Data-files acquired by reverse phase liquid chromatography coupled online to electrospray-ionisation mass spectrometry (Q Exactive HF, Thermo Scientific).(1) 20200213_PJ_GC_MS1: full-scan MS, 500-700 m/z(2) 20200213_PJ_GC_PRM: product ion scans by parallel reaction monitoring for cyclase substrate MgPME, product DV PChlide a, and catalytic intermediates 131-hydroxy-MgPME and 131-keto-MgPME at m/z 598, 611, 614 and 612 respectively.Thermo RAW files are included in mzML format which is readable by open source applications such as SeeMS (http://proteowizard.sourceforge.net/download.html).