2-Iminothiolane as a Useful Coupling Reagent for Polyamine Solid-Phase Synthesis (1)
Polyarginines and also polyamines are protonated at physiological pH thereby tightly interacting with negatively charged cell-membrane components such as proteoglycans and the phosphate groups of phospholipids. the cellular uptake is suggested to occur either via a polyamine transport system (PAT) or via endocytosis of proteoglycan-enriched membranes.
The on-bead coupling of the biomacromolecule or the nanomaterials is the coupling of a thiol to a maleimido group in a Michael addition yielding a thioether linkage. A very gentle method to introduce a thiol functionality in a biomacromole- cule is the modification of a primary amino group with 2-iminothiolane hydrochloride which is a popular crosslinking agent for peptides.
2-Chlorotrityl resin was loaded with N-monoprotected diaminopropane in order to avoid crosslinking of the symmetric diamine. The o-nosyl group was removed with 2-mercaptoethanol and DBU in DMF and the free primary amine was reacted with excess of 2-iminothiolane hydrochloride. The thiol modification with 2-iminothiolane is usually carried out in DMSO or in aqueous buffers at pH values above 7.0. With polystyrene resins, solvent mixture of water–THF (1:9). The clear liquid phase indicated that the 2-iminothiolane had entirely dissolved, while the resin was sufficiently swollen. To avoid the formation of byproducts and to optimize the protocol for the maleimidothiol coupling, the respective maleimide and 2-iminothiolane were simultaneously added to resin 3. The desired product 9 could be obtained in 95% purity after a short reaction with 3 equivalents of 1 and 2 equivalents of ethylmaleimide. The formation of both byproducts could be suppressed by adding the maleimide two minutes after the 2-iminothiolane, since the recyclization occured slower than the coupling reaction.
An alkoxytrityl resin was loaded with tris-Aloc-sper-mine. Protection of the primary amine prevented any coupling of 11 with 4 equivalents of 1 even after 4 hours. Deprotection resulted in nonsymmetrically immobilized spermine,5 and a variety of maleimides were crosslinked using the established protocol. The crosslink occurred efficiently with a variety of structurally diverse maleimides.
