Antimicrobial Peptides with Enhanced Salt Resistance and Antiendotoxin Properties.

A strategy was described to design antimicrobial peptides (AMPs) with enhanced salt resistance and antiendotoxin activities by linking two helical AMPs with the Ala-Gly-Pro (AGP) hinge. Among the designed peptides, KR12AGPWR6 demonstrated the best antimicrobial activities even in high salt conditions (NaCl ~300 mM) and possessed the strongest antiendotoxin activities. These activities may be related to hydrophobicity, membrane-permeability, and α-helical content of the peptide. Amino acids of the C-terminal helices were found to affect the peptide-induced permeabilization of LUVs, the α-helicity of the designed peptides under various LUVs, and the LPS aggregation and size alternation. A possible model was proposed to explain the mechanism of LPS neutralization by the designed peptides. These findings could provide a new approach for designing AMPs with enhanced salt resistance and antiendotoxin activities for potential therapeutic applications.

The Interactions between the Antimicrobial Peptide P-113 and Living Candida albicans Cells Shed Light on Mechanisms of Antifungal Activity and Resistance.

In the absence of proper immunity, such as in the case of acquired immune deficiency syndrome (AIDS) patients, Candida albicans, the most common human fungal pathogen, may cause mucosal and even life-threatening systemic infections. P-113 (AKRHHGYKRKFH), an antimicrobial peptide (AMP) derived from the human salivary protein histatin 5, shows good safety and efficacy profiles in gingivitis and human immunodeficiency virus (HIV) patients with oral candidiasis. However, little is known about how P-113 interacts with Candida albicans or its degradation by Candida-secreted proteases that contribute to the fungi's resistance. Here, we use solution nuclear magnetic resonance (NMR) methods to elucidate the molecular mechanism of interactions between P-113 and living Candida albicans cells. Furthermore, we found that proteolytic cleavage of the C-terminus prevents the entry of P-113 into cells and that increasing the hydrophobicity of the peptide can significantly increase its antifungal activity. These results could help in the design of novel antimicrobial peptides that have enhanced stability in vivo and that can have potential therapeutic applications.

A novel CXCL8 analog is effective in inhibiting the growth via cell cycle arrest and attenuating invasion of Lewis lung carcinoma.

PURPOSE: Lung cancer and other solid tumors contain not only tumor cells but various types of stromal cells, such as fibroblasts and endothelial cells. In addition, tumors are infiltrated by inflammatory cells (neutrophils, macrophages, and lymphocytes). Tumor cells, stromal cells, and the tumor-associated leukocytes are responsible for the production of chemokines inside the tumor and the maintenance of systemic circulating chemokine levels. CXCL8 and its receptors, CXCR1 and CXCR2, were found to play important roles in tumor proliferation, migration, survival, and growth. We have developed a novel ELR-CXC chemokine antagonist CXCL8-IP10 based on the structure of CXCL8 and IP10. PATIENTS AND METHODS: We assessed the anticancer efficacies of the blockade of CXCL8-CXCR1/2 axis in the Lewis lung carcinoma (LL/2) model using CXCL8-IP10. RESULTS: We found that CXCL8-IP10 markedly reduced LL/2 cell anchorage-independent growth and invasion. Moreover, we demonstrated that CXCL8-IP10 could significantly suppress tumor growth and improve survival rate as well as lifespan of C57BL/6 mice inoculated with LL/2 cells. CONCLUSION: Our results suggest that ELR-CXC chemokine antagonism would potentially be a useful therapeutic approach in patients with lung cancer.

Characteristic genes in THP­1 derived macrophages infected with Mycobacterium tuberculosis H37Rv strain identified by integrating bioinformatics methods.

