MAGIC-ANGLE-SPINNING NUCLEAR-MAGNETIC-RESONANCE IN SOLID-PHASE PEPTIDE-SYNTHESIS

Solid-phase peptide synthesis of certain sequences (commonly called '' difficult sequences'') suffers from the occurrence of incomplete coupl ing reactions and/or partial unmaskings of Na-protection. The underlyi ng reasons for these problems are thought to be a structuration and/or a poor solvation of the growing peptide chains. Few methods are avail able to study the structural aspects of the peptide chains when still anchored to the solid support. In most cases, they rely on the incorpo ration of a specific label and examine therefore a modified peptide an alog. We describe the complete characterization by homonuclear and het eronuclear magic angle spinning nuclear magnetic resonance (MAS NMR) o f the solid-phase synthesis of a 10-residue peptide. A detailed second ary structure determination of the growing peptide on the resin beads, based on the NOE analysis and the H-1 and C-13 chemical shift deviati ons, indicated an extended structure on the whole length of the sequen ce. At critical synthesis steps, a correlation between the coupling di fficulties and the aggregation of the peptide chains was established b y 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 t he MAS NMR spectra. This increased mobility is linked to an enhanced p eptidyl-resin solvation as reflected by the better coupling efficiency at the critical synthesis steps.