Free cysteine and iodoacetamide method:

Make up a stock solution of saturated cysteine (free acid) in 2M tris.

Using the stock solution, bring the sample up to 40mM cysteine (approx. 50mM tris) and adjust pH to 7.8 (if necessary).

Incubate 30 minutes on ice.

Add iodoacetamide to 100mM and adjust pH to 8.0 (if necessary).

Incubate 60 minutes on ice.

Dialyse or desalt into desired buffer.

For QC, check purity/homogeneity by PAGE.