Difference in the structures of alanine tri‐ and tetra‐peptides with antiparallel β‐sheet assessed by X‐ray diffraction, solid‐state NMR and chemical shift calculations by GIPAW

Alanine oligomers provide a key structure for silk fibers from spider and wild silkworms.We report on structural analysis of l ‐alanyl‐ l ‐alanyl‐ l ‐alanyl‐ l ‐alanine (Ala) 4 with anti‐parallel (AP) β‐structures using X‐ray and solid‐state NMR. All of the Ala residues in the (Ala) 4 are in equivalent positions, whereas for alanine trimer (Ala) 3 there are two alternative locations in a unit cell as reported previously (Fawcett and Camerman, Acta Cryst., 1975, 31, 658–665). (Ala) 4 with AP β‐structure is more stable than AP‐(Ala) 3 due to formation of the stronger hydrogen bonds. The intermolecular structure of (Ala) 4 is also different from polyalanine fiber structure, indicating that the interchain arrangement of AP β‐structure changes with increasing alanine sequencelength. Furthermore the precise 1 H positions, which are usually inaccesible by X‐ray diffraction method, are determined by high resolution 1 H solid state NMR combined with the chemical shift calculations by the gauge‐including projector augmented wave method. © 2013 Wiley Periodicals, Inc. Biopolymers 101: 13–20, 2014.