Oleksandr

Prior to the evolution of membrane proteins, intrinsic membrane stability and permeability to polar solutes are essential features of a primitive cell membrane. These features are difficult to achieve simultaneously in model protocells made of either pure fatty acid or phospholipid membranes, raising the intriguing question of how the transition from fatty acid to phospholipid membranes might have occurred while continuously supporting encapsulated reactions required for genomic replication. Here, the properties of a blended membrane system composed of both oleic acid (OA), a monoacyl fatty acid, and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), a diacyl phospholipid are described. This hybrid vesicle system exhibits high stability to divalent cations (Mg²⁺), while simultaneously maintaining its permeability to small charged molecules such as nucleotides and divalent ions such as Mg²⁺. This combination of features facilitates key reactions expected to occur during a transition from primitive to modern cells, including nonenzymatic RNA replication, and is also compatible with highly evolved functions such as the ribosomal translation of a protein. The observations support the hypothesis that the early transition from fatty acid to phospholipid membranes could be accomplished through intermediate states in which membranes are composed of amphiphile mixtures, and do not require protein transporters.

The initial transition from fatty acid membranes (gray) to phospholipid membranes (purple) in primitive cells may have been facilitated by the properties of membranes composed of both amphiphiles. Such hybrid membranes achieve high stability to divalent cations while maintaining permeability to small charged molecules, thus allowing critical chemical reactions to proceed within primitive cells.

Dissipative self-assembly leads to structures and materials that exist away from equilibrium by continuously exchanging energy and materials with the external environment. Although this mode of self-assembly is ubiquitous in nature, where it gives rise to functions such as signal processing, motility, self-healing, self-replication, and ultimately life, examples of dissipative self-assembly processes in man-made systems are few and far between. Herein, recent progress in developing diverse synthetic dissipative self-assembly systems is discussed. The systems reported thus far can be categorized into three classes, in which: i) the fuel chemically modifies the building blocks, thus triggering their self-assembly, ii) the fuel acts as a template interacting with the building blocks noncovalently, and iii) transient states are induced by the addition of two mutually exclusive stimuli. These early studies give rise to materials that would be difficult to obtain otherwise, including hydrogels with programmable lifetimes, vesicular nanoreactors, and membranes exhibiting transient conductivity.

Although dissipative self-assembly is ubiquitous in nature, it is largely absent from man-made systems. Recent progress in designing structures and materials that require chemical fuels to exist is reviewed. These studies give rise to unique materials, such as self-destructing gels and membranes exhibiting transient conductivity.

Photoswitchable acid-base pairs, allowing for a reversible alteration of their pKa values, are attractive molecular tools to control chemical and biological processes by light. A significant, light-induced pKa change of three units in aqueous medium has been realized involving two thermally stable states, which can be interconverted using UV and green light. The light-induced pKa modulation is based on incorporating a 3-H-thiazol-2-one moiety into the framework of a diarylethene photoswitch, which upon photochemical ring-closure loses the heteroaromatic stabilization of the corresponding, negatively charged base and hence, becomes significantly less acidic. In addition, the efficiency of the photoreactions in the deprotonated state is drastically increased, thereby giving rise to catalytically enhanced photochromism. It appears that protonation has a significant influence on the shape of ground and excited state potential energy surface as indicated by quantum chemical calculations. Using this concept of interconnected photoisomerization and acid-base equilibria allows to adjust the degree of photoconversion by the pH of the solution and constitutes a powerful tool to precisely and remotely control the charge state by light.

Today we are poised for a transition from the highly customized crafting of specific molecular targets by hand to the increasingly general and automated assembly of different types of molecules with the push of a button. Creating machines that are capable of making many different types of small molecules on demand, akin to that which has been achieved on the macroscale with 3D printers, is challenging. Yet important progress is being made toward this objective with two complementary approaches: 1) Automation of customized synthesis routes to different targets by machines that enable the use of many reactions and starting materials, and 2) automation of generalized platforms that make many different targets using common coupling chemistry and building blocks. Continued progress in these directions has the potential to shift the bottleneck in molecular innovation from synthesis to imagination, and thereby help drive a new industrial revolution on the molecular scale.

Automation is enabling a new Industrial Revolution, this time on the molecular scale. This Minireview provides insights into the ongoing transition from manual to automated organic synthesis. Akin to a 3D printer for molecules, continued progress in this frontier area will democratize innovation on the molecular scale and shift the rate-limiting step in new molecular function discovery from synthesis to imagination.

Chiral catenanes and rotaxanes are the subject of the Minireview by N. H. Evans on page 3101 ff. Mechanically interlocked molecules may be chiral by either inclusion of a classical chiral element and/or as a consequence of the mechanical bond. Possessing 3D structures, chiral catenanes and rotaxanes are beginning to emerge as excellent candidates for applications such as chiral host-guest recognition and asymmetric catalysis.

John Wiley and Sons, Hoboken 2018. 464 pp., hardcover, € 182.00.—ISBN 978-1119075639