Although D-amino acids are found in various naturally occurring peptides and frequently used for structure-activity studies, not much is known about their impact on the helical secondary structures formed by L-amino acids. Although several previous accounts reported on the alpha-helical propensity of L-amino acids, the present contribution addresses this subject for the first time for the corresponding D-enantiomers of all of the proteinogenic amino acids. Thus, the helix-destabilizing abilities of the 19 D-amino acids in the host sequence acetyl-KLALKLALxxLKLALKLA-amide (x(9,10)-KLA) were evaluated by means of circular dichroism (CD) spectroscopy, nuclear magnetic resonance, and reversed-phase HPLC. CD and HPLC data enabled calculation of differences in the free energy of helix formation for D-amino acid x relative to glycine (Delta Delta G(t)) or to the corresponding L-amino acid (Delta Delta G(D-L)). The data show that the helix-destabilizing propensity is highly dependent on the amino acid side chain and not related to the structure propensity of the corresponding L-amino acid. In consequence, the D-amino acids can be grouped into (i) weak helix destabilizers (D-His, D-Asp, D-Glu, D-Cys, D-Gln, D-Asn, D-Ser), (ii) medium helix destabilizers (D-Leu, D-Arg, D-Met, D-Lys, D-Trp, D-Ala), and (iii) strong helix destabilizers (D-Thr, D-Phe, D-Val, D-Ile, D-Tyr, D-Pro). Accordingly, the D-isomers of bulky and beta-branched amino acids are the most effective in destabilizing the amphipathic KLA-helix by induction of turn-like structures. Such D-isomers disrupt the secondary structure in a manner similar to that of L-proline.