Mycobacterium tuberculosis (M. tb) is a highly successful pathogen that has co­existed with humans for 1,000's of years. As the cornerstone of the immune system, macrophages are a key part of innate immunity. They ingest and degrade foreign substances including aging cells and microorganisms, coordinate the inflammatory process, and are the first line of defense against M. tb infection. Recent advances in cellular mycobacteriology have indicated that M. tb uses an remarkably complex strategy to disrupt macrophage function, in order to counteract the antimicrobial mechanisms of the innate and adaptive immune responses, thereby achieving immune escape. With the popularity of microarray technology, a variety of public platforms have provided a variety of gene expression data associated with physiological and disease conditions. Meta­analysis can systematically and quantitatively analyze multiple independent data concerning the same disease, greatly improving the statistical significance and credibility of the gene expression data analysis performed. In the present study, 6 microarray expression datasets of human acute monocytic leukemia THP­1 cell line infected by M. tb H37Rv strain were collected from the GEO database. A total of 4 high­quality datasets were identified using meta­analysis methods in R language, and 306 differentially expressed genes with statistical significance were obtained. Then, a protein­protein interaction (PPI) network of these differentially expressed genes was constructed on the Search Tool for the Retrieval of Interacting Genes/Proteins Database online tool and visualized by Cytoscape v. 3.6.1 software. Using CentiScape and MCODE plugin in the Cytoscape software to mine the functional modules associated with M. tb infection process, 32 characteristic genes were identified. Gene ontology and Kyoto Encyclopedia of Genes and Genomes analysis was performed on the 32 characteristic genes, and it was demonstrated that these genes were primarily associated with the type I interferon (IFN) pathway. In the established model of THP­1­derived macrophages infected by M. tb, the actual differential expression levels of IFN­stimulated gene 15 (ISG15), 2'­5­oligoadenylate synthetase like (OASL), IFN regulatory factor 7 (IRF7) and DExD/H­box helicase 58 (DDX58), the first 4 genes of the 32 characteristic genes, were verified by reverse transcription quantitative polymerase chain reaction. The results were consistent with the results of microarray analysis. The association between ISG15, OASL and IRF7 and TB infection was also verified. Although a number of studies have identified that the type I IFN pathway may assist M. tb to achieve immune escape, the present study used a meta­analysis of microarray data and PPI network analysis to examine some of the novel genes identified in the IFN pathway. The results furthered the understanding of the molecular mechanisms of the TB immune response and provided a novel perspective for future therapeutic goals.

Dependence on size and shape of non-nature amino acids in the enhancement of lipopolysaccharide (LPS) neutralizing activities of antimicrobial peptides.

HYPOTHESIS: Release of lipopolysaccharides (LPS) from bacteria into bloodstream may cause serious unwanted stimulation of the host immune system. P-113 is a clinically active histidine-rich antimicrobial peptide. Nal-P-113, a ß-naphthylalanine-substituted P-113, is salt-resistant but has limited LPS neutralizing activity. We suspected the size and shape of the non-natural bulky amino acid may affect its LPS neutralizing activity. Herein, antimicrobial, LPS neutralizing, and antiproteolytic effects of phenylalanine- (Phe-P-113), ß-naphthylalanine- (Nal-P-113), ß-diphenylalanine- (Dip-P-113), and ß-(4,4'-biphenyl)alanine- (Bip-P-113) substituted P-113 were studied. EXPERIMENTS: Structure-activity relationships of P-113, Phe-P-113, Nal-P-113, Dip-P-113, and Bip-P-113 were evaluated using antimicrobial activity assays, serum proteolytic assays, peptide-induced permeabilization of large unilamellar vesicles, zeta potential measurements, dynamic light scattering measurement of LPS aggregation, and Limulus amebocyte lysate assays for measuring LPS neutralization. In vitro and in vivo LPS neutralizing activities were further confirmed by LPS-induced inflammation inhibition in an endotoxemia mouse model. FINDINGS: Bip-P-113 and Dip-P-113 had the longest and widest non-nature amino acids, respectively. Bip-P-113 enhanced salt resistance, serum proteolytic stability, peptide-induced permeabilization, zeta potential measurements, LPS aggregation, and in vitro and in vivo LPS neutralizing activities. These results could help design novel antimicrobial peptides that have enhanced stability in vivo and that can have potential therapeutic applications.

A novel CXCL8-IP10 hybrid protein is effective in blocking pulmonary pathology in a mouse model of Klebsiella pneumoniae infection.

