[신간의 별자리x] 우리/미술, 그리고 ‘슬픔의 박물관’

J. Appl. Cryst.の発刊に際して

Molecular self-assembly is the spontaneous association of molecules under equilibrium conditions into stable, structurally well-defined aggregates joined by noncovalent bonds. Molecular self-assembly is ubiquitous in biological systems and underlies the formation of a wide variety of complex biological structures. Understanding self-assembly and the associated noncovalent interactions that connect complementary interacting molecular surfaces in biological aggregates is a central concern in structural biochemistry. Self-assembly is also emerging as a new strategy in chemical synthesis, with the potential of generating nonbiological structures with dimensions of 1 to 10(2) nanometers (with molecular weights of 10(4) to 10(10) daltons). Structures in the upper part of this range of sizes are presently inaccessible through chemical synthesis, and the ability to prepare them would open a route to structures comparable in size (and perhaps complementary in function) to those that can be prepared by microlithography and other techniques of microfabrication.

Molecular self-assembly and nanochemistry: A chemical strategy for the synthesis of nanostructures

The widespread use of controlled molecular-level motion in key natural processes suggests that great rewards could come from bridging the gap between the present generation of synthetic molecular systems, which by and large rely upon electronic and chemical effects to carry out their functions, and the machines of the macroscopic world, which utilize the synchronized movements of smaller parts to perform specific tasks. This is a scientific area of great contemporary interest and extraordinary recent growth, yet the notion of molecular-level machines dates back to a time when the ideas surrounding the statistical nature of matter and the laws of thermodynamics were first being formulated. Here we outline the exciting successes in taming molecular-level movement thus far, the underlying principles that all experimental designs must follow, and the early progress made towards utilizing synthetic molecular structures to perform tasks using mechanical motion. We also highlight some of the issues and challenges that still need to be overcome.

Synthetic molecular motors and mechanical machines.

The widespread use of molecular machines in biology has long suggested that great rewards could come from bridging the gap between synthetic molecular systems and the machines of the macroscopic world. In the last two decades, it has proved possible to design synthetic molecular systems with architectures where triggered large amplitude positional changes of submolecular components occur. Perhaps the best way to appreciate the technological potential of controlled molecular-level motion is to recognize that nanomotors and molecular-level machines lie at the heart of every significant biological process. Over billions of years of evolution, nature has not repeatedly chosen this solution for performing complex tasks without good reason. When mankind learns how to build artificial structures that can control and exploit molecular level motion and interface their effects directly with other molecular-level substructures and the outside world, it will potentially impact on every aspect of functional molecule and materials design. An improved understanding of physics and biology will surely follow.

The first steps on the long path to the invention of artificial molecular machines were arguably taken in 1827 when the Scottish botanist Robert Brown observed the haphazard motion of tiny particles under his microscope.1,2 The explanation for Brownian motion, that it is caused by bombardment of the particles by molecules as a consequence of the kinetic theory of matter, was later provided by Einstein, followed by experimental verification by Perrin.3,4 The random thermal motion of molecules and its implications for the laws of thermodynamics in turn inspired Gedankenexperiments (“thought experiments”) that explored the interplay (and apparent paradoxes) of Brownian motion and the Second Law of Thermodynamics. Richard Feynman’s famous 1959 lecture “There’s plenty of room at the bottom” outlined some of the promise that manmade molecular machines might hold.5,6 However, Feynman’s talk came at a time before chemists had the necessary synthetic and analytical tools to make molecular machines. While interest among synthetic chemists began to grow in the 1970s and 1980s, progress accelerated in the 1990s, particularly with the invention of methods to make mechanically interlocked molecular systems (catenanes and rotaxanes) and control and switch the relative positions of their components.7−24

Here, we review triggered large-amplitude motions in molecular structures and the changes in properties these can produce. We concentrate on conformational and configurational changes in wholly covalently bonded molecules and on catenanes and rotaxanes in which switching is brought about by various stimuli (light, electrochemistry, pH, heat, solvent polarity, cation or anion binding, allosteric effects, temperature, reversible covalent bond formation, etc.). Finally, we discuss the latest generations of sophisticated synthetic molecular machine systems in which the controlled motion of subcomponents is used to perform complex tasks, paving the way to applications and the realization of a new era of “molecular nanotechnology”.

