# Roles of D-Amino Acids on the Bioactivity of Host Defense Peptides
## 4 Mechanism of Membrane Destabilization
Toroidal pores may also be formed; (**C**) Partial carpet mechanism, the primary interaction mechanism of all-L host defense peptides (HDPs). This mechanism can occur when disrupting both negative and neutral membranes. Both the carpet and membrane pore disruption mechanisms occur. The partial carpet mechanism is also employed by strong hemolytic peptides, such as paradxin, during the disruption of negative PE/PG membranes; (**D**) Barrel-stave mechanism, the mechanism employed by strong hemolytic HDPs, such as paradxin, for the disruption of zwitterionic PC/Cho membranes. The peptides insert deep into the hydrophobic core of the membrane and form well-defined trans-membrane ion channel, with the peptide hydrophobic face facing the membrane hydrophobic core and the polar face facing inward. Adapted with permission from [49]. Copyright (2004) American Chemical Society. The findings from [PERSON] and [PERSON] [49] provides a credible, although not necessarily complete explanation as to why diastereomeric AMPs frequently exhibit much better selectivity [44; 52] since the lower affinity of the diastereomers toward the neutral mammalian membranes reduces the amount of diastereomers that are bound to healthy mammalian cells and thus less likely to lyse them. Moreover, L-AA enantiomeric peptides were observed to insert into the PC/Cho bilayers, whereas the diastereomers were surface localized [49; 50], thereby providing an explanation on why the inner layer could contribute to the binding of all-L peptides. The outer surface of mammalian membranes is electrostatically neutral; thus, for the L-AA peptide to interact with the inner layer, hydrophobic interactions must dominate once the peptide is bound. Specifically, these interactions are required for the peptide to penetrate into the hydrophobic core of the membrane [22; 49]. This finding indicates that the activity of diastereomers depends almost exclusively on electrostatic interactions, whereas the all-L peptides were also capable of a considerable level of hydrophobic interaction for the binding and subsequent insertion into mammalian membranes [49]. Findings from [PERSON] and [PERSON] [49] as well as later works [18; 53] had indicated that high hydrophobicity, helicity and amphiphicity were correlated with strong hemolytic activity and cytotoxicity against mammalian cells and therefore exert low selectivity. Furthermore, it should also be noted that diastereomers have lower hydrophobicity and less stable helical structures despite having the same amino acid sequence as their all-L counterparts, which prevents hydrophobic interactions with mammalian membranes and thus imbues the diastereomers with high selectivity. The incorporation of d-AAs thus provides a method of adjusting the parameters that affect selectivity. Additionally, because diastereomers cannot insert into the hydrophobic core of the PC/Cho bilayer, they cannot form transmembrane pores on mammalian cells and must therefore rely on other mechanisms (e.g., carpet mechanism) for membrane destabilization [50]. FTIR analysis of the amphiphicity [16] of the same peptides as studied by [PERSON] and [PERSON] [49] showed that the all-L peptides had an \(\alpha\)-helical structure once bound to PC/Cho membranes. The diastereomers were also helical but had a distorted \(3_{10}\)-helical structure that prevented the formation of a well-defined hydrophobic face and hindering its binding to zwitterionic membranes, providing an explanation on why the hydrophobicity of the diastereomers was lower. A similar observation was made by previous works [50; 54], which showed that the introduction of amino acids with opposite chirality in the hydrophobic face of amphiphic secondary structures breaks the secondary structures, thus reducing the hydrophobic interaction necessary for interacting with mammalian cells. The behavior of both diastereomers and all-L peptides were markedly different on negative PE/PG membranes.