A synthetic peptide patterned after the sequence of the inactivating ball domain of the Shaker B K(+) channel, the ShB peptide, fully restores fast inactivation in the deletion Shaker BDelta6-46 K(+) channel, which lacks the constitutive ball domains. On the contrary, a similar peptide in which tyrosine 8 is substituted by the secondary structure-disrupting d-tyrosine stereoisomer does not. This suggests that the stereoisomeric substitution prevents the peptide from adopting a structured conformation when bound to the channel during inactivation. Moreover, characteristic in vitro features of the wild-type ShB peptide such as the marked propensity to adopt an intramolecular beta-hairpin structure when challenged by anionic phospholipid vesicles, a model target mimicking features of the inactivation site in the channel protein, or to insert into their hydrophobic bilayers, are lost in the d-tyrosine-containing peptide, whose behavior is practically identical to that of noninactivating peptide mutants. In the absence of high resolution crystallographic data on the inactivated channel/peptide complex, these latter findings suggest that the structured conformation required for the peptide to promote channel inactivation, as referred to above, is likely to be beta-hairpin.