four idoxifene orientations per domain of CaM are necessary to satisfy the intermolecular nuclear Overhauser effect (NOE) restraints, and this requires that the idoxifene molecules switch rapidly between positions. The CaM molecule is predominantly in the form where the N and C-terminal domains are in close proximity, allowing for the idoxifene molecules to contact both domains simultaneously. Hence, the
complex illustrates how high-affinity binding occurs without the loss of extensive positional dynamics.
A partir de la novela La ciudad ausente (1992) y el cuento “La loca y el relato del crimen” (1975), Ricardo Piglia organiza un espacio estructurado por distintas voces: las voces institucionales (el ...discurso policial, médico y jurídico), las cuales entran en tensión con las voces adyacentes y periféricas (de gauchos, inventores, locas y periodistas). Aquellas voces adquieren una corporalidad en las distintas máquinas de narrar, como la grabadora, el teléfono y el casete. Literatura y medios. El estudio de la voz (una voz que se simula en el texto) en ambas narraciones de Piglia es permeable a distintos discursos filosóficos y psicológicos. ¿Qué sucedería si los flujos de información (lo dicho) no pudiesen ser controlados, codificados y, por ende, reterritorializados, es decir, vueltos a poner en un lugar, rotularlos como, por ejemplo, lícitos o ilícitos? El objetivo del trabajo es analizar las voces presentes en los textos literarios y la dinámica de control, censura y autoridad que las teje, desde donde se propone pensar y trabajar la hipótesis de que Piglia construye a Junior y Renzi, personajes medulares de los textos, como oyentes atentos.
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