Effect of Drastic Sequence Alteration and d-Amino Acid Incorporation on the Membrane Binding Behavior of Lytic Peptides

Penetration of the Antimicrobial Peptide Dicynthaurin into Phospholipid Monolayers at the Liquid–Air Interface

This work focuses on the adsorption kinetics of dicynthaurin with lipid monolayers, the effect of peptide adsorption on the structure of the lipid condensed chain lattice, peptide orientation and secondary structure in the adsorbed state. The studies with DPPG as model system revealed strong adsorption and massive incorporation of the peptide into the monolayer. Infrared reflection absorption spectroscopy (IRRAS) experiments showed that the secondary structure of the peptide is maintained upon adsorption. Specular X-ray reflectivity showed the destabilization of the condensed phase of the pure lipid monolayer and revealed a tilted orientation of the long axis of the peptide helix of about 40 degrees from the surface normal. Incorporation of the peptide was found to be pressure dependent, and at high pressure a "squeeze-out" was observed; however, the peptide remained localized to the interface, as suggested by infrared data. These findings were supported by optical fluorescence microscopy measurements which showed the squeeze-out of the peptide on water, but not under physiological conditions. The results suggest that dicynthaurin is able to adsorb to the phosphatidylglycerol-rich inner cytoplasmic membrane of bacteria and alter membrane integrity. To identify and interact with membrane motifs that are characteristic of microbes, but which are absent in eukaryotic cells, might be an intrinsic ability of peptide antibiotics.

De Novo Design of Selective Antibiotic Peptides by Incorporation of Unnatural Amino Acids

The evolution of drug-resistant bacteria is one of the most critical problems facing modern medicine and requires the development of new drugs that exhibit their antibacterial activity via novel mechanisms of action. One potential source of new drugs could be the naturally occurring peptides that exhibit antimicrobial activity via membrane disruption. To develop antimicrobial peptides exhibiting increased potency and selectivity against Gram positive, Gram negative, and Mycobacterium bacteria coupled with reduced hemolytic activity, peptides containing unnatural amino acids have been designed, synthesized, and evaluated. These compounds were designed on the basis of the electrostatic surface potential maps derived from the NMR determined SDS and DPC micelle-bound conformations of (Ala8,13,18)magainin-2 amide. Unnatural amino acids were incorporated into the polypeptide backbone to control the structural and physicochemical properties of the peptides to introduce organism selectivity and potency. The methods and results of this investigation are described below.

Rationale for the design of shortened derivatives of the NK-lysin-derived antimicrobial peptide NK-2 with improved activity against Gram-negative pathogens

The peptide NK-2 is an effective antimicrobial agent with low hemolytic and cytotoxic activities and is thus a promising candidate for clinical applications. It comprises the alpha-helical, cationic core region of porcine NK-lysin a homolog of human granulysin and of amoebapores of pathogenic amoeba. Here we visualized the impact of NK-2 on Escherichia coli by electron microscopy and used NK-2 as a template for sequence variations to improve the peptide stability and activity and to gain insight into the structure/function relationships. We synthesized 18 new peptides and tested their activities on seven Gram-negative and one Gram-positive bacterial strains, human erythrocytes, and HeLa cells. Although all peptides appeared unordered in buffer, those active against bacteria adopted an alpha-helical conformation in membrane-mimetic environments like trifluoroethanol and negatively charged phosphatidylglycerol (PG) liposomes that mimick the cytoplasmic membrane of bacteria. This conformation was not observed in the presence of liposomes consisting of zwitterionic phosphatidylcholine (PC) typical for the human cell plasma membrane. The interaction was paralleled by intercalation of these peptides into PG liposomes as determined by FRET spectroscopy. A comparative analysis between biological activity and the calculated peptide parameters revealed that the decisive factor for a broad spectrum activity is not the peptide overall hydrophobicity or amphipathicity, but the possession of a minimal positive net charge plus a highly amphipathic anchor point of only seven amino acid residues (two helical turns).