Klebsiella pneumoniae (K. pneumoniae) is a hospital-acquired infectious agent that causes a range of diseases. Herein we have developed a novel CXCL8-IP10 hybrid protein and evaluated its efficacy in an animal model of K. pneumoniae infection. Neutrophil chemotaxis data revealed that CXCL8-IP10 could inhibit human neutrophil chemotactic responses induced by the ELR-CXC chemokine CXCL8. To evaluate the effect of CXCL8-IP10 on K. pneumoniae infection, C57BL/6 mice were challenged with K. pneumoniae followed by treatment with CXCL8-IP10 (500 µg/kg, i.p.), or dexamethasone (0.8 mg/kg, s.c.), or ceftazidime (200 mg/kg, s.c.) individually. CXCL8-IP10, dexamethasone or ceftazidime markedly inhibit Klebsiella-induced pulmonary inflammation as assessed by gross examination and histopathology. Moreover, the chemotactic responses of neutrophils was blocked by CXCL8-IP10 rather than dexamethasone or ceftazidime. Furthermore, the levels of inflammatory factors IL-1ß, IFN-γ and TNF-α were decreased after CXCL8-IP10, dexamethasone or ceftazidime treatment. Together, these results suggest that CXCL8-IP10 may provide a novel strategy for treating K. pneumoniae infection.

Solar spectrum matching with white OLED and monochromatic LEDs.

In this paper, the solar spectrum matching in the visible range of 380-780 nm with white organic light-emitting diode (OLED) and monochromatic light-emitting diodes (LEDs) is investigated. The correlation index (R2) is used to evaluate the difference between the matching spectrum and the solar spectrum. The optimal combination is obtained by the least squares method. We also perform subtraction experiments to find the optimal combination. We utilize a common white OLED device design and just change the species of monochromatic LEDs used. We report and evaluate different degrees of matching effects. The results show that the correlation index of the best combination can reach 94.09% with white OLED and 36 monochromatic LEDs. We define three levels of performance as an evaluation system in accordance with the matching effect. The level is excellent with an R2 above 90.14%. The good level is from 86.65% to 58.28%. From 42.08% to 33.06% is the reasonable level. Compared with other methods, using white OLED combined with monochromatic LEDs achieves the best solar spectrum matching effect. The results can be applied to different requirements of engineering practice.

High Level Expression and Purification of the Clinically Active Antimicrobial Peptide P-113 in Escherichia coli.

P-113, which was originally derived from the human saliva protein histatin 5, is a histidine-rich antimicrobial peptide with the sequence AKRHHGYKRKFH. P-113 is currently undergoing phase II clinical trial as a pharmaceutical agent to fight against fungal infections in HIV patients with oral candidiasis. Previously, we developed a new procedure for the high-yield expression and purification of hG31P, an analogue and antagonist of human CXCL8. Moreover, we have successfully removed lipopolysaccharide (LPS, endotoxin) associated with hG31P in the expression with Escherichia coli. In this paper, we have used hG31P as a novel fusion protein for the expression and purification of P-113. The purity of the expressed P-113 is more than 95% and the yield is 4 mg P-113 per liter of E. coli cell culture in Luria-Bertani (LB) medium. The antimicrobial activity of the purified P-113 was tested. Furthermore, we used circular dichroism (CD) and nuclear magnetic resonance (NMR) spectroscopy to study the structural properties of P-113. Our results indicate that using hG31P as a fusion protein to obtain large quantities of P-113 is feasible and is easy to scale up for commercial production. An effective way of producing enough P-113 for future clinical studies is evident in this study.

Preventive effects of the novel antimicrobial peptide Nal-P-113 in a rat Periodontitis model by limiting the growth of Porphyromonas gingivalis and modulating IL-1ß and TNF-α production.

BACKGROUND: P-113 (AKRHHGYKRKFH-NH2) is a 12-amino-acid histidine-rich peptide derived from histatin 5 that is highly degradable in high salt concentrations and biological fluids such as serum, plasma and saliva. Nal-P-113, a novel antimicrobial peptide whose histidine residues are replaced by the bulky amino acids ß-naphthylalanine, causes the antimicrobial peptide to retain its bactericidal activity even in physiological environments. This study evaluated the effect of the novel antimicrobial peptide Nal-P-113 in a rat periodontitis model and the mechanisms of action of Nal-P-113 for suppressing periodontitis. METHODS: Periodontitis was induced in mandibular first molars in rats receiving a ligature and infected with Porphyromonas gingivalis. Animals were randomly divided into six groups: a, P. gingivalis W83 alone; b, P. gingivalis W83 with 6.25 µg/mL of Nal-P-113; c, P. gingivalis W83 with 25 µg/mL of Nal-P-113; d, P. gingivalis W83 with 100 µg/mL of Nal-P-113; e, P. gingivalis W83 with 400 µg/mL of Nal-P-113; and f, control without P. gingivalis W83 or Nal-P-113. Morphometric analysis was used to evaluate alveolar bone loss. Microbiological assessment of the presence of Porphyromonas gingivalis and total bacteria was performed using absolute quantitative real-time PCR and scanning electron microscopy. Gingival tissue was collected for western blot and immunohistochemical assays of IL-1ß and TNF-α levels. RESULTS: Alveolar bone loss was inhibited by 100 µg/mL or 400 µg/mL of Nal-P-113 compared to the control group (P < 0.05). Lower amounts of P. gingivalis and total bacteria were found in groups d and e compared with group a (P < 0.05). A decrease in the levels of IL-1ß and TNF-α was detected in group d and group e compared to the control group (P < 0.05). The amount of P. gingivalis was positively correlated with IL-1ß and TNF-α expression in periodontal tissue (P < 0.05). CONCLUSIONS: Nal-P-113 exhibited protective effects on Porphyromonas gingivalis-induced periodontitis in rats by limiting the amount of bacteria and modulating IL-1ß and TNF-α production. The use of Nal-P-113 in vivo might serve as a beneficial preventive or therapeutic approach for periodontitis.

