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Structural Analyses of Alanine Trimer and Tetramer Crystals with Antiparallel and Parallel β-Sheet Structures Using
Akira Naito1, Shunsuke Kametani1, Akihiro Aoki1
1Department of Biotechnology , Tokyo University of Agriculture and Technology , Koganei , Tokyo 184-8588 , Japan.
The Journal of Physical Chemistry. B
|September 21, 2018
Summary
Poly-l-alanine (PLA) sequences are crucial in silk. This study used 1H spin-diffusion NMR on deuterated crystals to determine PLA structure, revealing packing arrangements are key to silk properties.
Area of Science:
- Materials Science
- Biophysics
- Spectroscopy
Background:
- Poly-l-alanine (PLA) sequences form crystalline domains in natural silks like spider dragline and wild silkworm silks.
- Understanding the precise structure of these PLA sequences is vital for correlating structure with silk mechanical properties.
Purpose of the Study:
- To develop an analytical method for determining the structure of poly-l-alanine sequences using NMR.
- To investigate the structural differences between antiparallel and parallel β-sheet conformations in PLA crystals.
Main Methods:
- Utilized 1H spin-diffusion two-dimensional (2D) correlation NMR spectroscopy on selectively deuterated (Ala)3 and (Ala)4 crystals.
- Analyzed build-up curves of cross peaks to determine spin-diffusion rate constants (kj,k).
- Calculated effective interproton distances (rj,keff) from experimental data and compared them with calculated distances from known crystal structures.
Main Results:
- Observed good agreement between experimental and calculated interproton distances for antiparallel (AP) β-sheet (Ala)3 and (Ala)4 crystals.
- Found poor agreement for parallel (P) β-sheet (Ala)3 and (Ala)4 crystals, attributed to interchain contributions from different packing arrangements.
- Spin-diffusion rate constants (kj,k) were successfully derived from relaxation master equations.
Conclusions:
- The packing arrangement of poly-l-alanine regions significantly influences the structural determination and mechanical properties of silks.
- 1H spin-diffusion NMR is an effective method for analyzing PLA structures and discerning conformational differences.
- Antiparallel β-sheet structures in PLA crystals are more accurately represented by current structural models compared to parallel structures.
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