To identify compound B, we generated nine candidate structures resulting from MDZ hydroxylation using the in silico enzymatic reaction tool BioTransformer . As discussed above, the strong H2Oloss observed in the MS/MS spectrum of compound B suggests that hydroxylation occurs on an aliphatic position, pointing in the direction of 1'-OH-MDZ and 4-OH-MDZ. Figure /e compare the IR spectrum of compound B to theoretical spectra of 1'-OH-MDZ and 4-OH-MDZ (see also Figure ). A comparison to the theoretical IR spectra of all other candidate structures (with the OH-group in an aromatic position) is shown in Figure in the SI. It is seen that the computed spectrum of 1'-OH-MDZ reproduces the experimental spectrum of compound B well, except for the OH-bending vibration of the 1'-OH group, which is blue-shifted in the experimental spectrum (as was also discussed above for the 1'-OH-MDZ reference spectrum). Computed spectra for most other candidates lack one or more diagnostic features observed in the experimental spectrum. For instance, several computed spectra fail to reproduce the intense vibrational band observed around 1000 cm -1 or the doublet around 1100 cm -1 . The spectrum predicted for aromatic OH-MDZ 4 (see SI) contains most of the observed features and shows a relatively good match to the experimental spectrum. However, relative band intensities and the overall band pattern deviate more from the experimental