| Synthetic 13C=O labeling of peptides provides a means by which NMR analysis elucidates chemical shifts that are indicative of site-specific secondary structure fold. The primary targets for analysis in this study are a helices and beta hairpins. The investigation into alpha helices involved alanine rich peptides, with 13C=O labeled alanines at many locations throughout the sequence, probing the effects of various residue point mutations at the N-terminus, middle and C-terminus of the helix. From these experiments, Lifson-Roig parameters were derived and applied to our helicity prediction algorithm Helix1.5. Additionally, the effects of various capping motifs (SEDE, Schellman and D-Arg) at different points along the length of the helix were probed using 13C=O labels. beta Hairpin studies involved 13C=O labeled valines incorporated into cross-strand bonding and nonbonding positions. The cross-strand bonded valines show an excellent correlation between 13C=O chemical shift deviations and fraction fold determined by 2D NMR, indicating that the chemical shift difference between outward non-bonded carbonyls and inward bonded carbonyls, in beta hairpins, is representative of 0 and 100 percent fold, whereby fraction fold can be determined with selectively placed 13C=O labels. The difference between outward non-bonded carbonyl and cross-strand bonded carbonyl chemical shift deviations in proteins was found by cross-referencing the PDB with the BMRB. The differences in proteins when compared to peptides are similar, but are statistically indistinguishable. The investigations performed in the course of my research establish 13C=O labeling as a simple, yet efficient site-specific NMR technique that can be used to probe secondary structure. |
