KEY WORDSElastin / (VPGVG) 6 / 2D Spin-Diffusion Nuclear Magnetic Resonance(NMR) under off Magic Angle Spinning / 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. . 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 13 C solid state NMR such as especially 2D spin-diffusion solid state NMR under off magic angle spinning (OMAS) coupled with 13 C 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 , (VPGV-order to obtain the information on the torsion angles of the backbone of two Val residues (Val 14 and Val 16 ) and one Gly residue (Gly 15 ) using both 2D spin-diffusion NMR under OMAS. Especially, the local conformation of the Gly 15 residue was analyzed in detail from the comparison of the calculated and observed 2D spin-diffusion NMR spectra.
EXPERIMENTALThe 13 C-labeled and un-labeled peptides mentioned above were synthesized by the solid-phase method. The samples are purified by HPLC with water and are freeze-dried. The 13 C CP/MAS NMR measurements were performed on a Chemagnetics Infinity 400 MHz spectrometer. A 1 H π/2 pulse of duration 3.0-6.0 µs and a contact time of 1 ms was used. The 2D spin-diffusion NMR spectra were obtained with Varian Unity INOVA 400 NMR spectrometer and 7 mm Jakobsen-type double-tuned MAS probe at off magic angle condition (θ m + 9 • ) and sample spinning of 6 kHz at room temperature. The scaling factor of the 2D spin-diffusion spectra were 1/2 (3 cos 2 (θ m + 9 • ) −1) = −0.206. The mixing time of 2 s was optimized for spin diffusion between intramolecular specific carbon atoms of selectively isotope-labeled Val and Gly residues; however, for no spin-diffusion between intermolecular carbon atoms. [5][6][7] The contact time was set to 2 ms using the Variable-Amplitude CP technique. 8 About 400 scans with a number of t 1 points of 150 and a recy- †