Peptides spin-lattice relaxation times

Spin-lattice relaxation times and 13C chemical shifts were used to study conformational changes of poly-L-lysine, which undergoes a coil-helix transition in a pH range from 9 to 11. In order to adopt a stable helical structure, a minimum number of residues for the formation of hydrogen bonds between the C = 0 and NH backbone groups is necessary therefore for the polypeptide dodecalysine no helix formation was observed. Comparison of the pH-dependences of the 13C chemical shifts of the carbons of poly-L-lysine and (L-Lys)12 shows very similar values for both compounds therefore downfield shifts of the a, / and peptide carbonyl carbons can only be correlated with caution with helix formation and are mainly due to deprotonation effects. On the other hand, a sharp decrease of the 7] values of the carbonyl and some of the side chain carbons is indicative for helix formation [854]. 

The determiiiation of spin-lattice relaxation time (Tj) and correlation time (r) using NMR spectroscopy (66) provides direct information about the intramcdec-ular motion of cydic peptides. Recently, Fossel et al. (67) determined r values with Cyclo-(Gly2), Qydo-(Gly-Pro), Cyclo-(Pr02), and Cydo- Pro-D-Pro) in D2O solution, and the r values were compared with those for the correspmiding linear dipeptides. 

Beside chemical shifts and coupling constants, spin-lattice relaxation times provide frequently another important source of information for structure elucidation, in particular for metal nuclei. In the same way as indirect detection allows effective chemical shift measurement for metal nuclei whose direct observation would be very time consuming or not feasible at all, it was also applied to relaxation time measurements.Since most of the studied organometallic compounds contained only a single metal nucleus, the use of ID sequences was preferred (however, analogous 2D schemes were applied to the characterization of peptides, " and P and F served beside H as observed nucleus. 

Bradbury and Warren have carried out further work on peptide sequence using proton magnetic resonance. On binding of gadolinium ions to the terminal carboxy-group of a peptide, the spin-lattice relaxation times of the a-protons are reduced sequentially. 

An interesting correlation has been presented relating peptide torsion angles to spin-lattice relaxation times (Bleich et a/., 1976a,b). The relaxation times relate to the peptide backbone torsion angles (j) and j/. The quaternary values depend on the side-chain torsion angle %. 

The terminal tripeptide of oxytocin has been uniformly enriched in C (85-90%), both at the proline-7 and at the leucine-8 position (Convert et al, 1977), as well as at the glycine-9 residue (Blumenstein and Hruby, 1976). Both C chemical shifts and spin-lattice relaxation times were measured for these peptides free in solution and in the presence of neurophysin. Neither of the above parameters is affected by the interaction of peptide and protein the proline peptide bond remains in the trans conformation. These studies also showed that the terminal tripeptide has considerable freedom and that mainly the first three residues in the ring portion interact with neurophysin. 

The Tip technique has only recently been applied to the study of intramolecular motion in peptides (Bleich and Glasel, 1978). The advantage of the technique is that time ranges of the order of 10" -10" s can be investigated, while chemical shifts and linewidths provide information on the 1- to 10 -s time scale and conventional spin-lattice relaxation data... 

---