DIFFICULT SEQUENCES AND SOLVATION OF PEPTIDE CHAINS IN SOLID-PHASE PEPTIDE-SYNTHESIS

Recently, ''difficult sequences'' have been often found as a key word in solid-phase peptide synthesis. ''Difficult sequences'' which cause the most serious potential problem during stepwise solid-phase peptide synthesis result from the intermolecular aggregation, namely, the formation of beta-sheet structure by intermolecular hydrogen bonds between protected peptide chains. This beta-sheet structure hinders the coupling reaction, the deprotection of the N-protecting group, and even the liberation of amino groups at each step for peptide chain elongation and results in the incomplete peptides with the properties similar to the target peptide. In liquid phase peptide synthesis, the ''difficult sequence'' is closely related to the solubility of protected peptides. The decreasing solubility of protected peptides in organic solvents along with increasing their chain length is due to the intermolecular beta-sheet structure formation. The conversion of ''difficult sequence'' to easy sequence by disrupting the beta-sheet structure can be achieved wherether the protected peptides are completely solvated by organic solvents or they are subject to peptide segment separation.