For mass spectrometric analysis, MOP samples were dissolved in methanol and directly analyzed using a Q Exactive Plus mass spectrometer (Thermo Scientific, Bremen, Germany) with negative mode electrospray ionization. Spectra were typically recorded in profile mode over a range of 15 1000 ≤ m/z ≤ 3000 at an instrument resolution of R = 280.000 (FWHM at m/z 200). For data depth and to achieve a good statistical representation of isotopic distributions, 20 to 50 single scans were averaged to give the final interpreted mass spectrum. For data evaluation, theoretical spectra of the different analytes were calculated using instrument vendor software (Thermo Xcalibur, Qual Browser version 4.7.69.37) at the given instrument resolution. Monoisotopic signals are typically of only minor abundance for these analytes, and therefore the most abundant isotopologue signal of the corresponding theoretical spectrum was used for mass measurement error estimation (exact mass, HRMS). Analyte abundance ratios for the different MOP cages were also calculated based on intensity of the most abundant isotopologue of the theoretical spectrum.