User:WKB321/Maleimide

Maleimides linked to polyethylene glycol chains are often used as flexible linking molecules to attach proteins to surfaces. The double bond readily undergoes a retro-Michael reaction with the thiol group found on cysteine to form a stable carbon-sulfur bond. '''Cysteines are often used for site-selective modifications for therapeutic purposes because of the rapid rate of complete bioconjugation with sulfhydryl groups, allowing for higher levels of cytotoxic drug incorporations. (cit)''' Attaching the other end of the polyethylene chain to a bead or solid support allows for easy separation of protein from other molecules in solution, provided these molecules do not also possess thiol groups.

'''The retro-Michael reactions resulting in maleimide-thiol adducts require precise control. The targeting ability of drugs containing the adducts can be easily hindered or lost due to their instability in vivo. (cit) The instability is mainly attributed to the formation of the thiosuccinimide which might be involved in thiol exchange reaction with glutathione. B-elimination reaction follows, resulting in off-target activity and a loss of efficacy of the drugs. (cit)'''

'''Currently, there is no general method for stabilizing thioesters, such as thiosuccinimides, so that their off-target effects can be eliminated in drugs. But mitigation of problems associated with thiol exchange is possible via several controlled techniques. Hydrolyzing the thiosuccinimide five-membered ring forms a stable hydrolysate, which prevents elimination of the maleimide-thiol bond. The process of ring-opening hydrolysis requires special catalysts and bases, which may not be biocompatible and lead to harsh conditions. Alternatively, cysteines in the positively charged environment or an electron-withdrawing group enable the thiosuccinimide ring to undergo self-hydrolysis. Complex molecular designs and syntheses make these processes potentially problematic.'''

'Another problem with hydrolysis arises if it is applied to N''-alkyl-substituted derivatives instead of the N-aryl-substituted derivatives because they hydrolyze at a rate that’s too slow to yield consistently stable adducts. (cit) An alternative method of stabilizing the maleimide-thiol adducts is to stretch the thiosuccinimide linkage through single-molecule force spectroscopy. (cit)'''