3.11 Limitations It is obviously ex

3.11 Limitations It is obviously extremely difficult to make generalized statements as to the limitations of any synthetic technique, particularly with one such as solid phase peptide synthesis, where there are so many factors other than just the success of chain assembly that can affect the overall overcome of the procedure. Nevertheless, it is not unreasonable to expect peptide sequences of up to 50 residues in length to be prepared in moderate to good purity by the standard methods, subject to the important caveat that aggregation during chain assembly did not occur. However, as the chain length increases, separation of the target peptide from the accumulated by-products becomes increasingly difficult. With particularly long peptides, the small and almost undetectable levels of impurities in protected amino acid derivatives and solvents, together with obvious by-products arising from incomplete Fmoc-removal and coupling reactions, begin to have an ever-increasing impact on the outcome of the synthesis. For these reasons, methods of chemoselective ligation, a technique whereby smaller, more manageable peptides are coupled together in aqueous media, and chemoselective purification, a method whereby the target peptide can be chemically tagged to assist purification, have been increasingly exploited for the production of large peptides and small proteins by chemical synthesis. These powerful techniques are dealt with in detail in Chapters 11 and 12.