[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

Powerful synthetic scaffolds mimicking natural proteinfunctions unlock the door to a world of interactive materialsparalleling that of biology. Numerous artificial systemsshowing a rich array of interconverting ion-channel conduc-tance states in phospholipid and polymeric membranes havebeen developed in the last decades.

Imidazole-Quartet Water and Proton Dipolar Channels†

Carbonic anhydrase activators. Part 14. Syntheses of mono and bis pyridinium salt derivatives of 2-amino-5-(2-aminoethyl)- and 2-amino-5-(3-aminopropyl)-1,3,4-thiadiazole and their interaction with isozyme II

G-quartets, formed by the hydrogen-bonding self-assembly of four guanosine (G) residues and stabilized by alkali-metal cations, play an important role in biology, in particular in nucleic acid telomers of potential interest to cancer therapy. The G-quartet architecture represents a nice example of a dynamic supramolecular system that has been used as a building block for gelators, columnar polymeric aggregates, self-organized surfaces, and prototypes of chemical dynamic devices. 5] . In the last few decades, G-quartets and the similar folic acid quartet have also been proposed as powerful scaffolds for building synthetic ion channels. Although stable in organic solvents, they do not seem to have defined transport functions in hydrophobic membranes. Barrel-stave, lipophilic, calix[4]arene, and 8-aromatic–guanosine conjugates have been used to stabilize the formation of G-quartets. Recently, a new strategy based on reversible metathesis was successfully used by Davis et al. to generate a rich array of interconverting ionchannel conductance states of a unimolecular G-quartet in a phospholipid membrane. Despite such impressive progress, considerable challenges still lie ahead and the more significant one is to improve the stability of G-quartet dynamic aggregates in polymeric devices, such as films or membranes, to extend (address) the transport studies to the macroscopic level. Several earlier studies reported the preparation of discrete supramolecular assemblies of nucleobases embedded in synthetic polymers and hybrid materials. However, the “dynamic communication” between the supramolecular self-assembly of nucleobases and the polymerization processes, which kinetically and stereochemically might communicate, is not so trivial. For all these reasons, in this study building blocks of guanosine (1; Scheme 1), 2-formylphenylboronic acid (2), and bis(3-aminopropyl)polytetrahydrofuran (PTHF, 3 ; numberaverage molecular weight Mn ca. 1100 gmol ) are used as molecular precursors to conceive G-quartet polymeric membrane materials at the macroscopic scale. Our efforts involve the successive synthesis of the ditopic bisiminoboronic 4 and bisiminoboronate-guanosine 5macromonomers, and then the self-assembly of 5 into G-quartet-type supramolecular superstructures (Scheme 1). The main strategy consists of generating (amplifying) dynamic supramolecular G-quartets by K ion templating, from a dynamic pool of oligomeric ribbon-type or cyclic supramolecular architectures. Then, the G-quadruplex architectures are fixed in self-supporting polymeric membrane films. A standard sample without potassium chloride, which resulted in the formation of an H-bond ribbon-type superstructure of the guanine moieties, was prepared as reference under the same conditions as described above (Figure 1). Scheme 1. Synthesis of bisiminoboronic 4 and bisiminoboronate-guanosine 5 macromonomers, and the self-assembly of 5 into G-quartettype superstructures.

Functional G‐Quartet Macroscopic Membrane Films

Imino-chitosan biodynamers

The polyheterocyclic strands 1-H and 2-H adopt a helical shape enforced by the pyridine-pyrimidine helicity codon. The crystal structure of 2-H shows the formation of stacks of dimers of right- and left-handed individual helices. Treatment of 1-H and 2-H with silver triflate results in the generation of double-helical entities 1-DH and 2-DH, containing two strands and two silver ions. NMR studies and determination of the crystal structure of 2-DH indicate that the duplex is stabilized by coordination of each Ag(+) ion to two terminal bipyridine units, one from each strand, and by pronounced pi-pi stacking interactions between the internal heterocycles of the strands, yielding a very robust double helical structure. Reversible interconversion of the single and double helix may be achieved by addition of a cryptand capable of sequestering Ag(+) and releasing it by protonation. Thus, successive addition of acid and base leads to reversible interconversion between the shorter ( approximately 3.6 A) single helix and the longer ( approximately 10.3 A) double helix, resulting in the generation of pronounced extension/contraction motion. The system 1,2-H/1,2-DH represents a dynamic chemical device undergoing ionic modulation of reversible molecular mechanical motion fueled by acid/base neutralization.

Mihail Barboiu

Papers

Imidazole-Quartet Water and Proton Dipolar Channels†

Carbonic anhydrase activators. Part 14. Syntheses of mono and bis pyridinium salt derivatives of 2-amino-5-(2-aminoethyl)- and 2-amino-5-(3-aminopropyl)-1,3,4-thiadiazole and their interaction with isozyme II

Functional G‐Quartet Macroscopic Membrane Films

Imino-chitosan biodynamers

Dynamic Chemical Devices: Generation of Reversible Extension/Contraction Molecular Motion by Ion-Triggered Single/Double Helix Interconversion