The selective nitration of tyrosine residues in three of the four biotin-binding sites of Neutralite avidin considerably reduces the affinity of the protein for biotinylated molecules above pH 9. Consequently, biotinylated probes can be adsorbed at neutral pH and released at pH ~10. Free biotin blocks the remaining high-affinity biotin-binding sites that have not been nitrated. Nitro-avidin is particularly useful for separating and purifying biotin conjugates from complex mixtures. Nitro-avidin also provides a regenerable capture reagent for functionalization of surface plasmon resonance (SPR) immunosensors2. In its neutral/deglycosylated form (NitroLite Avidin), it encompasses the advantage of reversibility of biotin-binding and the absence of nonspecific binding which may represent a real plus in affinity chromatography. The biotin-binding capacity of Nitroavidin derivatives is typically min 10 µg free biotin per mg protein.