Interaction of the Antimicrobial Peptide Dicynthaurin with Membrane Phospholipids at the Air−Liquid Interface

This paper reports the first study on the interaction of the antimicrobial peptide dicynthaurin with 1,2-dipalmitoyl-glycerophosphatidyl-glycerol investigated in monolayers at the air-liquid interface. The influence of the peptide on the two-dimensional phase behavior of the negatively charged lipid was elucidated by means of pressure-area isotherm measurements, fluorescence microscopy, and grazing incidence X-ray diffraction measurements. The pure peptide forms a stable monolayer at the air-liquid interface up to 30 mN/m as shown for both the monomeric and the dimeric cynthaurins. The peptide lipid interaction was monitored in isotherm measurements showing a strong adsorption of the peptide and stabilization at the interface promoted by the lipid monolayer. The X-ray diffraction measurements in agreement with fluorescence microscopy studies showed that the peptide destabilizes the condensed chain lattice, leading to a complete fluidization of the condensed lipid phase on physiological buffer. The adsorption of the peptide to the negatively charged lipid monolayer and the fluidization of the condensed chain lattice suggest a direct link to the peptides' ability to expand the bacterial membrane that would be relevant for the in vivo mode of action.

Optimising histidine rich peptides for efficient DNA delivery in the presence of serum

We recently showed that the antibacterial histidine rich amphipathic peptide LAH4 has significant DNA transfection capabilities in the absence of serum. To further understand the transfection process and to develop the peptides for future applications, we have combined a range of biochemical and biophysical techniques, including fluorescence assisted cell sorting and (2)H solid-state NMR, to characterise the initial binding of the peptide/DNA complexes to the cell surface and the subsequent release of the complexes from the endosome in the presence of serum. Our results show that both primary and secondary peptide structure play important roles in both of these processes. Specifically, we show that an ideal helix length and positioning of the histidine residues should be maintained to obtain optimal resistance to serum effects and release of DNA from the endosome. Inclusion of d-amino acids at the peptide termini does not reduce serum effects however further enrichment of the peptides with histidine residues can enhance transfection efficiency in the presence of serum. The detailed understanding of these two key stages in the transfection process shows that LAH4-L1 and its derivatives are likely to be highly efficient and robust vectors for a range of applications.

WLIP and tolaasin I, lipodepsipeptides from Pseudomonas reactans and Pseudomonas tolaasii, permeabilise model membranes

The activity of the White Line Inducing Principle (WLIP) and tolaasin I, produced by virulent strains of Pseudomonas reactans and Pseudomonas tolaasii, respectively, was comparatively evaluated on lipid membranes. Both lipodepsipeptides were able to induce the release of calcein from large unilamellar vesicles. Their activity was dependent on the toxin concentration and liposome composition and in particular it increased with the sphingomyelin content of the membrane. Studies of dynamic light scattering suggested a detergent-like activity for WLIP at high concentration (> 27 microM). This effect was not detected for tolaasin I at the concentrations tested (< 28 microM). Differences were also observed in lipodepsipeptides secondary structure. In particular, the conformation of the smaller WLIP changed slightly when it passed from the buffer solution to the lipid environment. On the contrary, we observed a valuable increment in the helical content of tolaasin I which was inserted in the membrane core and oriented parallel to the lipid acyl chains.

Inhibition of tumor growth and elimination of multiple metastases in human prostate and breast xenografts by systemic inoculation of a host defense-like lytic peptide

We report on a short host defense-like peptide that targets and arrests the growth of aggressive and hormone-resistant primary human prostate and breast tumors and prevents their experimental and spontaneous metastases, respectively, when systemically inoculated to immunodeficient mice. These effects are correlated with increased necrosis of the tumor cells and a significant decrease in the overall tumor microvessel density, as well as newly formed capillary tubes and prostate-specific antigen secretion (in prostate tumors). Growth inhibition of orthotopic tumors derived from stably transfected highly fluorescent human breast cancer cells and prevention of their naturally occurring metastases were visualized in real time by using noninvasive whole-body optical imaging. The exclusive selectivity of the peptide towards cancer derives from its specific binding to surface phosphatidylserine and the killing of the cancer cells via cytoplasmic membrane depolarization. These data indicate that membrane disruption can provide a therapeutic means of inhibiting tumor growth and preventing metastases of various cancers.

