Fig. 1.

The experimental design for determining the ¹³C incorporation level in peptides by its decimal place slope. In our study, P. putida ML2 was grown in the presence of [¹³C₆]benzene (sole energy and carbon source) and, as a control, in the presence of [¹²C₆]benzene. Subsequently, the proteins were extracted and subjected to 1-D gel electrophoresis and tryptic in-gel digestion, and the peptides were analyzed by nano-LC LTQ Orbitrap-MS. The difference in the isotopic pattern between the ¹²C/¹³C peptides allowed for the calculation of ¹³C incorporation levels for various labeled contents, using the highest intensity ¹³C peak and the decimal place slope script.