1) An octapeptide was divided into three samples and each sample was subjected to cyanogen bromide, chymotrypsin, or trypsin. The fragments obtained in each of the treatments were separated, hydrolyzed and subjected to amino acid analysis with the following results.

Amino acid composition of each fragment is listed in alphabetical rather than sequential order.

Cyanogen bromide fragment 1: cysteine, lysine, tryptophan

Cyanogen bromide fragment 2: methionine, tyrosine, valine

Cyanogen bromide fragment 3: leucine, methionine

Chymotrypsin fragment 1: cysteine, lysine, methionine, tryptophan

Chymotrypsin fragment 2: leucine, methionine, tyrosine, valine

Trypsin fragment 1: cysteine, tryptophan

Trypsin fragment 2: leucine, lysine, methionine (2), tyrosine, valine

Top of Form

What is the full sequence of the original octapeptide?

2) Explain why Edman degradation is superior to other methods of amino acid modification for protein sequencing?

3) A sample of a dimeric protein was treated with 1-fluoro-2,4-dinitrobenzene, however, 2,4-dinitrophenyl derivative of only one amino acid Ala was found. Explain why?

4) Two peptides, each containing two Arg residues were treated with trypsin. Explain why one peptide yielded three fragments and the other only two?