To a solution of bis-tris propane [2] buffer (70 mL, 50 mM bis-tris propane, pH 8.0, 300 mM NaCl) and a solution of 2,2-dimethoxyethanal X′ (10 mL, 0.5 M in bis-tris propane buffer as described above), an aqueous solution of DERA 02 (10 mL, prepared with analogue method to one described in Example 1) and a solution of acetaldehyde (10 mL, 1.0 M in bis-tris propane buffer as described above) was added. The mixture was stirred for 3 h at 37° C. and pH was regulated at 8.0 with NaOH. After 3 h the proteins in the sample (1 mL) from the reaction mixture (100 mL) were precipitated with acetonitrile (9 mL) and the solution was centrifuged. A sample from the supernatant was collected and analyzed directly by HPLC-MS/MS instrument:
