Mixture of Rectangular and Staggered Packing Arrangements of Polyalanine Region in Spider Dragline Silk in Dry and Hydrated States As Revealed by 13C NMR and X-ray Diffraction

Abstract

For the first time, we determined the relative percentages of “rectangular” and “staggered” packing arrangements in the crystalline polyalanine regions with antiparallel β-sheet structure within spider dragline silk fiber from Nephila clavata (NCF) and recombinant silk protein (RSP). The methods used included X-ray diffraction and 13C NMR coupled with selective 13C isotope labeling of the Ala Cβ carbons. From deconvolution analyses of the Ala Cβ peaks in the 13C CP/MAS NMR spectra, the relative percentages of the rectangular arrangements in [3-13C]Ala-NCF were determined to be 49 ± 8% and 40 ± 7% in the dry and hydrated states, respectively, and in [3-13C]Ala-RSP 62 ± 11% and 81 ± 5% in the dry and hydrated states, respectively. Thus, the packing structure changed significantly between the two spider silks and also between the two physical states. The use of NMR was critical in this analysis; from X-ray diffraction patterns alone it would have been difficult to obtain these quantitative data.