Modeling of the gas-phase ion chemistry of protonated arginine.

Arginine is often involved at the C-terminus of peptides obtained from tryptic digests of proteins. The very basic guanidine group of the side-chain of arginine has a large effect on the backbone fragmentation of protonated peptides. Furthermore, arginine exhibits specific fragmentation reactions involving its side-chain. Various tautomerization states, conformers and side-chain dissociation channels of protonated arginine were studied using theoretical methods. The guanidine loss of protonated arginine is proved to be an S(N)2 substitution on the delta-carbon of the side-chain, starting from species containing the N(epsilon)H-C(+)(N(eta)H(2))(N(eta')H(2)) or -N(epsilon) (+)H(2)-C(N(eta)H)(N(eta')H(2)) moieties and leads to formation to either protonated guanidine or protonated proline. In the corresponding transition structures the proline moiety is protonated. Under low-energy collision conditions the extra proton transfers to the guanidine moiety, leading to the formation of C(+)(NH(2))(3). On the other hand, the lifetime of the fragmenting species under high-energy collision conditions is shorter, resulting in enhanced formation of protonated proline and its dissociation products. The first step of ammonia loss is the leaving of a preformed NH(3) from tautomers containing the -N(epsilon)H-C(N(eta)H(3) (+))(N(eta')H) or -N(epsilon)-C(N(eta)H(3) (+))(N(eta')H(2)) moieties. The resulting protonated carbodiimide group can be stabilized by intramolecular nucleophilic attack, leading to ring formation. Overall, reactions involved in the ammonia loss from protonated arginine can be considered as an S(N)1 substitution on the central zeta-carbon of the guanidine group.