The product ion spectra of proline-containing peptides are commonly dominated by yn ions generated by cleavage at the N-terminal side of proline residues. This proline effect is investigated in the current work by collision-induced dissociation (CID) of protonated Ala-AlaXxx-Pro-Ala (Xxx includes Ala, Ser, Leu, Val, Phe, and Trp) in an electrospray/quadrupole/timeof-flight (QqTOF) mass spectrom...