Practice question

Question

Q140 An unknown peptide was isolated from the leaf of a medicinal plant and purified to homogeneity. The peptide did not yield any sequence when subjected to Edman degradation. However, tryptic digest of the peptide produced a unique sequence. The mass of the intact peptide was found to be 18 Da less than that obtained from the trypsin treated sample. The possible interpretation of the above experimental results could be that the A. the N-terminus of the peptide was blocked by acetylation or methylation. B. the peptide was cyclic and contained at least one internal arginine or lysine residue. C. the peptide was cyclic and contained a lysine or arginine residue at the C terminus. D. the peptide was cyclized by peptide bond formation between α- amino group and α-carboxyl group. Which of the above statements is true?