Herein, as trategy for the selective derivatization of 3-nitrotyrosine-containing proteins using the classic azo coupling reactiona st he keys tep is described. This novel approach featured multiple advantages and was successfully appliedt od etect picomole levelso fp rotein tyrosine nitrationi nb iological samples.Protein tyrosine nitration (PTN), that is, conversion of the tyrosine residue into 3-nitrotyrosine (3-NT)b yr eactive nitrogen species( RNS), is ac ommon protein post-translationalm odification, and is aw ell-establishedb iomarker of nitroxidative stress. [1] Elevationo fP TN level has been observed in multiple pathological conditions, and is closely associatedw itha ging [2] and many diseases, including neurodegeneration, cancera nd cardiovascular diseases. [1c, 3] The occurrence of 3-NT in proteome and the importantr ole of PTN have simulated increasing studies to establish specific and sensitivem ethodsf or detection of 3-NT in biological samples. [1a,d, 4] Traditionald etection methods of 3-NT in biological samples are generally divided to two types. One is based on instrumental analysis, such as HPLC, GC, HPLC-MS, tandem mass spectrometry. [5] However, this kind of detection methods requiresc omplicated sample pretreatment processes. The other is based on immunoassays, such as western blot, immunohistochemistry,i mmunoprecipitation and ELISA,b yu sing 3-NT antibodies. [6] However,t he specificity and affinity of anti-3-NT antibodiesi nr ecognizing different 3-NT-containing proteins are varying as most of thesea ntibodies are produced using 3-NT or specific 3-NT containing proteins as an immunogen without considering the surrounding amino acid sequence and the compatibility of the anti-body,r espectively.I nr ecent years, selective chemical derivatizationo f3 -NT for enriching or detecting PTN has gained increasinga ttention. [4, 5c] In general, 3-NT was first transferred to 3-aminotyrosine (3-AT)b yr educing agents, and subsequently different reagents were employed to selectively capture3 -AT. These3 -AT-reactive partners, such as N-hydroxysuccinimide (NHS)e ster [7] and aldehydes, [8] may capture the 3-AT-containing proteins to facilitatet he following instrumentala nalysis. These reagents react with amines promiscuously,a nd it thus requires blockingo ther protein aminog roups prior to capturing 3-AT. Besides, several coupling reactions that selectively react with 3-ATh ave been developed. For example, Schçneicha nd his colleagues used benzylamined erivatives to achieve at urn-on labeling of PTN under oxidative conditions. [9] Francis and coworkersd eveloped at wo-step chemoselective aniline-based oxidative coupling strategy to detect PTN in cell lysates. [10] However,t he oxidative conditions might cause undesired modifications of oxidative-sensitive residues (such as cysteine and methionine). And based on the mechanism they proposed, [9b, 10b, 11] the benzylamined erivatives or aniline might also react with 3-hydroxytyrosine.N ga nd Wong converted 3-ATt o 3-azidotyrosine, followed by c...