5092

A biologically important heptapeptide was found, upon vigorous hydrolysis, to give Cys, Gly, His₂, Leu₂, Ser as the amino acid residues. Mild acid hydrolysis produced the fragments Cys-Gly-Ser, His-Leu-Cys, Ser-His-Leu. Determine the amino acid sequence of this peptide.

5120

An important application of aromatic substitution to the structure proof of proteins involves the use of 2,4-dinitrofluorobenzene for determination of the amino end group of these polymers. The application is as follows: the terminal -NH₂ group of the polypeptide reacts smoothly with 2,4-dinitrofluorobenzene. This product is then hydrolyzed vigorously. The structure of the 2,4-dinitroaniline is characteristic of the N-terminal amino acid.

(a) Show a mechanism for the reaction of 2,4-dinitrofluorobenzene with an -NH₂ group. Discuss and classify the reaction

(b) Follow through the reaction, described in words above, of 2,4-DNF with the tripeptide

5126

Show all the steps in the preparation of the dipeptide Phe-Ala from the amino acids.

5129

The proton nmr spectrum of N,N-dimethylformamide at 25° is (in d, ppm): 2.88 (3, s), 2.97 (3, s), 8.02 (1,s). As the sample temperature is raised, the peaks at 2.88 and 2.97 ppm broaden and coalesce. At about 170° they have become one singlet with a relative area of 6. Explain the nmr result and relate it to the structure of the peptide bond.

5268

Complete hydrolysis of an unknown heptapeptide showed the following amino acid composition: 2 val, leu, pro, gly, phe, met. Reaction of the original peptide with 2,4-dinitrofluorobenzene followed by exhausive hydrolysis provided N-(2,4-dinitroflourophenyl)valine. Reaction of the peptide with cyanogen bromide, a reaction that specifically cleaves the carboxy terminus of methionine, yielded a dipeptide composed of glycine and valine along with other fragments. Mild acid hydrolysis of the original peptide gave the following fragments:

(phe, met, gly), (pro, leu), (val, leu), (met, phe, pro), (gly, val).

What is the structure of the original peptide?