Figure 4. Determination of the range within which reliable quantitative data can be obtained.

A, Soluble leaf protein extracts from plants grown in ¹⁴N - and ¹⁵N -medium, respectively, were mixed at different ratios (1∶9, 2∶8, 3∶7, 5∶5, 7∶3, 8∶2, 9∶1) and subjected to SDS-PAGE. Two bands containing Rubisco large subunit, and fructose-1,6-bisphosphate aldolase (FBPase) were excised from each lane and processed as described in Figure 2. B, Experimental data for one of the FBPase peptides (TAAYYQQGAR). C, Average q-values for 11 peptides (6 from Rubisco and 5 from FBPase, in total based on 94 spectra) are summarized (grey columns; bar = standard deviation) and compared to the theoretical values (black columns). The relative abundance of ¹⁴N - and ¹⁵N -labeled peptides is defined as the fraction of ¹⁵N -labeled species of total peptide = the q-value of the peptide (equation at bottom, right. I = Intensity).