structural assignment of compound D, we can exclude the presence of 1'-OH-MDZ-N-glucuronide 2 (yellow trace) based on the absence of an experimental band near 1550 cm -1 (corresponding to the C=N stretching vibration of the glucuronated N-atom), the absence of the band around 1300 cm -1 and the mismatch in the diagnostic 600-900 cm -1 region. The distinction between the 1'-OH-MDZ-O-glucuronide and the 1'-OH-MDZ-N-glucuronide 1 is more subtle. Compared with the computed spectrum for 1'-OH-MDZ-N-glucuronide 1 (Figure ), the experimental spectrum does not contain the bands predicted around 1200 cm -1 and the match in the 600-950 cm -1 range is poor. On the other hand, the predicted spectrum of 1'-OH-MDZ-Oglucuronide (Figure ) does not reproduce the relative intensities of the bands around 1100 and 1400 cm -1 . However, overlaying the IR spectra of compound D and compound E (see Figure ) shows that they are quite different, whereas computed spectra for structures with the same glucuronidation site are very similar; this suggests that compounds E and D do not share the same glucuronidation site. Additionally, the band predicted near 1200 cm -1 for 1'-OH-MDZ-N-glucuronide 1 (Figure ) is analogous to the feature predicted for MDZ-N-glucuronide 1 (Figure ), which is clearly observed in the experimental IR spectrum of compound D; this suggests that the computations perform properly for this band. The intensity of the band computed at 1400 cm -1 for protonated 1'-OH-MDZ-Oglucuronide is clearly at odds with the experimental spectrum; it corresponds to the OH-bending vibration of the carboxylic acid-group, which has a trans-configuration in the lowest-energy conformer. This causes the OH-group to be part of a strong hydrogen-bonding network, probably leading to an overestimation of the intensity of this band in the computation. Indeed, the higherenergy (+20.8 kJ/mol) conformer with a cis-configuration carboxylic acid group is predicted to show the correct relative intensity around 1400 cm -1 (see Figure in the SI). Moreover, the relative intensity around 1100 cm -1 is also better predicted for this conformer. Therefore, we suspect that the intensities of these bands are incorrectly predicted at the level of theory used and we assign the 1'-OH-MDZ-O-glucuronide isomer to compound E.