I am trying to perform a coupling reaction between amino-modified DNA and NHS-diazirine to attach the diazirine to the DNA for UV irradiation studies. My procedure involves dissolving NHS-diazirine in 100% DMSO and reacting it with amino-modified DNA (20 equivalents of NHS-diazirine) in phosphate buffer (pH 8.5). The reaction is incubated at 25°C for 2 hours, and I purify the product using a column purification kit. However, MALDI analysis does not show the expected peaks.

Should I consider avoiding the buffer during the reaction or switching to a different purification method? Any suggestions on how to improve this process?