1.1. The Language Used To Describe Molecular Machines
Terminology needs to be properly and appropriately defined and these meanings used consistently to effectively convey scientific concepts. Nowhere is the need for accurate scientific language more apparent than in the field of molecular machines. Much of the terminology used to describe molecular-level machines has its origins in observations made by biologists and physicists, and their findings and descriptions have often been misinterpreted and misunderstood by chemists. In 2007 we formalized definitions of some common terms used in the field (e.g., “machine”, “switch”, “motor”, “ratchet”, etc.) so that chemists could use them in a manner consistent with the meanings understood by biologists and physicists who study molecular-level machines.14

The word “machine” implies a mechanical movement that accomplishes a useful task. This Review concentrates on systems where a stimulus triggers the controlled, relatively large amplitude (or directional) motion of one molecular or submolecular component relative to another that can potentially result in a net task being performed. Molecular machines can be further categorized into various classes such as “motors” and “switches” whose behavior differs significantly.14 For example, in a rotaxane-based “switch”, the change in position of a macrocycle on the thread of the rotaxane influences the system only as a function of state. Returning the components of a molecular switch to their original position undoes any work done, and so a switch cannot be used repetitively and progressively to do work. A “motor”, on the other hand, influences a system as a function of trajectory, meaning that when the components of a molecular motor return to their original positions, for example, after a 360° directional rotation, any work that has been done is not undone unless the motor is subsequently rotated by 360° in the reverse direction. This difference in behavior is significant; no “switch-based” molecular machine can be used to progressively perform work in the way that biological motors can, such as those from the kinesin, myosin, and dynein superfamilies, unless the switch is part of a larger ratchet mechanism.14

Artificial Molecular Machines

Water is fundamental to life, playing a variety of functions related to its complex dynamic behavior at the supramolecular level. Most of the physiological processes depend on the selective exchange of ions or molecules between a cell and its environment, with water playing a crucial role in their translocation events. Artificial ion channels have been extensively studied with the hope of facilitating ionic conduction in bilayer membranes. However, there has been less progress in the area of synthetic water channels. In contrast to spherical ions, dipolar water molecules shed light on the inherent multiple interactions with biological environments through the combination of weak reversible bonds, namely hydrogen bonding, salt bridges, dipolar, and coordinative interactions. Moreover, the overall dipolar orientation of assembled water dipoles is of tremendous importance in the regulation of the selective transport of charged components across the cell membrane. For example, encapsulated water wires of opposite dipolar orientation control the electrochemical potential selectively along the aquaporin pore Aqp channel, thereby allowing the rapid diffusion of water, while protons and ions are excluded. The water permeability of Aqp-incorporated polymeric vesicles has been at least an order of magnitude larger than values obtained for classical polymeric membranes. Moreover, it has been shown that about five water molecules may flow per nanosecond through carbon nanotube membranes under an osmotic gradient comparable to those measured for biological water channels. Finally, the one-dimensional water wires have attracted a lot interest, and selective proton gating is the key function of the influenza A M2 proton channel. Different water clusters have been entrapped within complex superstructures. They have attracted a lot interest in terms of a variety of fundamental processes such as water structuration and also in the context of special aspects of water–cluster interactions with the host matrix or diffusional phenomena under confined conditions. Despite such an impressive development, only a few artificial hydrophobic, hydrophilic, and hybrid amphiphilic pores have been designed to selectively transport water very efficiently through bilayer membranes. Water transport across phospholipid vesicles can be monitored using optical microscopy or dynamic light scattering. The molecular-scale hydrodynamics of water through the channel will depend on channel–water and water–water interactions as well as on the in-pore electrostatic dipolar profile of the water within the channel. The diffusion of water and facilitated transport of protons with exclusion of the transport of other cations and anions through bilayer membranes were reported for the first time by Percec et al. The dendritic dipeptides 1 (Figure 1a) selfassemble through enhanced peripheral p stacking to form stable helical pores in bilayers. These pores, envisioned as “primitive aquaporins”, transport water but do not exclude protons. The ion-exclusion phenomena are based on hydrophobic effects which appear to be very important. Later, Barboiu and co-workers reported that imidazole (I) quartets can be mutually stabilized by inner dipolar water wires, reminiscent of G-quartets stabilized by cation templating. The I-quartets are stable in solution, in the solid state, and within bilayers, leading to the tubular channel-type chiral superstructures. These systems have provided excellent reasons to consider that the supramolecular chirality of Iquartets and water-induced polarization within the channels may be strongly associated. The confined water wires, similar to the situation in aquaporin channels, form one hydrogen bond with the inner wall of the I-quartet and one hydrogen bond with an adjacent water molecule. Moreover, the water molecules adopt a unique dipolar orientation and preserve the overall electrochemical dipolar potential along the channel. These results strongly indicate that water molecules and protons can permeate the bilayer membranes through Iquartet channels. The ion-exclusion phenomena are based on dimensional steric effects, whereas hydrophobic and hydrodynamic effects appear to be less important. The water-free “off form” superstructure of the I-quartet is reminiscent of the closed conformation of the proton gate of the influenza A M2 protein. The slight conformational adjustments allow formation of the water-templated I-quartet, through which protons can diffuse along a dipolar oriented water wire in the open-state pore–gate region. These artificial I-quartet superstructures obtained by using a simple chemical approach are in excellent agreement with structural X-ray and NMR spectroscopic results as well as theoretical results that might provide accurate mechanistic explanations for water/proton conductance through the influenza A M2 proton channel. In a very recent report Hou and co-workers proposed a very elegant artificial system that functions exclusively as single-molecular water channels. Polydrazide-substituted pillar[5]arenes were used to form tubular hydrogen-bonded [*] Dr. M. Barboiu Adaptive Supramolecular Nanosystems Group Institut Europeen des Membranes, ENSCM-UMII-UMR CNRS 5635 Place Eugene Bataillon CC047, 34095 Montpellier (France) E-mail: mihai.barboiu@iemm.univ-montp2.fr . Angewandte Highlights

