The interaction of a synthetically prepared mutant peptide of hevein (a well known chitin-binding lectin) Hev32S19D with chitin oligosaccharides (and chitosan analogues) has allowed us to estimate their affinity constants and associated thermodynamic data. The mutant peptide is able to bind chitin oligomers, but with significant decreases in the association constants with chito-oligosaccharides. The determination of the three-dimensional structure of the peptide mutant, by using NMR, has permitted us to deduce that the topology of the backbone is very similar to that of the parent Hev32 peptide. The same is true regarding the orientations of the key aromatic residues Trp21, Trp23, and Tyr30. The decrease in the association constants can be attributed to the different topological orientation of key side chains and to the importance of protein-sugar intermolecular essential hydrogen bonds and CH-pi stacking interactions. The analysis has permitted us to infer the free energy of binding associated with these interactions as well as to estimate the corresponding binding enthalpy.