Experimental and theoretical spectra of seglitide Cyclic(N-methyl-Ala, Tyr, D-Trp, Lys, Val, Phe (a) Cyclic diagram of seglitide. A⁺¹⁴ denotes a methylated alanine; the integer residue masses are 85, 163, 186, 128, 99 and 147. (b) MS² fragmentation of seglitide generates up to 6 linear peptides representing different linearized variants of the same cyclic peptide. (c) The theoretical spectrum for seglitide is a superposition of the fragment masses from the linearized peptide. (d) Experimental spectrum of seglitide (the peaks corresponding to prefix masses are shown in red). (e) The auto-convolution of the spectrum in insert d has prominent peaks for offsets corresponding to masses of amino acids (shown in red). The peak at 0 is truncated. (f) Generation of a gapped peptide from a theoretical spectrum of seglitide. The theoretical spectrum is colored to highlight various linear peptides. For illustration purposes only 3 linearized (A⁺¹⁴YWKV (blue), FA⁺¹⁴YWKV (red) and VFA⁺¹⁴YWK (green)) versions of the cyclic peptide are shown. The frequent 2-amino-acid tag YW is observed in 3 different locations in the spectrum. Additionally, the offsets between 3 consecutive locations of tag YW reveal the masses of amino acids F and V. (g) The gapped peptide constructed from f combines YW (derived from a frequent tag) with VF (derived from the inter distances between tag locations). A⁺¹⁴ and K are inferred from the flanking masses of YW and VF. The complete sequence A⁺¹⁴YWKVF is recovered, but gaps may be generated.