Peptide torsion angle measurements: Effects of nondilute spin pairs on carbon-observed, deuterium-dephased PM5-REDOR

Reintroducing dipolar coupling between spin-1/2, nuclei (e.g., C-13, N-15) and spin-1 H-2, using phase-modulated deuterium dephasing pulses, provides a simple and efficient basis for obtaining peptide backbone torsion angles (phi, psi) in specific stable-isotope enriched samples, Multiple homonuclea r spin-1/2 interactions due to isotopic enrichment can arise between neighb oring molecules or within a multiply labeled protein after folding. The con sequences of C-13 homonuclear interactions present during C-13-observed, H- 2-dephased REDOR measurements are explored and the theoretical basis of the experimentally observed effects is investigated. Two tripeptides are taken to represent both the general case of H-2(alpha)-alanine (in the tripeptid e LAF) and the special case of H-2(2)alpha-glycine (in the tripeptide LGF). The lyophilized tripeptides exhibit narrowed spectral linewidths over time due to reduced conformational dispersion. This is due to a hydration proce ss whereby a small fraction of peptides is reorienting and the bulk peptide fraction undergoes a conformational change. The new molecular packing arra ngement lacks homonuclear C-13 spin interactions, allowing determination of (phi, psi) backbone torsion angles. (C) 2001 Academic Press.