Formation of peptide-based oligomers in dimethylsulfoxide: identifying the precursor of fibril formation

• Published in: Soft matter Vol. 16; no. 33; pp. 786 - 7868
• Main Authors: Levine, Matthew S, Ghosh, Moumita, Hesser, Morgan, Hennessy, Nathan, DiGuiseppi, David M, Adler-Abramovich, Lihi, Schweitzer-Stenner, Reinhard
• Format: Journal Article
• Language: English
• Published: Cambridge Royal Society of Chemistry 26.08.2020
• Subjects: Chemical equilibrium; Conformation; Coupling; Dilution; Fibrils; Gelation; Hydrogels; Hydrogen bonding; Infrared spectroscopy; Light diffraction; Light scattering; Moisture content; NMR; Nuclear magnetic resonance; Oligomers; Peptides; Phenylalanine; Photon correlation spectroscopy; Polyproline; Raman spectra; Raman spectroscopy; Residues; Self-assembly; Water content; X ray powder diffraction
• ISSN: 1744-683X; 1744-6848
• DOI: 10.1039/d0sm00035c

The well-studied dipeptide fluorenylmethyloxycarbonyl-di-phenylalanine (FmocFF) forms a rigid hydrogel upon dissolving in dimethylsulfoxide (DMSO) and dilution in H 2 O. Here, we explored the pre-aggregation of the peptide in pure DMSO by vibrational spectroscopies, X-ray powder diffraction and dynamic light scattering. Our results show an equilibrium between a dominant population of amorphous oligomers (on a length scale of 2 nm) and a small number of protofibrils/fibrils (on a length scale of 30 nm in the centimolar and of 200 nm in the sub-molar region). To probe the mechanism underlying the formation of these protofilaments, we measured the 1 H-NMR, IR and visible Raman spectra of DMSO containing different FmocFF concentrations, ranging between 10 and 300 mM. Our data reveal that interpeptide hydrogen bonding leads to the self-assembly of FmocFF in the centimolar region, while π-π stacking between Fmoc-groups is observed above 100 mM. The high 3 J (H N H Cα ) coupling constant of the N-terminal amide proton indicates that the Fmoc end-cap of the peptide locks the N-terminal residue into a conformational ensemble centered at a -value of ca. −120°, which corresponds to a parallel β-sheet type conformation. The 3 J (H N H Cα ) coupling constant of the C-terminal residue is indicative of a polyproline II (pPII)/β t mixture. Our results suggest that the gelation of FmocFF caused by the addition of a small amount of water to DMSO mixtures is facilitated by the formation of disordered protofibrils in pure DMSO. The aromatic dipeptide fluorenylmethyloxycarbonyl-di-phenylalanine (FmocFF) self-assembles into amorphous oligomers and fibrils.