Backbone and tryptophan side-chain mobilities in the 26-residue, cytolytic peptide melittin (MLT) were investigated by
15N and
13C NMR. Specifically, inverse-detected
15N T
1 and steady-state NOE measurements were made at 30 and 51 MHz on MLT at 22 °C enriched with
15N at six amide positions and in the Trp
19 side chain. Both the disordered MLT monomer (1.2 mM peptide at pH 3.6 in neat water) and -helical MLT tetramer (4.0 mM peptide at pH 5.2 in 150 mM phosphate buffer) were examined. The relaxation data were analyzed in terms of the Lipari and Szabo model-free formalism with three parameters:
m, the correlation time for the overall rotation; S
2, a site-specific order parameter which is a measure of the amplitude of the internal motion; and
e, a local, effective correlation time of the internal motion. A comparison was made of motional parameters from the
15N measurements and from
13C measurements on MLT, the latter having been made here and previously [Kemple et al. (1997) Biochemistry, 36, 1678–1688].
m and
e values were consistent from data on the two nuclei. In the MLT monomer, S
2 values for the backbone N-H and C-H vectors in the same residue were similar in value but in the tetramer the N-H order parameters were about 0.2 units larger than the C-H order parameters. The Trp side-chain N-H and C-H order parameters, and
e values were generally similar in both the monomer and tetramer. Implications of these results regarding the dynamics of MLT are examined.
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