derives from alpha-amino trimethylated residue protonated L-serine (L-serinium) residue N,N,N-trimethyl-L-serine (from L-serinium) 1+ 131.094085693359375 131.1699981689453125 42.0464019775390625 42.0800018310546875 A protein modification that effectively converts an L-serinium (protonated L-serine) residue to an N,N,N-trimethyl-L-serine. C 3 H 6 N 0 O 0 C 6 H 13 N 1 O 2 For amino acids residues, amine trimethylation can effectively only be accomplished with an aminium, protonated primary amino, group. This process accounts only for trimethylation and not protonation. The alternative N2Me3+Ser process (MOD:00071) accounts for both protonation and trimethylation. MOD:01801 MOD:01802 N-term N2Me3Ala PSI-MOD natural