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

Abstract

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 beta-turns with a large distribution of the torsion angles and antiparallel beta-sheets. The relative proportion of the distorted beta-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 beta-sheets and beta-turns. Our statistical mechanical analysis is in excellent agreement with our Ala Cbeta 13C CP/MAS NMR data strongly suggesting that (AG)15 has a lamellar structure.

The Influence of Ser and Tyr Residues on the Structure of Bombyx Mori Silk Fibroin Studied Using High-resolution Solid-state 13C NMR Spectroscopy and 13C Selectively Labeled Model Peptides

“…The 13 C chemical shifts were calibrated indirectly through the methine peak of adamantane observed at 28.8 ppm relative to TMS at 0 ppm. 4,5 The peak fitting was performed as follows: After fixing the chemical shifts of three peaks of Ala C carbon, the fraction and width of each peak was changed to reproduce the observed line shape by assuming Gaussian for each peak. The uncertainty of the percentages is about AE1%.…”

“…The change in the peak intensity of 16.7 ppm depending on the 13 C labeled position of (AG) 15 has been used for study of the lamellar structure. 4,5 In this study, two Ala positions in I, Ala 11th and Ala 19th , were selected for the 13 C labeling position because of relatively high distorted -turn probabilities in the proposed lamellar structure of (AG) 15 . The most remarkable difference between (AG) 15 and (AGSGAG) 5 after 9M LiBr/dialysis treatment was that (AG) 15 took silk I form, 4 but (AGSGAG) 5 took silk II form as shown in Figure 1 (left).…”

“…However, the fraction of 16.7 ppm peak is not 100%, means that turns also occur at other positions frequently in the lamellar structure as was discussed in (AG) 15 . 5 The Ser C peak which gives structural information on the Ser residue, suggests that the Ser residues in (AGSGAG) 5 took -sheet judging from the chemical shift, 64 ppm, and the line shape. 7 We suspect that the nature of hydrogen bonding, involving the O H group of the Ser side-chain, might be critical to select silk I or silk II structures during the course of dialysis process against water.…”

“…Recently, 13 C solidstate NMR on selectively 13 C labeled 15 peptides labeled singly at different Ala methyl carbons was used to examine the silk II structure of (AG) 15 which was considered to be a model for the crystalline domain, with emphasis on a possible lamellar structure containing -turns. 4,5 However, there are also Ser and Tyr residues and the relative content is reasonably high; 12.2% and 4.8%, respectively. They stand third and fourth behind the Gly and Ala residues.…”

A structural study of the self-assembly of a palmitoyl peptide amphiphile

“…[21][22][23] In particular solid state NMR (ssNMR) on lyophilised samples was used. In ssNMR, the samples are spun very fast around a so-called magic angle (MAS, magic angle spinning) averaging anisotropic interactions to increase resolution.…”