The observed deviations between the modelled and experimental F ENDOR distances around G5 prompted us to trilaterate the F - position based on the experimental distances. This places the F - in a position between the pseudoknot and the unpaired bases between the pseudoknot and stem 2 (Fig. ). Here, the F --Mg 2+ cluster could not be stabilised by coordination from the phosphate backbone (Fig. ), strongly indicating a different structure or conformation around G5 well beyond the error of the F ENDOR measurement. The crystal structure shows significant intermolecular contacts (Fig. ) for the first two bases from the 5' end, suggesting a structural variation in absence of these contacts in solution. The deviation in the distance between the nitroxide spin label and the F -at G5 could be explained by a different orientation of the first five nucleotides at the 5' end. The distance between the G5 phosphate group and the F -is 6.3 Å in the crystal structure. The spin label rotamers have distances of 7 -7.5 Å between phosphate and NO group adding to a maximum NO-F distance of about 14 Å. Thus, the 12 Å distance seen in the F ENDOR measurement (Fig. ) could arise from a differently oriented phosphate group. Nevertheless, a high similarity between five crystal structures of the riboswitch in presence of various ions as well as with the NMR derived structure of the B. cereus riboswitch has been reported (Fig. ). Thus, the ENDOR and PELDOR results reveal different levels of structural heterogeneity -representing different functional dynamics -not directly obvious from crystal structures. and orientations (α, β) of all rotamers were extracted, a F ENDOR spectrum for each rotamer was simulated using SimSpec and the sum of the simulated