The interaction in solution between Man84 and L-SIGN and DC-SIGN was monitored by NMR. First, a chemical shift perturbation (CSP) analysis of the proteins upon Man84 binding was carried out. For L-SIGN, the addition of 10 equivalents of the ligand resulted in almost complete saturation of the protein, while the addition of 100 equivalents to DC-SIGN did not achieve saturation, highlighting the important difference in affinity for the two systems. Figure shows the CSP plot for the binding of L-SIGN (top) and DC-SIGN (below) to Man84. The differences are evident. While residues located at the long loops surrounding the Ca 2+ binding site are similarly perturbed in both lectins, some remote regions are exclusively perturbed in the case of L-SIGN. That is the case of the a2 helix and the beginning of b8-strand, which flanks residue Phe325, one of the most affected residues in L-SIGN. Curiously, the most perturbed residue was Lys270, at the N-terminus and far away from the protein binding site, an effect previously observed for the interaction with Mannose based ligands . Additionally, residues at the b6 and b7 strands are more affected in L-SIGN than in DC-SIGN. These results confirm that Man84 binds to DC-SIGN and L-SIGN in a different manner. While the same CSP profile is observed for Man84 and for a single Mannose residue (data not shown), where basically only amino acids at the primary Ca +2 binding site are perturbed upon binding, the experimental data obtained for L-SIGN unequivocally unravels the existence of further contacts between the protein and the studied glycomimetic. To obtain information of the recognition process from the ligand perspective, trNOESY experiments were performed. First, a NOESY spectrum of the ligand alone was acquired, which showed positive NOEs. The most interesting piece of information in this case are the NOEs of the proton of the triazole ring (H_Tz), which define its orientation with respect to the pyranose (Man) ring (Fig. ). H_Tz bond, this set of NOEs indicates a high degree of conformational averaging around this bond (Fig. ). Now, the H_Tz/H1 and H_Tz/H2 correlations are lost, while those for H_Tz/H4 and H_Tz/H6 are strong and very strong, respectively. This cross-peak pattern reveals a conformational selection process upon binding to L-SIGN. The experimental data show that Man84 is bound to L-SIGN in a particular, welldefined conformation, shown in Fig. , in which H_Tz points towards H6Man and H4Man and corresponding to the one observed in the X-ray structure (Fig. ). These NMR data, therefore, are in full agreement with the X-ray crystallographic structure of the Man84/L-SIGN complex, and strongly support the rationale for the L-SIGN versus DC-SIGN selectivity. For DC-SIGN, the aglycon in Man84