Sequence Determination of the Brain Peptide Leucine Enkephalin A group of peptides that influence nerve transmission in certain parts of the brain have been isolated from normal brain tissue. These peptides are known as opioids because they bind to specific receptors that also bind opiate drugs, such as morphine and naloxone. Opioids thus mimic some of the properties of opiates. Some researchers consider these peptides to be the brain's own painkillers. Using the information below, determine the amino acid sequence of the opioid leucine enkephalin. Explain how your structure is consistent with each piece of information.
(a) Complete hydrolysis by [tex]6 \mathrm{M} \mathrm{HCl}[/tex] at [tex]110^{\circ} \mathrm{C}[/tex] followed by amino acid analysis indicated the presence of Gly, Leu, Phe, and Tyr, in a [tex]2: 1: 1: 1[/tex] molar ratio.
(b) Treatment of the peptide with 1 -fluoro-2,4 -dinitrobenzene followed by complete hydrolysis and chromatography indicated the presence of the 2,4-dinitrophenyl derivative of tyrosine. No free tyrosine could be found.
(c) Complete digestion of the peptide with chymotrypsin followed by chromatography yielded free tyrosine and leucine, plus a tripeptide containing Phe and Gly in a 1: 2 ratio.
