The purity of the substance was 99.7% with an isotope-
labeling exclusively on the glutamate part of the molecule.
Acetonitrile (HPLC grade), formic acid (99%) and sodium hydroxide
were obtained from VWR (Darmstadt, Germany), 2-(cyclohexylamino)
ethanesulfonic acid (CHES), 4-(2-hydroxyethyl) piperazine-
1-ethanesulfonic acid (HEPES) and dithiothreitol (DTT) were
from Sigma Aldrich (Deisenhofen, Germany).