Characterization and cDNA cloning of hinnavin II, a cecropin family antibacterial peptide from the cabbage butterfly, Artogeia rapae

Hinnavins, together with lysozymes, are the main types of antibacterial peptides/proteins previously isolated from the larval haemolymph of the cabbage butterfly, Artogeia rapae as part of the humoral immune response to a bacterial invasion. One of these antibacterial peptides, named hinnavin II, was purified and characterized after cDNA cloning. The purified hinnavin II was more active against Gram negative than against Gram positive bacteria. Hinnavin II also showed a powerful synergistic effect on the inhibition of bacterial growth with purified lysozyme. The cDNA has a total length of 186 bp with a 114 coding region. The deduced protein sequence contains 38 amino acids with a coding capacity of 4142.8 Da. The result of a multiple sequence alignment and phylogenetic analysis with Clustal W indicated that mature hinnavin II showed an approximately 78.9% amino acid sequence identity with cecropin A and originated from a group containing mostly lepidopteran cecropins.

Solution Structures of Human LL-37 Fragments and NMR-Based Identification of a Minimal Membrane-Targeting Antimicrobial and Anticancer Region

To understand the structure and activity relationship of human LL-37, a series of peptide fragments was designed. The N-terminal fragment, LL-37(1-12), was not active, while the C-terminal fragment, LL-37(13-37), killed Escherichia coli, as well as drug-sensitive and drug-resistant cancer cells. A 13-residue core antibacterial and anticancer peptide, corresponding to residues 17-29 of LL-37, was identified based on total correlated spectroscopy by trimming nonessential regions (TOCSY-trim). Because LL-37 acts on bacterial membranes, three-dimensional structures of its fragments were determined in micelles by NMR, including structural refinement by natural abundance 15N and 13C chemical shifts. Aromatic-aromatic interactions in the N-terminal fragment were proposed to be essential for LL-37 aggregation. The LL-37 core peptide adopts a similar structure in the micelles of SDS or dioctanoyl phosphatidylglycerol. This structure is retained in the C-terminal fragment LL-37(13-37) and very likely in intact LL-37 based on peptide-aided signal assignments. The higher antibacterial activity of the LL-37 core peptide than aurein 1.2 was attributed to additional cationic residues. To achieve selective membrane targeting, D-amino acids were incorporated into LL-37(17-32). While the D-peptide showed similar antibacterial activity to the L-diastereomer, it lost toxicity to human cells. Structural analysis revealed hydrophobic defects in the new amphipathic structure of the D-peptide, leading to a much shorter retention time on a reversed-phase HPLC column. It is proposed that hydrophobic defects as a result of incoherent hydrophobic packing provide a structural basis for the improvement in cell selectivity of the LL-37 fragment.

Utilizing ESEEM spectroscopy to locate the position of specific regions of membrane-active peptides within model membranes

Membrane-active peptides participate in many cellular processes, and therefore knowledge of their mode of interaction with phospholipids is essential for understanding their biological function. Here we present a new methodology based on electron spin-echo envelope modulation to probe, at a relatively high resolution, the location of membrane-bound lytic peptides and to study their effect on the water concentration profile of the membrane. As a first example, we determined the location of the N-terminus of two membrane-active amphipathic peptides, the 26-mer bee venom melittin and a de novo designed 15-mer D,L-amino acid amphipathic peptide (5D-L9K6C), both of which are antimicrobial and bind and act similarly on negatively charged membranes. A nitroxide spin label was introduced to the N-terminus of the peptides and measurements were performed either in H2O solutions with deuterated model membranes or in D2O solutions with nondeuterated model membranes. The lipids used were dipalmitoyl phosphatidylcholine (DPPC) and phosphatidylglycerol (PG), (DPPC/PG (7:3 w/w)), egg phosphatidylcholine (PC) and PG (PC/PG (7:3 w/w)), and phosphatidylethanolamine (PE) and PG (PE/PG, 7:3w/w). The modulation induced by the 2H nuclei was determined and compared with a series of controls that produced a reference "ruler". Actual estimated distances were obtained from a quantitative analysis of the modulation depth based on a simple model of an electron spin situated at a certain distance from the bottom of a layer with homogeneously distributed deuterium nuclei. The N-terminus of both peptides was found to be in the solvent layer in both the DPPC/PG and PC/PG membranes. For PE/PG, a further displacement into the solvent was observed. The addition of the peptides was found to change the water distribution in the membrane, making it "flatter" and increasing the penetration depth into the hydrophobic region.

