I read that only proteins from 20 kDa to 200 kDa can be labeled with your ChromaLink kit. Our protein has a MW of 5 kDa. Does that mean that the kit will not work for us?

While our ChromaLink™ reagent is certainly compatible with small proteins or biomolecules, our kit is comes with several desalting apparatuses that have a protein cut-off of 7,000 Da (7 kDa). We could certainly modify our kit for you and add some diafiltration units with a cut-off of 3 kDa. We would only recommend the diafiltration units if you have significant amount of your small protein, we are a bit concerned about the protein crashing out on the membrane. If you have an abundance of protein, we would certainly recommend trying the kit with this diafiltration procedure. Also the recommended amount of ChromaLink™ added to your small protein would, of course, have to be adjusted.