Conformational Characterization of (Val-Pro-Gly-Val-Gly)6 with 13C Solid State NMR

Abstract

The repeated pentapeptide sequence, (Val-Pro-GlyVal-Gly)n, has been considered to be an essential repeated sequence for producing the elastic character of elastin.1 The previous structural studies have been concentrated to clarify whether the unit, Pro-Gly, in the sequence takes β-turn structure.2–4 However, there are very limited structural information on the other parts of the pentapeptide sequence. By employing the mathematical methods of deriving a linear helical correlation structures from a conformation of cyclo-(Val1Pro-Gly-Val-Gly)3 obtained by X-Ray diffraction analysis,3 Urry’s group reported the torsion angles as Val1(φ, ψ) = (−120◦, 100◦), Val4(φ, ψ) = (−120◦, 80◦) and Gly5(φ, ψ) = (−150◦, −130◦) together with the ProGly unit; (Pro2(φ, ψ) = (−60◦, 120◦) and Gly3(φ, ψ) = (120◦, −40◦)). However, in general, cyclo-peptides take limited conformations because of the presence of many constraints such as steric constraints and hydrogen bonding formation as well as “cyclic” without the end groups. The extension of the information on the structures of cyclo-peptides to linear peptide with similar sequence should be performed very carefully. Thus, it is better to obtain the information on the torsion angles directly for the linear pentapeptide sequence in the solid state if it is possible. In our previous papers,5–7 13C solid state NMR such as especially 2D spin-diffusion solid state NMR under off magic angle spinning (OMAS) coupled with 13C isotope double labeling of specific residues has been successfully used to determine the torsion angles of the backbone amino acid residues for the silk model peptides, (Ala-Gly)15 and (Ala-Gly-Gly)10 in the several solid state forms. Especially, a new structural model of (Ala-Gly)15 as a model of Bombyx mori silk fibroin before spinning could be proposed.5 In this paper, we synthesized four kinds of the repeated pentapeptide sequences, (VPGVG)6, (VPGVG)2VP[1-C]G13[1-C]V14G(VPGVG)3, (VPGVG)2VPG[1-C]V14[1-C]G15(VPGVG)3, (VPGVG)2VPGV[1-C]G15[1-C]V16PGVG(VPGVG)2 in order to obtain the information on the torsion angles of the backbone of two Val residues (Val14 and Val16) and one Gly residue (Gly15) using both 2D spin-diffusion NMR under OMAS. Especially, the local conformation of the Gly15 residue was analyzed in detail from the comparison of the calculated and observed 2D spin-diffusion NMR spectra.