Mechanistic and Functional Studies of the Interaction of a Proline-rich Antimicrobial Peptide with Mammalian Cells

Mammalian antimicrobial peptides provide rapid defense against infection by inactivating pathogens and by influencing the functions of cells involved in defense responses. Although the direct antibacterial properties of these peptides have been widely characterized, their multiple effects on host cells are only beginning to surface. Here we investigated the mechanistic and functional aspects of the interaction of the proline-rich antimicrobial peptide Bac7(1-35) with mammalian cells, as compared with a truncated analog, Bac7(5-35), lacking four critical N-terminal residues (RRIR) of the Bac7(1-35) sequence. By using confocal microscopy and flow cytometry, we showed that although the truncated analog Bac7(5-35) remains on the cell surface, Bac7(1-35) is rapidly taken up into 3T3 and U937 cells through a nontoxic energy- and temperature-dependent process. Cell biology-based assays using selective endocytosis inhibitors and spectroscopic and surface plasmon resonance studies of the interaction of Bac7(1-35) with phosphatidylcholine/cholesterol model membranes collectively suggest the concurrent contribution of macropinocytosis and direct membrane translocation. Structural studies with model membranes indicated that membrane-bound Bac7(5-35) is significantly more aggregated than Bac7(1-35) due to the absence of the N-terminal cationic cluster, thus providing an explanation for hampered cellular internalization of the truncated form. Further investigations aimed to reveal functional implications of intracellular uptake of Bac7(1-35) demonstrated that it correlates with enhanced S phase entry of 3T3 cells, indicating a novel function for this proline-rich peptide.

Basis for selectivity of cationic antimicrobial peptides for bacterial versus mammalian membranes

Novel cationic antimicrobial peptides typified by structures such as KKKKKKAAXAAWAAXAA-NH2, where X = Phe/Trp, and several of their analogues display high activity against a variety of bacteria but exhibit no hemolytic activity even at high dose levels in mammalian erythrocytes. To elucidate their mechanism of action and source of selectivity for bacterial membranes, phospholipid mixtures mimicking the compositions of natural bacterial membranes (containing anionic lipids) and mammalian membranes (containing zwitterionic lipids + cholesterol) were challenged with the peptides. We found that peptides readily inserted into bacterial lipid mixtures, although no insertion was detected in model "mammalian" membranes. The depth of peptide insertion into model bacterial membranes was estimated by Trp fluorescence quenching using doxyl groups variably positioned along the phospholipid acyl chains. Peptide antimicrobial activity generally increased with increasing depth of peptide insertion. The overall results, in conjunction with molecular modeling, support an initial electrostatic interaction step in which bacterial membranes attract and bind peptide dimers onto the bacterial surface, followed by the "sinking" of the hydrophobic core segment to a peptide sequence-dependent depth of approximately 2.5-8 A into the membrane, largely parallel to the membrane surface. Antimicrobial activity was likely enhanced by the fact that the peptide sequences contain AXXXA sequence motifs, which promote their dimerization, and possibly higher oligomerization, as assessed by SDS-polyacrylamide gel analysis and fluorescence resonance energy transfer experiments. The high selectivity of these peptides for nonmammalian membranes, combined with their activity toward a wide spectrum of Gram-negative and Gram-positive bacteria and yeast, while retaining water solubility, represent significant advantages of this class of peptides.

Rational combinatorial design of pore-forming beta-sheet peptides

Exogenous polypeptides that self-assemble on biological membranes into pores are abundant and structurally diverse, functioning as transporters, toxins, ion channels, and antibiotics. A means for designing novel pore-forming sequences would unlock new opportunities for the development and engineering of protein function in membranes. Toward this goal, we designed a 9,604-member rational combinatorial peptide library based on the structural principles of known membrane-spanning beta-sheets. When the library was screened under stringent conditions for sequences with pore-forming activity, a single active motif was found, which is characterized by aromatic residues at the lipid-exposed interfacial positions and basic residues in the pore-lining portion of the sequence. Peptides with this motif assembled on bilayer membranes into beta-sheets and formed transient peptide/lipid pores of approximately 1-nm diameter. The mechanism of action is very similar to that of natural, pore-forming peptides. These methods provide a powerful means for selecting and engineering novel pore-forming sequences and will open prospects for designing peptide antibiotics, biosensors, and new membrane protein structures.

