The Role of Solid-Phase Fragment Condensation (SPFC) in Peptide Synthesis

Condensation of fully protected peptide fragments on a solid support is an efficient alternative to stepwise solid-phase peptide synthesis (SPPS). While stepwise SPPS is often efficient for assembly of peptides up to 20 or 30 amino acids in length, for longer peptides and small proteins (50 amino acid residues and more) separation of the target molecule from often closely related deletion, modification and termination sequences, generated at each single step by incomplete reaction, may lead to complete failure of the stepwise approach. This is where SPFC plays a key role by combining the advantages of solution synthesis (possibility of purification of the intermediate peptides) and solid-phase procedures (rapid reaction, high yield and easy removal of excess reagent). By limiting the number of coupling steps between fragments, SPFC helps to increase yield and purity of the target peptide with respect to the stepwise approach. Recent progress in rapid assembly of fully protected peptide fragments on specially designed linkers by stepwise protocols along with the development of efficient methods for fragment purification is now reported regularly and will further increase the importance of SPFC in peptide chemistry. 1. Introduction and Scope 2. Historical Development 3. Strategy and Tactics in SPFC 3.1. The Role of the Solid Support 3.2. Orthogonal Protection Schemes for Formation of Protected Peptides. Double Linker Strategy and Preformed Handle Approach 3.3. Handles and Linker-Resins for Synthesis of Protected Fragments and for Final Fragment Assembly: Barlos and Kaiser Resins 3.4. Proper Choice of Fragments 3.5. Activation of Peptide Acids: Comparison of Different Coupling Methods 3.6. Excess Reagent. Influence of Solvent, Additives, Side-Chain Protection and Temperature on Solubility. Repeated Coupling and Capping 3.7. Comparative Synthetic Studies Between Stepwise SPPS, Solution and SPFC 4. Special Developments in SPFC 4.1. Synthesis and Attachment of the Peptide to the resin. Side-Chain Attachment and Cyclisation Reactions 4.2. (C→N)- and (N→C)-Assembly in SPFC. Oxidation-Reduction Condensation 4.3. Towards Automatisation: Combined Stepwise and FC-Approach 5. Monitoring of Cleavage and of FC. Internal Reference AAs (IRAAs) 6. Purification Methods for Protected Peptide Fragments 7. SPFC-Procedures in the Literature Condensation of Fragments on Merrifield and PAM resins (HF-Cleavage, Transesterification) FC on Wang Resins (TFA-Cleavage) FC on Kaiser Resins (Cleavage by Nucleophiles) FC on Highly Acid-Labile Resins Procedures for Synthesis of Peptide Amides Special Procedures: Side-Chain Attachment of Peptides, (N→C)-Assembly of Fragments, Oxidation-Reduction Condensation 8. Outlook and Conclusions 9. Abbreviations