On the stability of a single-turn alpha-helix: the single versus multiconformation problem

The pentapeptide Ac-HAAAH-NH2, cyclized through its imidazoles by PdII to give [Pd(en)(peptide)]2+, has recently been evaluated by 2-D NMR and simulated annealing as a single alpha-helix conformation in solution. In the present work, we have questioned this assumption by developing Pd2+ parameters for AMBER*, performing an extensive conformational search for the [Pd(en)(peptide)]2+, and deconvoluting the averaged NMR data into eight rapidly equilibrating conformations with populations ranging from 2 to 55%. None of the latter correspond to the alpha-helix, although a 3% form possesses a related structure. As a critical component of interpreting an averaged NMR spectrum in terms of a single conformation, we advise testing this assumption with a method that permits conformational deconvolution.