Artificial water channels.

Self-Organized Heteroditopic Macrocyclic Superstructures

Inspired by biological proteins, artificial water channels (AWCs) can be used to overcome the performances of traditional desalination membranes. Their rational incorporation in composite polyamide provides an example of biomimetic membranes applied under representative reverse osmosis desalination conditions with an intrinsically high water-to-salt permeability ratio. The hybrid polyamide presents larger voids and seamlessly incorporates I–quartet AWCs for highly selective transport of water. These biomimetic membranes can be easily scaled for industrial standards (>m2), provide 99.5% rejection of NaCl or 91.4% rejection of boron, with a water flux of 75 l m−2 h−1 at 65 bar and 35,000 ppm NaCl feed solution, representative of seawater desalination. This flux is more than 75% higher than that observed with current state-of-the-art membranes with equivalent solute rejection, translating into an equivalent reduction of the membrane area for the same water output and a roughly 12% reduction of the required energy for desalination. Inspired by biological models, I–quartet artificial water channels can be incorporated in composite polyamide membranes synthesized via interfacial polymerization, providing biomimetic membranes for desalination.

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Biomimetic artificial water channel membranes for enhanced desalination

Aquaporins (AQPs) are biological water channels known for fast water transport (∼108–109 molecules/s/channel) with ion exclusion. Few synthetic channels have been designed to mimic this high water permeability, and none reject ions at a significant level. Selective water translocation has previously been shown to depend on water-wires spanning the AQP pore that reverse their orientation, combined with correlated channel motions. No quantitative correlation between the dipolar orientation of the water-wires and their effects on water and proton translocation has been reported. Here, we use complementary X-ray structural data, bilayer transport experiments, and molecular dynamics (MD) simulations to gain key insights and quantify transport. We report artificial imidazole-quartet water channels with 2.6 A pores, similar to AQP channels, that encapsulate oriented dipolar water-wires in a confined chiral conduit. These channels are able to transport ∼106 water molecules/s, which is within 2 orders of magnitude...

Mihail Barboiu

Papers

Artificial water channels.

Self-Organized Heteroditopic Macrocyclic Superstructures

Biomimetic artificial water channel membranes for enhanced desalination

Salt-Excluding Artificial Water Channels Exhibiting Enhanced Dipolar Water and Proton Translocation.

Constitutional Dynamic Materials—Toward Natural Selection of Function