Lamellar Structure in Poly(Ala-Gly) Determined by Solid-State NMR and Statistical Mechanical Calculations

Lamellar structure of poly(Ala-Gly) or (AG) n in the solid was examined using 13C solid-state NMR and statistical mechanical approaches. Two doubly labeled versions, [1-13C]Gly14[1-13C]Ala15- and [1-13C]Gly18[1-13C]Ala19 of (AG)15 were examined by two-dimensional (2D) 13C spin diffusion NMR in the solid state. In addition five doubly labeled [15N,13C]-versions of the same peptide, (AG) 15 and 15 versions labeled [3-13C] in each of the successive Ala residues were utilized for REDOR and 13C CP/MAS NMR measurements, respectively. The observed spin diffusion NMR spectra were consistent with a structure containing a combination of distorted β-turns with a large distribution of the torsion angles and antiparallel β-sheets. The relative proportion of the distorted β-turn form was evaluated by examination of 13C CP/MAS NMR spectra of [3-13C]Ala-(AG)15. In addition, REDOR determinations showed five kinds of atomic distances between doubly labeled 13C and 15N nuclei which were also interpreted in terms of a combination of β-sheets and β-turns. Our statistical mechanical analysis is in excellent agreement with our Ala Cβ 13C CP/MAS NMR data strongly suggesting that (AG)15 has a lamellar structure.