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Communication Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
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Formation of Giant and Small Cyclic Complexes from a Flexible Tripeptide Ligand Controlled by Metal Coordination and Hydrogen Bonds Ryosuke Miyake,*,†,‡ Akira Ando,† Manami Ueno,† and Takahiro Muraoka§
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Department of Chemistry and Biochemistry, Graduate School of Humanities and Sciences, Ochanomizu University, 2-1-1 Otsuka, Bunkyo-ku, Tokyo 112-8610, Japan ‡ JST-PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan § Institute of Global Innovation Research, Tokyo University of Agriculture and Technology, 2-24-16 Naka-cho, Koganei, Tokyo 184-8588, Japan S Supporting Information *
artificial systems. One of the most powerful approaches used to overcome the synthetic difficulties is the construction of metal complexes with small, well-designed units.2−7 Excellent examples of large cavities (diameter >1 nm)3−5 have been reported by using precisely designed rigid units3 or units that can form stable higher structures (such as peptides).4 However, owing to the disadvantageous enthalpy change (i.e., disadvantages of assembling a large number of units and the formation of a cavity), it is extremely difficult to form discrete complexes possessing large spaces from small, flexible units that can adapt a variety of conformations.6,7 The formation of large, flexible cavities with both conformational variety and designability, is essential for further development of dynamic properties that are similar to those observed in biological systems.8 To design giant complexes with large, flexible cavities, we focused on the use of highly flexible peptides possessing metal coordination sites in the main chain. The metal coordination at the sites would determine the conformation of the flexible peptides supported by inter- and/or intramolecular hydrogen bonds. We envisioned that the conformation of peptides stabilized by these interactions would be useful for constructing giant assembled structures of flexible units. Herein, we report the formation of giant tetradecanuclear cyclic complexes ([114Ni14]28+) with large cavities (diameter: ca. 2 nm) by using a very flexible tripeptide (1) with three methylene groups in the main chain (Figures 1 and 2a). The cyclic structure was formed through the construction of a mesh-like framework by connecting three peptides at each metal center. In the structure, the conformation of 1 is supported by the Ni(II) coordination and hydrogen bonds. We also demonstrate that, depending on the conditions of the assembly process, the tripeptide 1 forms other cyclic complexes, some with small differences in the cavity size and some with a” shrunk” cavity (1/10 size of that of the 114Ni14 complexes), because of the flexibility of 1. We designed a flexible tripeptide, 1, based on a β-dipeptide ligand 29 as the Ni(II)-macrocycle of 2 shows cooperative structural changes in the crystalline state leading to heterotopic
ABSTRACT: Formation of giant cyclic complexes by the assembly of small, flexible units is demonstrated by connecting 14 artificial tripeptides (1) with 14 Ni(II) ions. Although tripeptide 1 is very flexible because of the presence of three CH2 groups in the main chain, it formed a tetradecanuclear cyclic complex ([114Ni14]28+) with a large cavity (diameter: ca. 2 nm). In this structure, three tripeptides are coordinated to each Ni(II) center by three different coordination sites in 1, forming a mesh-like structure. Crystal structure analysis and theoretical calculations indicate that the conformation of 1 was controlled by the formation of metal coordination bonds and intramolecular hydrogen bonds. Because of the structural flexibility, the cyclic framework formed both circular and ellipsoidal structures in the crystalline state, depending on the packing structure. In addition, by the conditions of the assembly process, the size of the cavities could be tuned either with a small decrement (dodecanuclear complex [112Ni12]24+) or a large decrement (octanuclear complexes [(1-3H+)4Ni8]4+), in which “shrunk” cavities with a 10-fold difference in diameter ( 2σ(I)). CCDC 1895031. (12) Crystal data for plate-like crystal of [114Ni14]28+ as NO3 salt (−100 °C): C168H392N133.5Ni14O212.4, Monoclinic, space group P21C, a = 29.431(3), b = 28.319(3), c = 29.007(3) Å, β = 114.755(3) °, V = 21954(4) Å3, Z = 2, GOF = 1.087, R1 = 0.0924 (I > 2σ(I)), wR2 = 0.2576 (I > 2σ(I)). CCDC 1895033. (13) As the TfO salt of the 114Ni14 complex formed from aqueous solution does not exhibit catenane structures in its crystal, we propose that the hydrophobic effect is key for the formation of catenane structures. For a detailed discussion, please refer to the Supporting Information. (14) (a) Forgan, R. S.; Sauvage, J.-P.; Stoddart, J. F. Chemical Topology: Complex Molecular Knots, Links, and Entanglements. Chem. Rev. 2011, 111, 5434−5464. (b) Huang, S.-L.; Hor, T. S. A.; Jin, G.-X. Metallacyclic assembly of interlocked superstructures. Coord. Chem. Rev. 2017, 333, 1−26. (c) Gil-Ramírez, G.; Leigh, D. A.; Stephens, A. J. Catenanes: Fifty Years of Molecular Links. Angew. Chem., Int. Ed. 2015, 54, 6110−6150. (15) (a) Sawada, T.; Yamagami, M.; Ohara, K.; Yamaguchi, K.; Fujita, M. Peptide [4]Catenane by Folding and Assembly. Angew. Chem., Int. Ed. 2016, 55, 4519−4522. (b) Sawada, T.; Saito, A.; Tamiya, K.; Shimokawa, K.; Hisada, Y.; Fujita, M. Metal-peptide rings form highly entangled topologically inequivalent frameworks with the same ring-and crossing-numbers. Nat. Commun. 2019, 10, 921.
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DOI: 10.1021/jacs.9b01541 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
