Solid-State NMR Characterization of the Structure of Self-Assembled Ile–Phe–OH

Izuru Kawamura,Yumi Ozawa,Kazuyoshi Ueda,Hiroki Shirakata,Batsaikhan Mijiddorj

doi:10.3390/magnetochemistry4030030

Abstract

Solid-state nuclear magnetic resonance (NMR) spectroscopy provides significant structural information regarding the conformation and dynamics of a variety of solid samples. In this study, we recorded the 13C and 15N solid-state NMR spectra of a self-assembled isoleucine-phenylalanine (Ile–Phe–OH) dipeptide. Immediately after the addition of hexane to a solution of concentrated peptide in ethyl acetate, the peptide visually aggregated into a nanofiber. Then, we obtained well-resolved 13C and 15N NMR signals of the natural, isotopic-abundant Ile–Phe–OH peptide in the nanofiber. Furthermore, we calculated the chemical shift values of the reported crystal structure of the Ile–Phe dipeptide via the density functional theory (DFT) calculation and compared these results with the experimental values. Notably, the two sets of values were in good agreement with each other, which indicated that the self-assembled structure closely reflected the crystal structure. Therefore, herein, we demonstrated that solid-state NMR characterization combined with DFT calculations is a powerful method for the investigation of molecular structures in self-assembled short peptides.

Highlights

IntroductionThe self-assembly of small molecules into highly ordered architectures has attracted significant interest
In supramolecular chemistry, the self-assembly of small molecules into highly ordered architectures has attracted significant interest
Scanning electron microscope (SEM) images of the dipeptides have been characterized by 13 C solid-state nuclear magnetic resonance (NMR) experiments [16,17]

Summary

Introduction

The self-assembly of small molecules into highly ordered architectures has attracted significant interest. When diphenylalanine peptide (Phe–Phe–OH) is dissolved in 1,1,1,3,3,3-hexafluoro-2-isopropanol (HFIP) and water is added as a poor solvent, the peptide rapidly self-assembles into a nanofiber [2]. With respect to the dipeptide, hydrogen bonding and π–π interactions among the inter-peptides play a crucial role in the growth of associated components [3]. Studies on self-assembled dipeptides involve the use of HFIP as a first solvent, which dissolves the peptide at high concentrations, and the addition of water, which triggers the rapid self-association. After the addition of n-hexane to the formed solution, the peptide visually aggregated. In aqueous or methanol solutions, Phe–Phe–OH self-assembles to form hollow tubular structures [4,5].

Methods

Results

Conclusion