An HPLC-ESMS study on the solid-phase assembly of C-terminal proline peptides.

DKP formation is a serious side reaction during the solid-phase synthesis of peptide acids containing either Pro or Gly at the C-terminus. This side reaction not only leads to a lower overall yield, but also to the presence in the reaction crude of several deletion peptides lacking the first amino acids. For the preparation of protected peptides using the Fmoc/tBu strategy, the use of a ClTrt-Cl-resin with a limited incorporation of the C-terminal amino acid is the method of choice. The use of resins with higher loading levels leads to more impure peptide crudes. The use of HPLC-ESMS is a useful method for analysing complex samples, such as those formed when C-terminal Pro peptides are prepared by non-optimized solid-phase strategies.