Effects of K11R and G31P Mutations on the Structure and Biological Activities of CXCL8: Solution Structure of Human CXCL8(3-72)K11R/G31P.

The ELR-CXC chemokines are important to neutrophil inflammation in many acute and chronic diseases. Among them, CXCL8 (interleukin-8, IL-8), the expression levels of which are elevated in many inflammatory diseases, binds to both the CXCR1 and CXCR2 receptors with high affinity. Recently, an analogue of human CXCL8, CXCL8(3-72)K11R/G31P (hG31P) has been developed. It has been demonstrated that hG31P is a high affinity antagonist for both the CXCR1 and CXCR2. Herein, we have determined the solution structure and the CXCR1 N-terminal peptide binding sites of hG31P by NMR spectroscopy. We have found that the displacement within the tertiary structure of the 30 s loop and the N-terminal region and more specifically change of the loop conformation (especially H33), of hG31P may affect its binding to the CXCR1 receptor and thereby inhibit human neutrophil chemotactic responses induced by ELR-CXC chemokines. Our results provide a structural basis for future clinical investigations of this CXCR1/CXCR2 receptor antagonist and for the further development of CXCL8 based antagonists.

Role of ß-naphthylalanine end-tags in the enhancement of antiendotoxin activities: Solution structure of the antimicrobial peptide S1-Nal-Nal in complex with lipopolysaccharide.

Lipopolysaccharide (LPS, endotoxin) is the major component of Gram-negative bacterial outer surface membrane. LPS released from bacteria into bloodstream during infection may cause serious unwanted stimulation of host's immune system and lead to septic shock of the patient. Recently, we have developed a strategy to increase salt resistance and LPS neutralization of short antimicrobial peptides by adding ß-naphthylalanine end-tags to their termini. Herein, correlations between membrane immersion depth, orientation, and antiendotoxin activities of the antimicrobial peptides S1 and S1-Nal-Nal have been investigated via solution structure, paramagnetic resonance enhancement, and saturation transfer difference NMR studies. Unlike the parent peptide S1, S1-Nal-Nal rotated its two terminal ß-naphthylalanine residues into the hydrophobic lipid A motif of LPS micelles. The LPS-induced inflammation may then be prohibited by the blocked lipid A motif.

Molecular pathways underlying inhibitory effect of antimicrobial peptide Nal-P-113 on bacteria biofilms formation of Porphyromonas gingivalis W83 by DNA microarray.

BACKGROUND: Wound-related infection remains a major challenge for health professionals. One disadvantage in conventional antibiotics is their inability to penetrate biofilms, the main protective strategy for bacteria to evade irradiation. Previously, we have shown that synthetic antimicrobial peptides could inhibit bacterial biofilms formation. RESULTS: In this study, we first delineated how Nal-P-113, a novel antimicrobial peptide, exerted its inhibitory effects on Porphyromonas gingivalis W83 biofilms formation at a low concentration. Secondly, we performed gene expression profiling and validated that Nal-P-113 at a low dose significantly down-regulated genes related to mobile and extrachromosomal element functions, transport and binding proteins in Porphyromonas gingivalis W83. CONCLUSIONS: These findings suggest that Nal-P-113 at low dose is sufficient to inhibit the formation of biofilms although Porphyromonas gingivalis W83 may maintain its survival in the oral cavity. The newly discovered molecular pathways may add the knowledge of developing a new strategy to target bacterial infections in combination with current first-line treatment in periodontitis.

