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[0065] 

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[0066] Dithiobis(sulfosuccinimidylpropionate) (DTSSP) (commercially available from Pierce Chemical Co., Rock› ford, Ill.) is taken up in phosphate-buffered saline at 50 nM/20 µLand the pH is adjusted to 7 .7 with 1 N NaOH. The solution (20 µL) is added to 1 µL of peptide HLGLAR (at 1 nM/µL) (SEQ ID NO: 3) and allowed to react for 30 minutes. The reaction is quenched with tris-hydroxymethyl› aminomethane (0.lM, 20 µL). The sample is desalted and oxidized with 10 µL of formic acid (88%):H$_{2}$ 0$_{2 }$ (30%) prepared at a ratio of 19:1 (v:v). Oxidation proceeds for 30 minutes at room temperature, and the derivatized analyte is dried prior to mass spectral analysis. 

[0067] 

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[0068] Dithiodiglycolic acid (0.93 g, 5.1 mmol, commer› cially available from Sigma Chemical Co., St. Louis, Mo.) ( alternatively, a polymer thereof of anhydride thereof ( cyclic or acyclic) may be used) is dissolved in dichloromethane (20 mL) and placed under inert atmosphere. 

[0069] Dicyclohexylcarbodiimide (1.05 g, 5.1 mmol) is added in one portion. After about 96 hours, the precipitate is removed by filtration and the filtrate is concentrated in vacuo. The resulting material is taken up in diethylether and 

filtered. Again, the filtrate is concentrated in vacuo to provide dithiodiglycolic anhydride. 

[0070] Dithiodiglycolic anhydride (the cyclic form is shown in this example) is prepared at a concentration of0.1 Min dry tetrahydrofuran prior to use. ASHLGLAR (1 nmol) (SEQ ID NO: 1) is diluted into 20 µL of 0.05 M trimethy› lamine. The dithiodiglycolic anhydride solution (5 µL) is added and the reaction mixture is vortexed for 30 seconds. The reaction proceeds for about two minutes at room temperature. The resulting product is oxidized with 2 µL of formic acid (88%):H$_{2}$ 0$_{2 }$ (30%) prepared at a ratio of 19:1 (v:v). Oxidation proceeds for 30 minutes at room tempera› ture, and the derivatized analyte is dried prior to mass spectral analysis. The concentration of the acidic moiety reagent is decreased by a factor of as much as 100 when derivatizing smaller quantities. 

[0071] 

[0072] 3-Sulfopropionic anhydride is prepared at a con› centration of 0.1 M in dry tetrahydrofuran prior to use. ASHLGLAR (1 nmol) (SEQ ID NO: 1) is diluted into 20 µL of tetrahydrofuran:diisopropylethylamine 4:1 (v:v). The 3-sulfopropionic anhydride solution (2 µL) is added and the reaction mixture is vortexed for 30 seconds. The reaction proceeds for about two minutes at room temperature prior to dilution and mass spectral analyses. The concentration of the acidic moiety reagent is decreased by a factor of as much as 100 when derivatizing smaller quantities. 

[0073] 

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[0074] Tetrafluorosuccinic anhydride is prepared at a con› centration of 0.1 M in dry tetrahydrofuran prior to use. 

