Influence of a gamma amino acid on the structures and reactivity of peptide a3 ions

► CID of protonated AGabaAIG leads to an unusually abundant a3 ion. ► The AGabaA b3 ion undergoes head-to-tail cyclization while AAA does not. ► Barriers to the a3 → b2 and a3 → a3* reactions are significantly higher for the AGabaA sequence than for AAA. ► The AGabaA a3 ion undergoes head-to-tail cyclization while the AAA does not.