α- and β- aspartyl peptide ester formationvia aspartimide ring opening

Abstract

The undesirable reaction of aspartimide formation has been proved to occur under both acid and base conditions in solid-phase peptide synthesis and is dependent on the beta-carboxyl protecting group, the acid or base used during the synthesis, as well as the peptide sequence. The hydrolysis of aspartimide-containing peptides, especially during HPLC purification, yields a mixture of alpha- and beta-aspartyl peptides that can not be purified easily. A previous study demonstrated that treatment of aspartimide-containing peptides with methanol in the presence of 2% diisopropylethylamine in solution leads to alpha- and beta-aspartyl peptide methyl esters. Taking advantage of these results and aiming at elucidating the optimal conditions for aspartimide ring opening, the effect of different types and concentrations of alcohols (primary and secondary) and bases (diisopropylethylamine, collidine, 4-pyrrolidinopyridine, 1-methyl-2-pyrrolidone, piperidine and KCN) was tested at various temperatures and reaction times. The best results were obtained with a combination of a primary alcohol and diisopropylethylamine, while aspartimide ring opening by secondary alcohols occurred only at high temperatures. The optimal conditions were also applied to solid-phase peptide synthesis.

New t‐butyl based aspartate protecting groups preventing aspartimide formation in Fmoc SPPS

“…Ring‐opening by piperidine gives a mixture of d / l ‐ α ‐ and β ‐piperidides that are characterised in MS as signals 67u greater than the expected peptide . Whilst in most cases, the aspartimides and α ‐ and β ‐piperidides may be easily separated from the target peptide by HPLC, the d / l ‐ β ‐aspartyl peptides and d ‐ α ‐aspartyl peptide are often impossible to remove as these will frequently have same retention times as the target peptide . Moreover, as these impurities have the same mass as the target, the absence or presence of these side products in the product cannot be determined by mass spectrometry.…”

Fmoc Methodology: Cleavage from the Resin and Final Deprotection

“…This undesired sidereaction has been observed during the acid-catalyzed cleavage/protection step [67][68][69]. This undesired sidereaction has been observed during the acid-catalyzed cleavage/protection step [67][68][69].…”

Abbreviations and symbols in peptide science: a revised guide and commentary

“…Bridges involving peptide backbone nitrogen atoms call for special care. The abbreviation Asi has been suggested [4] for the aspartimide 'residue' formed by the side chain to backbone cyclization 9 to 10. This is a slight abuse of the way Xaa should really be used, but has the great merit of simplicity.…”