Parallel β-Sheet Structure of Alanine Tetrapeptide in the Solid State As Studied by Solid-State NMR Spectroscopy

Area of Science:

• Biophysics
• Structural Biology
• Materials Science

Background:

• Alanine oligopeptides are crucial for understanding silk structures and diseases linked to polyalanine expansions.
• While antiparallel beta-sheet (AP) structures of alanine tripeptide (Ala3) and tetrapeptide (Ala4) are known, the parallel beta-sheet structure of Ala4 (P-Ala4) remains uncharacterized.

Purpose of the Study:

• To establish a preparation protocol for P-Ala4.
• To determine the atomic-level structure of P-Ala4 using solid-state NMR.
• To elucidate the structural differences between parallel and antiparallel beta-sheet alanine tetrapeptides.

Main Methods:

• Preparation of P-Ala4 from the more stable AP-Ala4.
• Solid-state NMR spectroscopy (13C, 15N, 1H) on isotopically labeled Ala4 samples.
• NMR spectral assignment and structural constraint acquisition.
• Molecular dynamics (MD) calculations.

Main Results:

• A preparation protocol for P-Ala4 was successfully established.
• Complete assignments of 13C, 15N, and 1H solid-state NMR spectra for P-Ala4 were achieved.
• Structural analysis revealed significantly longer intermolecular hydrogen bonds (2.21–2.34 Å) in P-Ala4 compared to AP-Ala4 (1.8–1.9 Å).
• NMR data indicated that amide proton peaks in P-Ala4 are observed at a higher field (approx. 7.4 ppm) than in AP-Ala4 (8.7–9.1 ppm).

Conclusions:

• The atomic-level structure of P-Ala4 was determined for the first time.
• P-Ala4 exhibits distinct structural features, notably longer intermolecular hydrogen bonds, compared to AP-Ala4.
• These findings contribute to the understanding of beta-sheet formation in alanine peptides relevant to biological systems and material science.