Efficacy of a novel antimicrobial peptide against periodontal pathogens in both planktonic and polymicrobial biofilm states.

Streptococcus gordonii, Fusobacterium nucleatum and Porphyromonas gingivalis represent the early, middle and late colonizers of the bacterial accretion in dental plaque biofilms. These sessile communities constitute a protected mode of growth that promotes survival in a hostile environment. This study describes a novel and unrecognized role for a synthetic cationic antimicrobial peptide, Nal-P-113, which inhibits and kills periodontal bacteria in planktonic state, inhibits the formation of biofilms and eradicates polymicrobial biofilms. Nal-P-113 is also stable in saliva, serum and saline solution. At a concentration less than 320 µg/mL which is harmless to normal oral cells, Nal-P-113 can kill bacteria in planktonic state. At a concentration of antimicrobial peptide Nal-P-113 (1280 µg/mL) which only causes slight damages to normal oral cells is needed to kill bacteria in biofilm state. It is worth mentioning that this concentration of Nal-P-113 is harmless to rat oral mucosa compared to chlorhexidine. The mechanism of Nal-P-113 inhibiting and killing periodontal bacteria might rely on the abilities to permeabilize and/or to form pores within the cytoplasmic membranes, thus causes the death of bacteria. Here, we provided a novel and stable antimicrobial peptide with very low mammalian cytotoxicity, which can inhibit and kill periodontal bacteria in both planktonic and polymicrobial biofilm states. STATEMENT OF SIGNIFICANCE: Nal-P-113 is a potent antimicrobial peptide with strong antimicrobial ability, improved deficiency compared with other antibacterial peptides, and remains stable in phosphate buffered saline, saliva, brain-heart infusion medium and bovine calf serum. Nal-P-113 exhibits a broad spectrum of bacteriocidal activity with excellent eradicating capability on oral pathogens and the respective biofilms. In this study, we used propidium iodide staining, scanning electron microscopy and transmission electron microscopy to confirm that Nal-P-113 can perforate plasmalemma thereby resulting in the death of oral pathogens and disintegrate the respective biofilms. Nal-P-113 also showed effective anti-plaque biofilms and cytotoxicity in the rat periodontitis model. No adverse effects can be observed on the gingivomucosa tissue. In short, the antimicrobial peptide Nal-P-113 presented to be an effective yet have low mammalian cytotoxicity agent with potential application in the clinic. This study provides a proof of concept in applying antimicrobial peptides in the clinical perspective.

Ultrashort Antimicrobial Peptides with Antiendotoxin Properties.

Release of lipopolysaccharide (LPS) (endotoxin) from bacteria into the bloodstream may cause serious unwanted stimulation of the host immune system. Some but not all antimicrobial peptides can neutralize LPS-stimulated proinflammatory responses. Salt resistance and serum stability of short antimicrobial peptides can be boosted by adding ß-naphthylalanine to their termini. Herein, significant antiendotoxin effects were observed in vitro and in vivo with the ß-naphthylalanine end-tagged variants of the short antimicrobial peptides S1 and KWWK.

Novel antimicrobial peptides with high anticancer activity and selectivity.

We describe a strategy to boost anticancer activity and reduce normal cell toxicity of short antimicrobial peptides by adding positive charge amino acids and non-nature bulky amino acid ß-naphthylalanine residues to their termini. Among the designed peptides, K4R2-Nal2-S1 displayed better salt resistance and less toxicity to hRBCs and human fibroblast than Nal2-S1 and K6-Nal2-S1. Fluorescence microscopic studies indicated that the FITC-labeled K4R2-Nal2-S1 preferentially binds cancer cells and causes apoptotic cell death. Moreover, a significant inhibition in human lung tumor growth was observed in the xenograft mice treated with K4R2-Nal2-S1. Our strategy provides new opportunities in the development of highly effective and selective antimicrobial and anticancer peptide-based therapeutics.

Correlations between membrane immersion depth, orientation, and salt-resistance of tryptophan-rich antimicrobial peptides.

