SPring-8, the large synchrotron radiation facility

Release Date

30 Apr, 2010

The University of Tokyo

Many viruses are known to have spherical shells. These shells are distinct structures made of several hundred to several thousand protein subunits that assemble spontaneously. Such spontaneous assembly of biological structures is called self-assembly and is familiar in the field of biology. How do living organisms achieve distinct structures through the self-assembly of several hundred to several thousand components? In the field of biology, some answers to this question have been proposed. However, in the field of chemistry directly related to manufacturing, we can only understand the self-assembly mechanism once we succeed in the artificial self-assembly of distinct structures consisting of several hundred molecules that are artificially synthesized by chemical methods. Moreover, clarification of the self-assembly mechanism will lead to the development of new manufacturing methods in which the assembly mechanism of biological structures is replaced by an artificial system.

In this study, we combined metal ions (M) with slightly bent organic ligands^*2 (L) to successfully form giant M₂₄L₄₈ coordination spheres by self-assembly (Fig. 1). This distinct structure was generated with 100% accuracy through the self-assembly of these components without any distribution in the size and number of components used.

The key findings of this study are as follows.
(1) The unprecedented self-assembly of 72 components was artificially realized, resulting in the formation of giant spheres with a distinct structure (the largest distinct structure ever formed in the world).

(2) The obtained aggregate of the many components had considerable stability. The results experimentally demonstrated that living organisms achieve stability through the self-assembly mechanism.

(3) Owing to the combination of two types of ligand, the structure of the aggregate critically changes from M₂₄L₄₈ to M₁₂L₂₄ when the bent angle is slightly increased to a certain value.

The phenomenon described in (3) can be explained using geometry. It has been observed in the generation of viruses that, when many components are assembled, highly symmetric regular and semiregular polyhedra (three-dimensional figures made up of a combination of regular polygons) are generated. While the structures of products seem to be numerous, according to geometry textbooks there are only five types of structure (n = 6, 12, 24, 30 and 60 for M_nL_2n coordination: Fig. 2(A)). Namely, when the initial structure of the components is slightly and successively changed, the final structure of the product changes discontinuously from M₆L₁₂ to M₁₂L₂₄ and then to M₂₄L₄₈ at certain thresholds (Figs. 2(B) and 2(C)). Subtle analogue changes generate a major digital change. We found that this is how living organisms achieve distinct structures through the self-assembly of many components.

The phenomenon wherein a small difference in an initial condition results in an incommensurable difference is called "emergence" and has recently been attracting attention from various fields of natural science. Our research results may be the first example of "chemical emergence" on the basis of a clear molecular system and has academic significance. Through an experiment using artificial molecules, we verified that living organisms achieve high stability and distinct biological structures using the phenomenon of emergence.

These research results were achieved under the following projects and research topics.

(1) Core Research for Evolutional Science and Technology (CREST)
Research area: "Development of the Foundation for Nano-Interface Technology" (Research supervisor: Seiji Shinkai, Professor of Sojo University)
Research topic: Chemistry of Self-Assembled Finite Nano-Interfaces
Lead researcher: Makoto Fujita (Professor of School of Engineering, The University of Tokyo)
Term of project: FY2007-2012

(2) Grant-in-Aid for Scientific Research on Innovative Areas by the Ministry of Education, Culture, Sports, Science and Technology
Research area: "Emergence in Chemistry of Nano-Scale Molecular Systems" (Research supervisor: Tomoji Kawai, Professor of Osaka University)
Research topic: Emergent Self-Assembly of Distinct Structures Consisting of Several Dozen or Hundreds of Components
Lead researcher: Makoto Fujita (Professor of School of Engineering, The University of Tokyo)
Term of project: FY2008-2012

(3) The X-ray crystal structure analysis data were obtained using the Structural Biology III Beamline (BL38B1) at SPring-8 and the PF-AR NW2A Beamline at the High Energy Accelerator Research Organization (KEK) Photon Factory.

<Figure>




Fig. 1 M₂₄L₄₈ coordination compound determined by X-ray crystal structure analysis







Fig. 2 A) Five types of M_nL_2n coordination structures. B) and C) A small difference in the initial structure (1 and 3) results in the formation of critically different final structures, namely, M₂₄L₄₈ (2) and M₁₂L₂₄ coordination structures (4)




<Glossary>

*1 Distinct structure
A strictly defined structure. The number and assembly behavior of the protein subunits that make up the structure are strictly defined without any distribution in their shape, size and weight (molecular weight).

*2 Ligands
Molecules that form weak bonds with metal ions. Owing to the cooperation of these weak bonds, the 72 components are bound to each other to form an M₂₄L₄₈ coordination compound.