Figure 3

Mass spectra of a major fragment ion of derivatized Ala at different time points of the chase experiment. Cells were initially grown in TAP medium made with 99 atom % [¹⁵N] salts as the only source of nitrogen. The cells were collected by centrifugation and resuspended in fresh ¹⁴N- TAP medium. Aliquots (1.5 ml) of the cultures were removed for each of the time points. The cells were centrifuged at 14,000 g for 30 s. The medium discarded and the cell pellet quickly frozen in liquid nitrogen and stored at −80°C until sampling for amino acid analysis using GC-MS. The top panel depicts the ¹⁵N-labeled fragment ion of derivatized Ala (m/z 103). The bottom panel depicts the natural abundance spectrum of unlabeled Ala control (m/z 102). At different time points, the mass spectrum depicts the degradation of the ¹⁵N-labeled fragment (m/z 103) and emergence of the lighter fragment (m/z 102) during the isotopic dilution period.