The min. oligosaccharide structure required for binding to the potent HIV-inactivating protein cyanovirin-N (CV-N) was detd. by satn.-transfer difference (STD) NMR spectroscopy. Despite the low mol. mass of the protein (11 kDa), STD-NMR spectroscopy allowed the precise at. mapping of the interactions between CV-N and various di- and trimannosides, substructures of Man-9, the predominant oligosaccharide on the HIV viral surface glycoprotein gp120. Contacts with mannosides contg. the terminal Mana(1->2)Mana unit of Man-9 were obsd., while (1->3)- and (1-6)-linked di- and trimannosides showed no interactions, demonstrating that the terminal Mana(1->2)Mana structure plays a key role in the interaction. Precise epitope mapping revealed that, for Mana(1->2)ManaOMe, Mana(1->2)Mana(1->3)ManaOMe, and Mana(1->2)Mana(1->6)ManaOMe, the protein is in close contact with H2, H3, and H4 of the nonreducing terminal mannose unit. In contrast, the STD-NMR spectrum of the CV-N/trisaccharide Mana(1->2)Mana(1->2)ManaOMe complex was markedly different, with resonances on all sugar units displaying equal enhancements, suggesting that CV-N is able to discriminate between the three structurally related trisaccharides. [on SciFinder (R)]