Combined solid-phase and solution approach for the synthesis of large peptides or proteins

In the synthesis of large peptides or proteins, highly homogeneous segments are indispensable for a convergent strategy either on a solid-phase resin or in solution. Employing Boc/Bzl chemistry to prepare fully protected segments with a free alpha-carboxyl group from the solid support, base-labile linkers are profitable for practical peptide synthesis since they require no special equipment. For this purpose, an N-[9-(hydroxymethyl)-2-fluorenyl]succinamic acid (HMFS) linker was adopted. Consequently, there must be high compatibility between the protecting groups of the segment and the anchoring group which is cleavable by treatment with morpholine or piperidine in DMF. Instead of using the 2-bromobenzyloxycarbonyl (BrZ) group for the Tyr residue and the formyl (For) group for the Trp residue, both of which are the most susceptible protecting groups under these base-catalysed conditions, the base-resistant 3-pentyl (Pen) and cyclohexyloxycarbonyl (Hoc) groups were introduced to the respective side-chain functional groups. By applying the present strategy, the authors were able to rapidly synthesize homogeneous protected segments for use in the subsequent segment coupling in solution. In the present paper, the utility of the combined solid-phase and solution approach is demonstrated by synthesizing muscarinic toxin 1 (MTX1) which binds to the muscarinic acetylcholine receptors.