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The fimbrial lectin FimH from uro- and enteropathogenic <i>Escherichia coli</i> binds with nanomolar affinity to oligomannose glycans exposing Man&#945;1,3Man dimannosides at their non-reducing end, but only with micromolar affinities to Man&#945;1,2Man dimannosides. These two dimannoses play a significantly distinct role in infection by <i>E. coli</i>. Man&#945;1,2Man has been described early on as shielding the (Man&#945;1,3Man) glycan that is more relevant to strong bacterial adhesion and invasion. We quantified the binding of the two dimannoses (Man&#945;1,2Man and Man&#945;1,3Man to FimH using ELLSA and isothermal microcalorimetry and calculated probabilities of binding modes using molecular dynamics simulations. Our experimentally and computationally determined binding energies confirm a higher affinity of FimH towards the dimannose Man&#945;1,3Man. Man&#945;1,2Man displays a much lower binding enthalpy combined with a high entropic gain. Most remarkably, our molecular dynamics simulations indicate that Man&#945;1,2Man cannot easily take its major conformer from water into the FimH binding site and that FimH is interacting with two very different conformers of Man&#945;1,2Man that occupy 42% and 28% respectively of conformational space. The finding that Man&#945;1,2Man binding to FimH is unstable agrees with the earlier suggestion that <i>E. coli</i> may use the Man&#945;1,2Man epitope for transient tethering along cell surfaces in order to enhance dispersion of the infection.