The efficacies of many antimicrobial peptides are greatly reduced in the presence of high salt concentrations therefore limiting their development as pharmaceutical compounds. PEM-2-W5K/A9W, a short Trp-rich antimicrobial peptide developed based on the structural studies of PEM-2, has been shown to be highly active against various bacterial strains with less hemolytic activity. Here, correlations between membrane immersion depth, orientation, and salt-resistance of PEM-2 and PEM-2-W5K/A9W have been investigated via solution structure and paramagnetic resonance enhancement studies. The antimicrobial activities of PEM-2-W5K/A9W and PEM-2 against various bacterial and fungal strains including multidrug-resistant and clinical isolates under high salt conditions were tested. The activities of the salt-sensitive peptide PEM-2 were reduced and diminished at high salt concentrations, whereas the activities of PEM-2-W5K/A9W were less affected. The results indicated that the strong salt-resistance of PEM-2-W5K/A9W may arise from the peptide positioning itself deeply into microbial cell membranes and thus able to disrupt the membranes more efficiently.

Boosting salt resistance of short antimicrobial peptides.

The efficacies of many antimicrobial peptides are greatly reduced under high salt concentrations, therefore limiting their use as pharmaceutical agents. Here, we describe a strategy to boost salt resistance and serum stability of short antimicrobial peptides by adding the nonnatural bulky amino acid ß-naphthylalanine to their termini. The activities of the short salt-sensitive tryptophan-rich peptide S1 were diminished at high salt concentrations, whereas the activities of its ß-naphthylalanine end-tagged variants were less affected.

Effect of the antimicrobial peptide D-Nal-Pac-525 on the growth of Streptococcus mutans and its biofilm formation.

Streptococcus mutans is the primary etiological agent of dental caries. The antimicrobial peptide D-Nal-Pac-525 was designed by replacing the tryptophans of the Trp-rich peptide Pac-525 with D-ß-naphthyalanines. To assess the effect of D-Nal-Pac-525 on cariogenic bacteria, the activity of D-Nal-Pac-525 on the growth of S. mutans and its biofilm formation were examined. D-Nal- Pac-525 showed robust antimicrobial activity against S. mutans (minimum inhibitory concentration of 4 µg/ml). Using scanning electron microscopy and transmission electron microscopy, it was shown that D-Nal-Pac-525 caused morphological changes and damaged the cell membrane of S. mutans. D-Nal-Pac-525 inhibited biofilm formation of S. mutans at 2 µg/ml. The results of this study suggest that D-Nal-Pac-525 has great potential for clinical application as a dental caries-preventing agent.

Easy strategy to increase salt resistance of antimicrobial peptides.

The efficacies of many antimicrobial peptides are greatly reduced under high salt concentrations, limiting their development as pharmaceutical compounds. Here, we describe an easy strategy to increase salt resistance of antimicrobial peptides by replacing tryptophan or histidine residues with the bulky amino acids ß-naphthylalanine and ß-(4,4'-biphenyl)alanine. The activities of the salt-sensitive peptide P-113 were diminished at high salt concentrations, whereas the activities of its ß-naphthylalanine and ß-(4,4'-biphenyl)alanine-substituted variant were less affected.

Rational design of tryptophan-rich antimicrobial peptides with enhanced antimicrobial activities and specificities.

Trp-rich antimicrobial peptides play important roles in the host innate defense mechanism of many plants and animals. A series of short Trp-rich peptides derived from the C-terminal region of Bothrops asper myothoxin II, a Lys49 phospholipase A(2) (PLA(2)), were found to reproduce the antimicrobial activities of their parent molecule. Of these peptides, KKWRWWLKALAKK-designated PEM-2-was found to display improved activity against both Gram-positive and Gram-negative bacteria. To improve the antimicrobial activity of PEM-2 for potential clinical applications further, we determined the solution structure of PEM-2 bound to membrane-mimetic dodecylphosphocholine (DPC) micelles by two-dimensional NMR methods. The DPC micelle-bound structure of PEM-2 adopts an α-helical conformation and the positively charged residues are clustered together to form a hydrophilic patch. The surface electrostatic potential map indicates that two of the three tryptophan residues are packed against the peptide backbone and form a hydrophobic face with Leu7, Ala9, and Leu10. A variety of biophysical and biochemical experiments, including circular dichroism, fluorescence spectroscopy, and microcalorimetry, were used to show that PEM-2 interacted with negatively charged phospholipid vesicles and efficiently induced dye release from these vesicles, suggesting that the antimicrobial activity of PEM-2 could be due to interactions with bacterial membranes. Potent analogues of PEM-2 with enhanced antimicrobial and less pronounced hemolytic activities were designed with the aid of these structural studies.