Within each biological cell, surface- and volume-confined enzymes control a highly complex network of chemical reactions. These reactions are efficient, timely, and spatially defined. Efforts to transfer such appealing features to in vitro systems have led to several successful examples of chemical reactions catalysed by isolated and immobilized enzymes. In most cases, these enzymes are either bound or adsorbed to an insoluble support, physically trapped in a macromolecular network, or encapsulated within compartments. Advanced applications of enzymatic cascade reactions with immobilized enzymes include enzymatic fuel cells and enzymatic nanoreactors, both for in vitro and possible in vivo applications. In this Review, we discuss some of the general principles of enzymatic reactions confined on surfaces, at interfaces, and inside small volumes. We also highlight the similarities and differences between the in vivo and in vitro cases and attempt to critically evaluate some of the necessary future steps to improve our fundamental understanding of these systems.

Photosynthesis is responsible for the photochemical conversion of light into the chemical energy that fuels the planet Earth. The photochemical core of this process in all photosynthetic organisms is a transmembrane protein called the reaction center. In purple photosynthetic bacteria a simple version of this photoenzyme catalyzes the reduction of a quinone molecule, accompanied by the uptake of two protons from the cytoplasm. This results in the establishment of a proton concentration gradient across the lipid membrane, which can be ultimately harnessed to synthesize ATP. Herein we show that synthetic protocells, based on giant lipid vesicles embedding an oriented population of reaction centers, are capable of generating a photoinduced proton gradient across the membrane. Under continuous illumination, the protocells generate a gradient of 0.061 pH units per min, equivalent to a proton motive force of 3.6 mV⋅min Remarkably, the facile reconstitution of the photosynthetic reaction center in the artificial lipid membrane, obtained by the droplet transfer method, paves the way for the construction of novel and more functional protocells for synthetic biology.

This article deals with artificial vesicles and their membranes as reaction promoters and regulators. Among the various molecular assemblies which can form in an aqueous medium from amphiphilic molecules, vesicle systems are unique. Vesicles compartmentalize the aqueous solution in which they exist, independent on whether the vesicles are biological vesicles (existing in living systems) or whether they are artificial vesicles (formed in vitro from natural or synthetic amphiphiles). After the formation of artificial vesicles, their aqueous interior (the endovesicular volume) may become - or may be made - chemically different from the external medium (the exovesicular solution), depending on how the vesicles are prepared. The existence of differences between endo- and exovesicular composition is one of the features on the basis of which biological vesicles contribute to the complex functioning of living organisms. Furthermore, artificial vesicles can be formed from mixtures of amphiphiles in such a way that the vesicle membranes become molecularly, compositionally and organizationally highly complex, similarly to the lipidic matrix of biological membranes. All the various properties of artificial vesicles as membranous compartment systems emerge from molecular assembly as these properties are not present in the individual molecules the system is composed of. One particular emergent property of vesicle membranes is their possible functioning as promoters and regulators of chemical reactions caused by the localization of reaction components, and possibly catalysts, within or on the surface of the membranes. This specific feature is reviewed and highlighted with a few selected examples which range from the promotion of decarboxylation reactions, the selective binding of DNA or RNA to suitable vesicle membranes, and the reactivation of fragmented enzymes to the regulation of the enzymatic synthesis of polymers. Such type of emergent properties of vesicle membranes may have been important for the prebiological evolution of protocells, the hypothetical compartment systems preceding the first cells in those chemical and physico-chemical processes that led to the origin of life.

Kinetics and particle size distributions in the spontaneous micelle-vesicle transformation of oleate vesicles are investigated by dynamic light scattering and optical density measurements. It is shown that, depending upon the conditions, there are two different kinetic processes for vesicles formation, a cooperative and a uncooperative one. In particular, the time progress of vesicle formation follows a sigmoid behavior and it is accelerated by the presence of preformed vesicles, confirming that a cooperative micelle-vesicle interaction takes place. This effect is investigated by adding fresh oleate surfactant to preformed, extruded (50 and 100 nm) oleate vesicles as well as to 50 and 100 nm extruded POPC (POPC is 1-palmitoyil-2-oleyl-sn-glycero-3-phosphocholine) liposomes. It is shown that both in the case of oleate and POPC the rate process of vesicle formation is remarkably accelerated with respect to the control (no preformed vesicles); and the final size distribution is very close to the narrowly distribution curve of the preformed vesicles, whereas in the control experiment (no preadded vesicles) a very broad distribution is obtained. We called this phenomenon the matrix effect to stress that preexisting vesicles or liposomes act as a matrix on the formation of the new ones. The time evolution of the particles average radius after addition of fresh surfactant is followed by means of the dynamic light scattering cumulant analysis, and thus when oleate micelles are added to oleate vesicles, a slight increase of the average dimensions is observed, whereas in the case of oleate added to POPC liposomes, the final radius is actually smaller than the initial radius. This confirms that under certain conditions, fission of vesicles is possible. Finally, some theoretical considerations on the kinetics of the micelle-vesicle cooperative transformation are drawn.

Self-assembled monolayers (SAMs) derived of 4-methoxy-terphenyl-3'',5''-dimethanethiol (TPDMT) and 4-methoxyterphenyl-4''-methanethiol (TPMT) have been prepared by chemisorption from solution onto gold thin films and nanoparticles. The SAMs have been characterized by spectroscopic ellipsometry, Raman spectroscopy and atomic force microscopy to determine their optical properties, namely the refractive index and extinction coefficient, in an extended spectral range of 0.75-6.5 eV. From the analysis of the optical data, information on SAMs structural organization has been inferred. Comparison of SAMs generated from the above aromatic thiols to well-known SAMs generated from the alkanethiol dodecanethiol revealed that the former aromatic SAMs are densely packed and highly vertically oriented, with a slightly higher packing density and a absence of molecular inclination in TPMT/Au. The thermal behavior of SAMs has also been monitored using ellipsometry in the temperature range 25-500 degrees C. Gold nanoparticles functionalized by the same aromatic thiols have also been discussed for surface enhanced Raman spectroscopy applications. This study represents a step forward tailoring the optical and thermal behavior of surfaces as well as nanoparticles.