Overall, these data suggest an important avidity phenomenon for these divalent compounds, and a chelating effect in the binding to the two lectins. Contrary to PM26, an hexavalent rod-based construct that we described as a potent DC-SIGN ligand and characterized recently, no cumulative avidity effect coming from statistical rebinding can be expected here, since a unique spearhead is presented at each extremity of the rod core. Rather, in the present case, there is probably an optimization of chelation properties with an improved access to two adjacent CRDs within the L-SIGN tetramer. However, potential clustering effect between two adjacent tetramers on the SPR chip cannot be excluded, since a single surface density was used for this characterization in the SPR assay. The higher efficiency of the ligand with the longer linker could be explained by the topology of DC-SIGN and L-SIGN tetramers as showed in Figure . SPR direct interaction (Fig. ). SPR inhibition experiments (Fig. ). As defined in . KDapp or IC50 ratio of DC-SIGN over L-SIGN. As measured by ITC (Fig. ) * KD of the monovalent ligand not determined. βfactor calculated using the IC50 associated with the monovalent ligand and the KDapps determined for the dimeric PM compounds. Despite the high overall homology between the two lectins (77% identity ), their CRDs, and thus binding sites, are not oriented in the same direction of space: while those of DC-SIGN are oriented upwards, those of L-SIGN are rather turned to the lateral side, increasing the gap between adjacent and opposite sites (between 60 and 80 Å for L-SIGN versus 40 to 60 Å for DC-SIGN). Such differential spacing and topology of the L-SIGN tetramer, with respect to DC-SIGN has been recently documented experimentally with results consistent with a larger tetramer and more outwardly exposed CRDs . A linker that is too may cover only a single portion of the side and not connect two sites that are facing each other, thus limiting chelation properties. Moreover, in a detailed molecular dynamic study performed recently, we have shown, for DC-SIGN multivalent binders, that distances between two internal sites can vary, due to internal flexibility between CRDs, and that the size of the linker helps to keep chelation-binding available and buffers distance fluctuation between sites 39 . That effect might be at work here also with PM74 vs PM69 increasing the dynamic situations where the chelation binding is still operative.