This dissertation presents studies of gas-phase fragmentation mechanism of doubly-charged a7 ions from basic amino acid containing model peptides under low-energy collision-induced dissociation (CID). The study includes three sets of C-terminal amidated model peptides which are alanine series containing basic amino acids (His – Arg – Lys). Position of His, Arg and Lys residue is varied from N-to-C terminal. Both positional effect and peptide sequence effect were examined for the fragmentation reactions of doubly-protonated a7 ions for these heptapeptides. The CID-MS4 mass spectra of doubly-protonated a7 ions have internal amino acid losses which provide an evidence for macrocyclization reaction. The proposed reaction mechanism involves production of doubly-charge a ions and charge-separation reaction of doubly-protonated a ions in the gas-phase which generates a protonated direct and non-direct a ion. All model peptides were also studied to understand behavior of doubly-protonated a ions better. Direct and non-direct sequence fragmentations which are singly or doubly protonated were observed for all studies. The reactions mechanisms were adjusted according to the results. In conclusions, the results presented in this dissertation can be used to elucidate the correct and reliable peptide sequences, and this improve protein identification strategies which is required for high-throughput proteomic studies.