Packing Arrangements and Intersheet Interaction of Alanine Oligopeptides As Revealed by Relaxation Parameters...

Alanine oligopeptides provide a key structure of the crystalline domains of the silks from spiders and wild silkworm and also the sequences included in proteins such as antifreeze proteins and amyloids. In this paper, the local dynamics of alanine oligopeptides, (Ala)₃, (Ala)₄, and (Ala)₆ were examined by high-resolution ¹³C solid-state NMR. The ¹³C spin-lattice relaxation times (T₁'s) for the Cβ4 carbons of antiparallel (AP)-β-sheet (Ala)₄ significantly prolonged and the correlation time was estimated as 3.6 × 10^-11 s which was shorter than those of other carbons in the AP-β-sheet (Ala)₄ (2.8 × 10^-10 s). The T₁ values for the Cβ carbons of (Ala)₆ showed significantly longer correlation time (8.8 × 10^-9 s) than those of AP-β-sheet (Ala)₄. It is thus revealed that AP-β-sheet (Ala)₆ exhibited stronger intersheet interaction than those of AP-β-sheet (Ala)₄. The ¹³C spin-spin relaxation times (T₂'s) for the Cβ4 carbons showed longer than those of the other Cβ1-3 carbons of AP-β-sheet (Ala)₄. T₂ values of Cβ carbons reflect the slow time-scale (∼70 kHz) backbone motions. The C-terminal forms strong hydrogen bonds with water molecules and thus the backbone motion is slower than ∼70 kHz, while the central backbone motions are faster than ∼70 kHz in the AP-β-sheet (Ala)₄.