How can we determine that which method should be followed for conjugation of a toxin to proteins? If the toxin has more than one reacting groups to make derivative. Please comment.
Pierce (now Thermo) has a huge array of reactive linking agents, usually with amine, thiol, carboxy or glycol reactivity and an array of spacer arm lengths, often including hydrophilic PEG. Its just a matter of finding the conjugate combination which works best for you. The book "Bioconjugate Techniques", 3rd Edition (2013) by Greg T. Hermanson is a good place to start, but Thermo has quite useful WEB pages as well.