Arginine derivatives
In terms of biochemical analysis, one of the most problematic metabolites of plant primary N metabolism is arginine, because it can be converted to several by-products upon derivatisation. It is particularly so during silylation, since the guanidium group of arginine may be cleaved to form ornithine, which in turn can lead to a cyclic form, ornithine lactam. In addition, one N atom of arginine can be lost and form citrulline. Taken as a whole, arginine derivatisation generates ornithine lactam 2TMS, ornithine 3TMS, arginine 3TMS and citrulline 3TMS (Fig. 4a). Arginine 3TMS is the minor derivative and has a retention time extremely close to citrulline 3TMS, and therefore it has often been overlooked. Here, arginine 3TMS is visible on the TIC, can also be identified with certainty with exact mass and thus cannot be confused with citrulline 3TMS. Although small, features typical of arginine 3TMS and citrulline 3TMS can be found at 187.108676 Da (guanidium group 2TMS) and 171.08278 Da (decarboxylated citrulline), respectively (Fig. 4b-e).