Helix-helix interconversion rates of short13C-labeled helical peptides as measured by dynamic NMR spectroscopy

The rates at which a peptide hexamer and a peptide octamer interconvert between left- and right-handed helical forms in CD2Cl2 solution have been characterized by 13C dynamic NMR (DNMR) spectroscopy. The peptide esters studied are Fmoc-(Aib)n-OtBu (n = 6 and 8), where Fmoc is 9-fluorenylmethyoxycarbonyl and Aib is the strongly helix-forming residue alpha-aminoisobutyric acid. Because the Aib residue is itself achiral, homooligomers of this residue form a 50/50 mixture of enantiomeric 3(10)-helices in solution. It has been demonstrated (R.-P. Hummel, C. Toniolo, and G. Jung, Angewandte Chemie International Edition, 1987, Vol. 26, pp. 1150-1152) that oligomers of Aib interconvert on the millisecond timescale. We have performed lineshape analysis of 13C-NMR spectra collected for our peptides enriched with 13C at a single residue. Rate constants for the octamer range from 6 s(-1) at 196 K to about 56,500 s(-1) at 320 K. At all temperatures, the hexamer interconverts about three times faster than the octamer. Eyring plots of the data reveal experimentally indistinguishable DeltaH++ values for the hexamer and octamer of 37.8 +/- 0.6 and 37.6 +/- 0.4 kJ mol(-1) respectively. The difference in the rates of interconversion is dictated by entropic factors. The hexamer and octamer exhibit negative DeltaS++ values of -29.0(-1) +/- 2.5 and -37.3 +/- 1.7 J K(-1) mol(-1), respectively. A mechanism for the helix-helix interconversion is proposed. and calculated DeltaG++ values are compared to the estimate for a decamer undergoing a helix-helix interconversion.

Survey of the year 2004 commercial optical biosensor literature

The year 2004 represents a milestone for the biosensor research community: in this year, over 1000 articles were published describing experiments performed using commercially available systems. The 1038 papers we found represent an approximately 10% increase over the past year and demonstrate that the implementation of biosensors continues to expand at a healthy pace. We evaluated the data presented in each paper and compiled a 'top 10' list. These 10 articles, which we recommend every biosensor user reads, describe well-performed kinetic, equilibrium and qualitative/screening studies, provide comparisons between binding parameters obtained from different biosensor users, as well as from biosensor- and solution-based interaction analyses, and summarize the cutting-edge applications of the technology. We also re-iterate some of the experimental pitfalls that lead to sub-optimal data and over-interpreted results. We are hopeful that the biosensor community, by applying the hints we outline, will obtain data on a par with that presented in the 10 spotlighted articles. This will ensure that the scientific community at large can be confident in the data we report from optical biosensors.

Antimicrobial peptides: New candidates in the fight against bacterial infections

Antimicrobial peptides (AMPs) of innate origin are agents of the most ancient form of defense systems. They can be found in a wide variety of species ranging from bacteria through insects to humans. Through the course of evolution, host organisms developed arsenals of AMPs that protect them against a large variety of invading pathogens including both Gram-negative and Gram-positive bacteria. At a time of increasing bacterial resistance, AMPs have been the focus of investigation in a number of laboratories worldwide. Although recent studies show that some of the peptides are likely to have intracellular targets, the vast majority of AMPs appear to act by permeabilization of the bacterial cell membrane. Their activity and selectivity are governed by the physicochemical parameters of the peptide chains as well as the properties of the membrane system itself. In this review, we will summarize some of the recent developments that provide us with a better understanding of the mode of action of this unique family of antibacterial agents. Particular attention will be given to the determinants of AMP-lipid bilayer interactions as well as to the different pore formation mechanisms. The emphasis will be on linear AMPs but representatives of cysteine-bridged AMPs will also be discussed.

Two-step mechanism of binding of apolipoprotein E to heparin: implications for the kinetics of apolipoprotein E-heparan sulfate proteoglycan complex formation on cell surfaces

