Poly-l-alanine (PLA) sequences are a key element in the structure of the crystalline domains of spider dragline silks, wild silkworm silks, antifreeze proteins, and amyloids. To date, no atomic-level structures of antiparallel (AP)-PLA longer than Ala have been reported using the single-crystal X-ray diffraction analysis. In this work, dipolar-assisted rotational resonance solid-state NMR spectra were observed to determine the effective internuclear distances of C uniformly labeled alanine tetramer with antiparallel (AP) β-sheet structure whose atomic coordinates are determined from the X-ray crystallographic analysis. Initial build-up rates, R , were obtained from the build-up curves of the cross peaks by considering the internuclear distances arising in the master equation. Subsequently, experimentally obtained effective internuclear distances, r(obs), were compared with the calculated r(calc) values obtained from the X-ray crystallographic data. Fairly good correlation between r(obs) and r(calc) was obtained in the range of 1.0-6.0 Å, with the standard deviation of 0.244 Å, without considering the zero-quantum line-shape functions. It was further noted that the internuclear distances of intermolecular contributions provide details relating to the molecular packing in solid-state samples. Thus, the present data agree well with AP-β-sheet packing but do not agree with P-β-sheet packing.
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