sentence: The cross peak of methine proton at δ 6.17 and methyl group at δ 1.70 in COSY spectrum and their HMBC correlations to the quaternary carbons C-10 ( δ 130.7 ) and C-13 ( δ 163.5 ) indicate an α , β - unsaturated amino acid , called dehydrobutyrine ( Dhb ) , of which two additional units were recognized in the NMR spectra .