the 1658 cm-1 band with d-proline was also assigned to the c = n stretching mode of an imino acid derived from d-proline , i.e. , delta 1-pyrrolidine-2-carboxylate , since the band shifts to 1633 cm-1 upon [ 15n ] -substitution and its stretching frequency is generally found in this frequency region .
C0020931
imino acid
C0033382
proline