The interaction of apolipoprotein E (apoE) with cell-surface heparan sulfate proteoglycans is an important step in the uptake of lipoprotein remnants by the liver. ApoE interacts predominantly with heparin through the N-terminal binding site spanning the residues around 136-150. In this work, surface plasmon resonance analysis was employed to investigate how amphipathic alpha-helix properties and basic residue organization in this region modulate binding of apoE to heparin. The apoE/heparin interaction involves a two-step process; apoE initially binds to heparin with fast association and dissociation rates, followed by a step exhibiting much slower kinetics. Circular dichroism and surface plasmon resonance experiments using a disulfide-linked mutant, in which opening of the N-terminal helix bundle was prevented, demonstrated that there is no major secondary or tertiary structural change in apoE upon heparin binding. Mutations of Lys-146, a key residue for the heparin interaction, greatly reduced the favorable free energy of binding of the first step without affecting the second step, suggesting that electrostatic interaction is involved in the first binding step. Although lipid-free apoE2 tended to bind less than apoE3 and apoE4, there were no significant differences in rate and equilibrium constants of binding among the apoE isoforms in the lipidated state. Discoidal apoE3-phospholipid complexes using a substitution mutant (K143R/K146R) showed similar binding affinity to wild type apoE3, indicating that basic residue specificity is not required for the effective binding of apoE to heparin, unlike its binding to the low density lipoprotein receptor. In addition, disruption of the alpha-helix structure in the apoE heparin binding region led to an increased favorable free energy of binding in the second step, suggesting that hydrophobic interactions contribute to the second binding step. Based on these results, it seems that cell-surface heparan sulfate proteoglycan localizes apoE-enriched remnant lipoproteins to the vicinity of receptors by fast association and dissociation.

Regulation of PutA-membrane associations by flavin adenine dinucleotide reduction

Proline utilization A (PutA) from Escherichia coli is a multifunctional flavoprotein that is both a transcriptional repressor of the proline utilization (put) genes and a membrane-associated enzyme which catalyzes the 4-electron oxidation of proline to glutamate. Previously, proline was shown to induce PutA-membrane binding and alter the intracellular location and function of PutA. To distinguish the roles of substrate binding and FAD reduction in the mechanism of how PutA changes from a DNA-binding protein to a membrane-bound enzyme, the kinetic parameters of PutA-membrane binding were measured under different conditions using model lipid bilayers and surface plasmon resonance (SPR). The effects of proline, FAD reduction, and proline analogues on PutA-membrane associations were determined. Oxidized PutA shows no binding to E. coli polar lipid vesicles. In contrast, proline and sodium dithionite induce tight binding of PutA to the lipid bilayer with indistinguishable kinetic parameters and an estimated dissociation constant (K(D)) of <0.01 nM (pH 7.4) for the reduced PutA-lipid complex. Proline analogues such as L-THFA and DL-P5C also stimulate PutA binding to E. coli polar lipid vesicles with K(D) values ranging from approximately 3.6 to 34 nM (pH 7.4) for the PutA-lipid complex. The greater PutA-membrane binding affinity (>300-fold) generated by FAD reduction relative to the nonreducing ligands demonstrates that FAD reduction controls PutA-membrane associations. On the basis of SPR kinetic analysis with differently charged lipid bilayers, the driving force for PutA-membrane binding is primarily hydrophobic. In the SPR experiments membrane-bound PutA did not bind put control DNA, confirming that the membrane-binding and DNA-binding activities of PutA are mutually exclusive. A model for the regulation of PutA is described in which the overall translocation of PutA from the cytoplasm to the membrane is driven by FAD reduction and the subsequent energy difference ( approximately 24 kJ/mol) between PutA-membrane and PutA-DNA binding.

Suppression of human prostate tumor growth in mice by a cytolytic D-, L-amino Acid Peptide: membrane lysis, increased necrosis, and inhibition of prostate-specific antigen secretion

Gene-encoded host defense peptides are used as part of the innate immunity, and many of them act by directly lysing the cell membrane of the pathogen. A few of these peptides showed anticancer activity in vitro but could not be used in vivo because of their inactivation by serum. We designed a 15-amino acid peptide, composed of D- and L-amino acids (diastereomer), which targets both androgen-independent and androgen-dependent human prostate carcinoma cell lines (CL1, 22RV1, and LNCaP). Most importantly, we observed a complete arrest of growth in CL1 and 22RV1 xenografts treated intratumorally with the diastereomer. This was also accompanied by a lowering of prostate-specific antigen serum levels secreted by the 22RV1 xenograft. Furthermore, the diastereomer synergized with conventional chemotherapeutics. In contrast, the parental all l-amino acids peptide was highly active only in vitro and could not discriminate between tumor and nontumor cells. Fluorescent confocal microscopy, histopathologic examination, and cell permeability studies (depolarization of transmembrane potential and release of an encapsulated dye) suggest a necrotic mechanism of killing, after a threshold concentration of peptide has been reached. Its destructive killing effect and the simple sequence of the diastereomer make it an attractive chemotherapeutic candidate possessing a new mode of action, with potential to be developed additionally for the treatment of prostate carcinoma.