Intra- and Intermolecular Effects on ¹H Chemical Shifts in a Silk Model Peptide Determined by High-Field Solid State ¹H NMR and Empirical Calculations

Abstract

A combination of solid state 1H NMR chemical shift measurements and empirical chemical shift calculations has been used to interpret 1H solid state chemical shifts of a model peptide (Ala-Gly)15 for the crystalline domain of Bombyx mori silk fibroin in silk I and silk II structures, including a treatment of both intra- and intermolecular arrangements. Silk I and silk II are the structures of silk fibroin before and after spinning, respectively. Two peaks with equal intensity were observed for the amide protons of (AG)15 in silk I, whereas only one broad peak was observed for silk II, reflecting a difference of 1.1 ppm in Ala HN shift between silk I and silk II, but a difference of only 0.2 ppm in Gly HN shift. Chemical shift calculations predicted chemical shifts that are in good agreement with the experimental observations and showed that the origin of these chemical shift differences was predominantly the magnetic anisotropy effect from the C=O bond that hydrogen bonds with HN, which has a more favorable geometry for Ala HN in silk II than for the other HN. This result shows that we could distinguish between proton chemical shift effects arising from intermolecular interactions and those from intramolecular interactions by combining observation of the solid state 1H NMR chemical shift and empirical chemical shift calculation.

Multidimensional Magic Angle Spinning NMR Spectroscopy for Site-Resolved Measurement of Proton Chemical Shift Anisotropy in Biological Solids

“…11,20,25,26 To gain deeper understanding on the relationship of the 1 H CSA to hydrogen bond parameters much more experimental information is needed that will include a broad range of proteins and in particular a detailed survey of the CSA of various secondary structure kinds.…”

“…26 A more complete understanding of the effects of molecular structure on the anisotropy of the proton chemical shift will undoubtedly be served by a broad database of experimental results, the current scarcity of which has often been held responsible for stagnant progress in this area. 11,18,19,26 …”

A Magic‐Angle‐Spinning NMR Spectroscopy Method for the Site‐Specific Measurement of Proton Chemical‐Shift Anisotropy in Biological and Organic Solids

“…[14] Other calculations showed that the origin of isotropic chemical-shift differences among protein amide protons is predominantly the magnetic anisotropy effect from the C=O acceptor. [15] In liquid water, the proton chemical-shift tensor was shown to be strongly dependent on the hydrogen-bond distances and angles, a relationship that allowed the temperature-dependent geometry to be derived from experimental chemical-shift tensor results. [16–17] In benzene the tensor was shown to differ greatly between the individual molecule and the crystal.…”

Structural characterization of silk-polyurethane composite material for biomaterials using solid-state NMR

“…To overcome the early formation of thrombosis for small-diameter vascular grafts, various tissueengineered vascular grafts have been developed 2 and their utility and clinical experience have also been reported. [3][4][5][6] However, there are still no commercially available grafts to date that fully satisfy the requirements for functional, small-diameter vascular grafts.…”

“…SF consists of highly organized b-sheet structure in the crystalline region. 3,4 Moreover, SF also has several semi-crystalline regions, which are responsible for its elasticity compared with fibers of similar tensile integrity. 5,6 We have already reported excellent results for the development of small-diameter vascular grafts made from SF.…”