Comparison of 15N- and 13C-determined parameters of mobility in melittin.

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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 T1 and steady-state NOE measurements were made at 30 and 51 MHz on MLT at 22 degrees C enriched with 15N at six amide positions and in the Trp19 side chain. Both the disordered MLT monomer (1.2 mM peptide at pH 3.6 in neat water) and alpha-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: tau m, the correlation time for the overall rotation; S2, a site-specific order parameter which is a measure of the amplitude of the internal motion; and tau 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]. tau m and tau e values were consistent from data on the two nuclei. In the MLT monomer, S2 values for the backbone N-H and C alpha-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 alpha-H order parameters. The Trp side-chain N-H and C-H order parameters, and tau e values were generally similar in both the monomer and tetramer. Implications of these results regarding the dynamics of MLT are examined.