44921-G4
Controlled Ring-Opening of NCAs for the Synthesis of Polypeptides
Polypeptides have been extensively utilized in drug delivery, tissue engineering, sensing and catalysis. To prepare polypeptides for these applications, it is essential to control their molecular weights (MWs) as well as their end groups during the ring-opening polymerizations (ROPs) of amino acid N-carboxyanhydrides (NCAs). We recently reported hexamethyldisilazane (HMDS)-mediated, controlled NCA polymerization (Journal of American Chemical Society, 2007, 129, 14114; ACF-PRF acknowledged). This polymerization proceeds via a unique, trimethylsilyl carbamate (TMS-CBM) propagating group, which involves the cleavage of the Si-N bond of HMDS during the initiation step. The resulting TMS-amine opens the NCA ring at its CO-5 position to form a TMS-amide at the C-end while the TMS group is attached to the N-end to form a TMS-CBM (the propagating chain end). The propagation of polypeptide chains proceeds through the transfer of the TMS group from the terminal TMS-CBM to the incoming monomer and forms a new TMS-CBM propagating chain end. We postulate that when a N-TMS amine is used as the initiator, the cleavage of its Si-N bond will generate an amine and a TMS group that will subsequently form the corresponding amide at the C-end and a TMS-CBM at the N-end after NCA ring opening. Thus, chain propagation should proceed in the same manner as the HMDS-mediated polymerization. Because a large variety of N-TMS amines are readily available, this method should allow facile functionalization of the C-termini of polypeptides. This report will mainly focus on the recent progress on the use of N-TMS amine initiated controlled polymerization. To demonstrate this concept, N-TMS allylamine (1-TMS) was utilized as the initiator for the polymerization of gamma-benzyl-L-glutamate NCA (Glu-NCA). We found that 1-TMS had remarkable control of Glu-NCA polymerization, which gave poly(gamma-benzyl-L-glutamate) (PBLG) with the expected MWs and narrow MWDs over a broad range of monomer/initiator (M/I) ratios (M/I = 20-300). The obtained Mns of PBLG at an M/I ratio of 20 and 300 were 4600 g/mol and 70100 g/mol, respectively, both of which were nearly identical to the expected Mns (4400 g/mol and 65700 g/mol, respectively). The Mns of PBLG showed linear correlation with the conversions of Glu-NCA, which were in good agreement with the expected Mns. This experiment demonstrated that the propagation of PBLG chains proceeded through a living chain end. 1-TMS also showed remarkable control of polymerizations of Cbz-L-lysine NCA (Lys-NCA) and resulted in poly(Cbz-L-lysine) (PZLL) with the expected MWs and very narrow MWDs. Block co-polypeptides, such as PZLL-b-PBLG, can also be readily prepared with the anticipated MWs and narrow MWDs via sequential addition of Lys- and Glu-NCA. We next compared the polypeptides obtained through 1- with those obtained through 1-TMS-initiated Glu-NCA polymerizations. The PBLG resulting from 1-initiated polymerization at an M/I ratio of 20 had a broad MWD (Mw/Mn=1.52) based on the MALDI-TOF MS analysis. In contrast, a similar polymerization mediated by 1-TMS resulted in PBLG with a much narrower MWD (Mw/Mn=1.17). The MALDI-TOF MS spectrum of the latter polymer showed a Poisson distribution centered at the exact theoretical MW of the PBLG containing the anticipated allyl amide end group (MW = allylamine + (Glu)20). This MS distribution pattern differed dramatically from that of the former PBLG. The bimodal GPC curve of the former PBLG compared to the monomodal curve of the latter PBLG further demonstrated the improved control of polymerization through subtle modification of the amine initiator. To evaluate whether this 1-TMS-mediated, controlled NCA polymerization can be extended to other N-TMS amines, we selected benzylamine (2), morpholine (3), propargylamine (4), N-(aminoethylene)-5-norbornene-endo-2,3-dicarboximide (5), and mPEG2000 amine (6) to represent primary (2) and secondary amines (3), amines containing functional groups that can be used for further reactions such as click chemistry (4) and ring-opening metathesis polymerization (5), and terminal amines of polymers (6). N-TMS’s of 2-6 were prepared and used to initiate Glu-NCA polymerization. As expected, all initiators gave excellent control of PBLG MWs. At an M/I ratio of 100, the Mns of PBLG were 23500, 21800, 21900, 23800, 28500 g/mol for PBLG derived from 2-TMS through 6-TMS mediated Glu-NCA polymerizations, respectively, which were in nearly perfect agreement with the expected Mns. Polymerizations of Glu- and Lys-NCAs with these initiators over a broad range of M/I ratios all gave corresponding polypeptides with controlled MWs and narrow MWDs. In conclusion, N-TMS amines can initiate controlled NCA polymerizations and allow facile functionalization at the C-termini of polypeptides. Polymerizations initiated by N-TMS amines are fast, give quantitative monomer conversion, and do not require excessive monomer purification. This methodology is useful for controlled synthesis of functional polypeptides, polypeptide macromonomers, and polypeptide copolymers. This work has been published in Journal of American Chemical Society, 2008, 130, 12562; ACF-PRF acknowledged)
