Cross correlation and spin-rotation effects on methyl spin-lattice relaxation in peptides. Tetragastrin

Abstract

I H and "C relaxation data relating to the methionine methyl group i n the peptide tetragastrin are reported. An analysis of methyl spin-lattice relaxation times and the heteronuclear Overhauser effect is presented. This analjsis illustrates an approach to the determination of motional information when dipolar and spin-rotation interactions dominate the relaxation mechanism and when the effect of cross correlations must be considered. The importance of these effects in studies of peptides in solution by N M R relaxation is emphasized.We have recently reported the existence of nonexponential relaxation behavior for the protons of the methyl group of tetragastrin2 (I) and have attributed this behavior to cross cor-H-Trp-Met-Asp-Phe-NHl I relations between the relaxing spin pairs. In addition, we have presented a detailed account of the relaxation of 'H and I3C nuclei in other groups of this molecule and have emphasized the conformation information implicit in the proper use of both ' H and 13C relaxation times and the 'H-I3C heteronuclear Overhauser e f f e~t .~ This paper is an amplification of our original report,2 and its importance stems from theoretical and experimental ~o r k~-~ which indicates that accurate measurements of cross-correlation effects may be an aid in determining the nature of overall anisotropic molecular tumbling in molecules of the size with which we are dealing. Moreover, we recently have found nonexponential spin-lattice relaxation for the methionyl methyl protons of another peptide, the pharmacologically active pentapeptide, "methionine enkephalin".lO.l Thus, nonexponential methyl proton relaxation and observable cross-correlation effects may be a general feature of methionine containing peptides. Also, specific I3C enrichment of S -C H j sites has recently been reported,I2 and thus the increased use of methyl carbons as NMR probes of protein conformation seems likely. Accurate utilization of data gathered from biophysical application of I 3C methyl probes will potentially require consideration of auto-and cross-correlation effects in both the dipolar and spin-rotation interactions, since spin-rotation frequently plays a role in methyl relaxation.I3 In such a situation analysis of the relaxation is not straightforward, and in the present publication we illustrate an attack on the problem utilizing both IH and I3C relaxation data.The approach taken here utilizes analysis of the two different spin systems '*CH3 and I3CH3, the former relating to the proton spin system and the latter relating to the proton decoupled I3C system. An alternative method involves analyzing the ' H and I3C relaxation directly in the fully coupled I3CH3 specie^,^ which has the advantage of potentially providing a complete description of the relaxation kinetics and thereby providing additional information from which motional parameters may be obtained. However, full utilization of this alternative method requires that I3C enriched species be available and that facilities exist with which various kinds of spin system ...

Accounting for the temperature dependence of 13C spin–lattice relaxation of methyl groups in the glycyl–alanyl-leucine model system under MAS with spin diffusion

“…Our calculations have also ignored the effects that 1 H– 1 H to 13 C– 1 H dipole–dipole cross correlation might have on the relaxation rates. It has been suggested that cross correlation can affect the measurement of correlation times by 6–15% (Cutnell and Glasel 1976). The effect of cross correlation is ordinarily to retard spin–lattice relaxation rates (Kumar et al 2000).…”