Studies on the Chemical Synthesis and the Solution Structures of Proline-Containing Cyclic Peptides

Peptides have the potential to contribute significantly to a sustainable society, due to their biomass origin, functionality, and unique properties. The ability to precisely control sequence, length, and stereochemistry in the synthesis of peptides has led to the generation of a variety of materials. However, the diversity of chemical topologies of peptides remains largely unexplored, and cyclic peptides, the simple class of topology, have just been applied to piezoelectric materials and drugs [1,2]. In the field of synthetic polymers, rotaxanes and catenanes have been extensively studied, and their materials with excellent mechanical properties via sliding ring mechanisms have been studied by various groups [3]. In nature, lasso peptides, which adopt a topology similar to that of a threaded lasso or knot, may have therapeutic functions ranging from antimicrobial activity to receptor antagonism and enzyme inhibition [4]. As shown above, peptide topology engineering has the potential to provide a multitude of functional benefits. However, there are still limited examples of the artificial synthesized mechanically interlocked peptide, because the flexible ring skeleton of cyclic peptides and the formation of intramolecular hydrogen bonds prevent cyclic peptides from maintaining an inner diameter large enough for an axial molecule to penetrate. Kimura et al. prepared cyclic hexa-b-peptides composed of b-glucosamino acid. This cyclic peptide has high planarity and large internal pores due to the rigidity derived from the pyranose ring in the side chain, which leads to the synthesis of the polypseudorotaxane with poly(ethylene glycol) [5]. Also, Thomson et al. prepared rotaxanes using cyclo(Pro-Gly)₄, which has a relatively large inner diameter and a planar structure, predominantly in an all-trans conformation in acetonitrile [6]. This cyclic peptide has a solution structure with the glycine carbonyls pointing toward the center and the proline carbonyls directed away, allowing it to interact with cations, which lead to the synthesis of rotaxanes. Therefore, it is important for rotaxane synthesis that the cyclic peptides adopt a solution structure with a large inner diameter. However, the cyclic peptides’ sequence-structure relationships have not been established until now [7].<br/>Here, we aim to establish a guideline for the molecular design of cyclic peptides with a large inner diameter in solution for rotaxane synthesis. Proline, one of our target amino acids, is the only natural amino acid that is N-alkylated and has a five-membered ring at the side chain. Because of the reduction of intramolecular hydrogen bonds and motional restrictions, the presence of proline residue leads to rigid and planer structure and greatly reduces the available conformation of cyclic peptides. Therefore, proline-containing cyclic peptides are potential candidates for cyclic peptides that allow for rotaxane synthesis. In this study, we synthesized cyclic peptides consisting of alternating sequences of L-proline and D-proline and cyclic peptides consisting of alternating sequences of L-proline and glycine by the conventional solid-phase and liquid-phase methods. To clarify the correlation between solution structure and peptide sequence for de novo cyclic peptide designs, the solution structures of the synthesized cyclic peptides were analyzed by circular dichroism measurements and molecular dynamics simulations. In the current presentation, we will present the details of the molecular design and structural information of the resultant cyclic peptides.<br/><br/>[1] T. Kurita, <i>et al</i>. <i>Biomacromolecules</i> <b>2021</b>,<i> 22</i>, 2815-2821.<br/>[2] D. Price, <i>et al</i>. <i>Chem. Biol. Drug Des.</i> <b>2013</b>, <i>81</i>, 136-147.<br/>[3] K. Ito, <i>et al.</i> <i>Adv. Mater</i>. <b>2001</b>, <i>13</i>, 485-487.<br/>[4] A. Link, <i>et al</i>. <i>Nat. Prod. Rep. </i><b>2012,</b><i> 29, </i>996-1006.<br/>[5] S. Kimura,<i> et al</i>. <i>Chemcomun</i>. <b>2007</b> <i>10</i><b>,</b> 1023-1025.<br/>[6] A. Thomson, <i>et al</i>. <i>J. Am. Chem. Soc.</i> <b>2006</b>, <i>128</i>, 1784–1785.<br/>[7] Y. Lin, <i>et al</i>.<i> Chem. Rev. </i><b>2021,</b> <i>121</i>, 2292-2324.