Problem 2, 135 - iii Dansylation followed by hydrolysis...

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Problem 2 (Due Mon., March 1) a) Calculate the isoelectric point of the dodecapeptide TALKINGHEADS, assuming solution values for the amino acid side chain pK a s, a value of 7.5 for the pK a of the amino terminus, and using the information that the C-terminal carboxyl group of the peptide exists as the methyl ester. In what direction will this peptide migrate in an electric field if the pH of the medium is 7.0? b) The following data was gathered in an attempt to sequence an oligopeptide (I) containing nine residues: i) Amino acid analysis showed the presence of Arg, Cys, Gly, Leu, Met, Pro, Tyr, and Val. ii) Neither Carboxypeptidase A or B had any effect on I; however a brief treatment with Carboxypeptidase Y produced free Pro. (The activity of the latter enzyme is not effected by the presence of Pro adjacent to the scissile peptide bond.)
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Unformatted text preview: iii) Dansylation followed by hydrolysis produced N(CH 3 ) 2 SO 2 NHCHCO 2 H CH(CH 3 ) 2 iv) Although treatment of I with either Trypsin or Chymotrypsin alone produced no fragmentation, simultaneous incubation with both enzymes resulted in the release of free Arg and an octapeptide (II) v) CNBr treatment of II produced a dipeptide and a six residue fragment (III) containing Cys, Gly, Pro, Tyr and Val. vi) Treatment of III with 2-mercaptoethanol produced two tripeptides (P 1 and P 2 ). One round of Edman degradation of P 1 produced PTH-Val, while the same treatment of P 2 produced PTH-Cys. In addition, P 1 showed a UV absorbance maximum near 280 nm. Use the above data to determine the amino acid sequence of I....
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