The Helix-Destabilizing Propensity Scale of d-Amino Acids: The Influence of Side Chain Steric Effects

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 α-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 (x9,10-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 (ΔΔGt) or to the corresponding l-amino acid (ΔΔGd-l). The data show that the helix-destabilizing propensity is highly dependent on the amino acid side chain and not related to the structu...