Influence of Amino Acid Side Chains on Apparent Selective Opening of Cyclic b5 Ions

In this study, the possible influence of acidic, basic, and amide side chains on the opening of a putative macrocyclic b ion (b(5)(+)) intermediate was investigated. Collision induced dissociation (CID) of b(5) ions was studied using a group of hexapeptides in which amino acids with the side chains of interest occupied internal sequence positions. Further experiments were performed with permuted isomers of glutamine (Q) containing peptides to probe for sequence scrambling and whether the specific sequence site of the residues influences opening of the macrocycle. Overall, the trend for (apparent) preferential/selective opening of the cyclic b(5)(+), presumably due to the side chain, followed by the loss of the amino acid with active side group is: QKDN approximately E.