[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

https://hal.archives-ouvertes.fr/hal-03028982/document

Single-walled carbon nanotubes-G-quadruple hydrogel nanocomposite matrixes for cell support applications.

Coulombically repulsive cationic guests can be encapsulated in the multicomponent capsule hosts via favorable ion pairing, H-bonding intermolecular interactions. The inclusion of protonated and diprotonated 2-pycolyl amine guests in the calix[4]arene tetrasulfonate, tetrasodium salt C[4]) host is reminiscent with complex dynamic equilibria in solution. The addition of AG aminoguanidinium cations to such mixtures results in the formation of distinct crystalline bilayer and capsule architectures in the solid state. This show that libraries of interexchanging supramolecular components may convert into robust solid state multicomponent capsules via constitutional crystallization.

Crystallization-Driven Multicomponent Encapsulation of Coulombically Repulsive Guests

Experimental evidence presented in this paper shows that long-range crystalline order as observed in the macrocyclic heteroditopic receptors’ single-crystal structures is preserved when embedded in the bilayer membranes. Investigations presented here showed that MCM41-type mesoporous hydrophobic hybrids can be used as biomimetic platforms for ion-channel biomimetic structure elucidation, while showing similar hydrophobic interactions with confined ligands/receptors as in bilayer membranes. This gives access to information concerning the structural pictures of the bilayer’s confined ion-channel structures, as well as of their relatives in the presence of ion effectors.

X-ray Structure Determination of Ion-Channel Crystalline Architectures Close to Bilayer Membrane-Confined Conditions

Biogenic formation of silica is a fundamental process occurring under mild conditions. Here we report autocatalytic generation of silica hybrid materials. Self-organized siloxanes containing imidazole self-catalytic groups can be transformed to hybrid silica by using mild biomimetic conditions similar to those under which primitive organisms make silica.

Biomimetic Autocatalytic Synthesis of Organized Silica Hybrids

We describe here a study of the coordination binding of Co 2+ , Cu 2+ and Pb 2+ ions to the ditopic ligand 1, leading to the formation of rack-type and double helical monomers. We report the crystal structures of three such complexes 5-7, which further self-organize into complementary duplex-rack, square and double helical architectures in the solid state. The ligand 1 discussed in this paper operates under conditions in which the two terpyridine (terpy) coordination sites resulted from Pyridine-Pyridine-Pyridazine connected sequences can be involved in the orthogonal binding events of the octahedral metal ions. In terms of molecular adaptability the formation of rack-type and double helical architectures represents the best coordination frameworks of the rigid ligand 1 presenting unfavorable orientation of coordination vectors. In terms of programmed self-assembly the formation of the different metallo-architectures 5, 6 and 7 underlines the fact that, despite the role of the strength of the coordination interactions and of maximal site occupation, other factors like stacking, binding of anions (triflate or tetrafluoroborate anions stabilizing the network of metallic ions) and of solvent molecules may interfere and influence the nature of the favored output species.

Mihail Barboiu

Papers

Single-walled carbon nanotubes-G-quadruple hydrogel nanocomposite matrixes for cell support applications.

Crystallization-Driven Multicomponent Encapsulation of Coulombically Repulsive Guests

X-ray Structure Determination of Ion-Channel Crystalline Architectures Close to Bilayer Membrane-Confined Conditions

Biomimetic Autocatalytic Synthesis of Organized Silica Hybrids

Rack, square and double helical dinuclear metallosupramolecular architectures : Coordination behavior, anion binding and stacking interactions