Hiroyasu Yamaguchi

Self‐Assembly of One‐ and Two‐Dimensional Hemoprotein Systems by Polymerization through Heme–Heme Pocket Interactions

Supramolecular protein polymers: When a heme moiety was introduced to the surface of an apo-cytochrome b(562)(H63C) mutant, supramolecular polymers formed through noncovalent heme-heme pocket interactions. The incorporation of a heme triad as a pivot molecule in the protein polymer further led to a two-dimensional protein network structure, which was visualized by tapping-mode atomic force microscopy (see picture).

Ring-opening polymerization of cyclic esters by cyclodextrins.

Synthetic polymers, typically prepared by addition polymerization or stepwise polymerization, are used constantly in our daily lives. In recent years, polymer scientists have focused on more environmentally friendly synthetic methods such as mild reaction conditions and biodegradable condensation polymers, including polyesters and polyamides. However, challenges remain in finding greener methods for the synthesis of polymers. Although reactions carried out in water are more environmentally friendly than those in organic solvents, aqueous media can lead to the hydrolysis of condensation polymers. Furthermore, bulk polymerizations are difficult to control. In biological systems, enzymes synthesize most polymers (proteins, DNAs, RNAs, and polysaccharides) in aqueous environments or in condensed phases (membranes). Most enzymes, such as DNA polymerases, RNA polymerases, and ribosomes, form doughnutlike shapes, which encircle the growing polymer chain. As biopolymers form, the active sites and the substrate-combining sites are located at the end of the growing polymer chain and carefully control the polymerization. Therefore, a synthetic catalyst that could insert the monomers between the active site and binding site would create an ideal biomimetic polymerization system. In this Account, we describe cyclodextrins (CDs) as catalysts that can polymerize cyclic esters (lactones and lactides). CDs can initiate polymerizations of cyclic esters in bulk without solvents (even water) to give products in high yields. During our studies on the polymerization of lactones by CDs in bulk, we found that CDs function not only as initiators (catalysts) but also as supporting architectures similar to chaperone proteins. CDs encircle a linear polymer chain so that the chain assumes the proper conformation and avoids coagulation. The CDs can mimic the strategy that living systems use to prepare polymers. Thus, we can obtain polyesters tethered to CDs without employing additional solvents or cocatalysts. Although CD has many hydroxyl groups, only one secondary hydroxyl group attaches to the polyester chain. In addition, the polymerization is highly specific for monomer substrates. We believe that this is the first system in which the catalyst includes monomers initially and subsequently activates the included monomers. The catalyst then inserts the monomers between the binding site and the growing chain. Therefore, this system should provide a new environmentally friendly route to produce biodegradable functional polymers.

Asymmetric hydrogenation with antibody-achiral rhodium complex

Monoclonal antibodies have been elicited against an achiral rhodium complex and this complex was used in the presence of a resultant antibody, 1G8, for the catalytic hydrogenation of 2-acetamidoacrylic acid to produce N-acetyl-L-alanine in high (>98%) enantiomeric excess.

Nanospheres with Polymerization Ability Coated by Polyrotaxane

BETA-cyclodextrin (beta-CD)-based nanosphere 1 initiated the oligomerization of delta-valerolactone (delta-VL) on the surface of 1 to give oligo(delta-VL)-tethered beta-CD nanosphere 2 in bulk. Atomic force microscopy indicated that the molecular size of 2 is twice that of 1. The addition of alpha-CD to 2 leads to the formation of poly-pseudo-rotaxane on the surface of 2 to give a nanosphere with poly-pseudo-rotaxane (alpha-CD[symbol: see text]2). 2D-NOESY NMR experiments showed correlation peaks between the inner protons of alpha-CD and the oligo(delta-VL) chains in an aqueous solution, indicating that the oligo(delta-VL) chains are included in the alpha-CD cavity. Alpha-CD[symbol: see text]2 has a core of beta-CDs with poly-pseudo-rotaxanes on the surface. It should be noted that 2 did not show polymerization ability for delta-VL, but after the formation of poly-pseudo-rotaxanes, oligo(delta-VL) of alpha-CD[symbol: see text]2 repropagated upon the addition of delta-VL. Alpha-CD[symbol: see text]2 is significantly larger than nanospheres 1 and 2. Additionally, postpolymerization increases the size of alpha-CD[symbol: see text]2. These behaviors are reminiscent of the function of a spherical virus, which forms an ordered spherical structure and releases RNA chains from the capsid surface.