Magic Angle Spinning Nuclear Magnetic Resonance in Solid-Phase Peptide Synthesis

Abstract

Solid-phase peptide synthesis of certain sequences (commonly called “difficult sequences”) suffers from the occurrence of incomplete coupling reactions and/or partial unmaskings of Nα-protection. The underlying reasons for these problems are thought to be a structuration and/or a poor solvation of the growing peptide chains. Few methods are available to study the structural aspects of the peptide chains when still anchored to the solid support. In most cases, they rely on the incorporation of a specific label and examine therefore a modified peptide analog. We describe the complete characterization by homonuclear and heteronuclear magic angle spinning nuclear magnetic resonance (MAS NMR) of the solid-phase synthesis of a 10-residue peptide. A detailed secondary structure determination of the growing peptide on the resin beads, based on the NOE analysis and the 1H and 13C chemical shift deviations, indicated an extended structure on the whole length of the sequence. At critical synthesis steps, a correlation between the coupling difficulties and the aggregation of the peptide chains was established by chemical measurements and MAS NMR. Upon titration with the hydrogen bond-accepting solvent DMSO, the mobility of the peptide chains on the resin beads increased, resulting in a significant line narrowing of the MAS NMR spectra. This increased mobility is linked to an enhanced peptidyl-resin solvation as reflected by the better coupling efficiency at the critical synthesis steps.

Peptide synthesis, characterization and 68Ga-radiolabeling of NOTA-conjugated ubiquicidin fragments for prospective infection imaging with PET/CT

“…Steric hindrance due to the presence of bulky protecting groups on amino acids may also be a factor that inhibits extension of the peptide [26,27]. Peptides with sequences that give rise to beta sheet secondary structures, and those with very hydrophobic regions are most prone to aggregation and are generally the most problematic to synthesize [23,[28][29][30][31][32].…”

Peptide structural analysis by solid-state NMR spectroscopy

“…MAS nmr spectroscopy therefore allows for the detailed analysis of the growing peptide chain and concomitantly the monitoring of the synthetic progress. [141][142][143] Magic angle spinning results in well-resolved nmr spectra in which sample homogeneity determines to a large extent the half width of the nmr resonances. The 15 N and 13 C chemical shifts have been used as an analytical tool to determine polypeptide main chain conformations.…”

Evaluation of the Trifluoromethanosulfonic Acid/Trifluoroacetic Acid/Thioanisole Cleavage Procedure for Application in Solid-Phase Peptide Synthesis.

“…25,26) Spectroscopic techniques of NMR, 27,28) IR 29,30) or EPR 31,32) with the use of a paramagnetic amino acid 33) or associated with fluorescence 34) have been intensively applied to achieve the same objective. In this context, the decrease in overall synthesis yield due to premature chain detachment in the TFA step combined with possible incomplete peptide cleavage from the resin has been routinely neglected.…”