Main chain and side chain dynamics of peptides in liquid solution from ¹³C NMR: Melittin as a model peptide

Peptide backbone and lysine and tryptophan side chain mobilities in the synthetic, 26-residue peptide melittin (MLT) enriched with ¹³C were investigated in liquid solution by ¹³C T₁ and steady state nuclear Overhauser effect measurements at two magnetic fields and by Trp fluorescence anisotropy measurements and were analyzed using the Lipari and Szabo model- free approach. The overall rotational correlation times at 20 °C were 1.28, 1.4, 2.8, and 4.2 ns for monomeric random coil MLT, for monomeric helical MLT (in CD₃OD), for tetrameric MLT in neat D₂O, and for the tetramer in 50 mM phosphate buffer, respectively. Motion of the backbone in the interior of the sequence was most restricted in the monomeric helix and least restricted in the tetramer. In the monomeric disordered peptide, relatively less restricted backbone motion extending from the N terminus to the fourth residue was observed. Such 'end effects' continued only to the third residue in the monomeric helix and were observed just in the amino terminus glycine in the tetramer. The three Lys side chains showed the least restricted motion in the monomers and a differential restriction in the tetramers consistent with the tetramer structure. The motion of the Trp side chain was more restricted than that of Lys side chains and generally as restricted as that of the interior backbone atoms. The effective correlation times for the local motion of the backbone atoms were in the motional narrowing limit and showed distinct patterns. Agreement between NMR relaxation and Trp fluorescence anisotropy data was good for the monomer but not for the tetramer. Implications of these results for peptide dynamics in general are examined.