1. (a)(b) The one on the left is the D- isomer and the one of the right, the L-configuration, is the one that occurs in proteins.(c) Glycine (0), isoleucine (2), threonine (2)2. (a) Cis/trans isomerisation and oxidation. (b) Both have extended conjugation so absorb in the visible region. Only cis-retinal contains a carbonyl group (C=O), which absorbs at approximately 1700-1750 cm-1 in the infrared region of the spectrum.