Preferential Equilibrium Partitioning of Positively Charged Tryptophan into Phosphatidylcholine Bilayer Membranes

The interaction of tryptophan enantiomers with model membranes is modulated by polar head type and physical state of phospholipids

The mutual influence of chiral bioactive molecules and supramolecular assemblies is currently being studied in many research fields, including medical-pharmaceutical applications. Model membranes of phospholipids, such as the zwitterionic dipalmitoylphosphatidylcholine (DPPC) and the anionic dipalmitoylphosphatidylglycerol (DPPG), interact with a variety of chiral compounds that include amino acids. In this work, the interaction of tryptophan enantiomers, L-Trp and D-Trp, on DPPC and DPPG bilayers was investigated by using differential scanning calorimetry, attenuated total reflectance-Fourier transform infrared and spin-label electron spin resonance spectroscopies as well as molecular docking simulations. The results show that Trp enantiomers slightly perturb the bilayer thermotropic phase transitions. For both membranes, O atoms in the carbonyl groups have a propensity to act as acceptors of a (weak) hydrogen bond. The Trp chiral forms also promote formation of hydrogen bonds and/or hydration in the PO₂^- moiety of the phosphate group, especially for the DPPC bilayer. In contrast, they interact more closely with the glycerol group of DPPG polar head. Only for DPPC bilayers, both enantiomers increase the packing of the first hydrocarbon chain segments for temperatures through the gel state, whereas they do not affect the lipid chain order and mobility in the fluid state. The results are consistent with a Trp association in the upper region of the bilayers without permeation in the innermost hydrophobic region. The findings suggest that neutral and anionic lipid bilayers are differently sensitive to amino acid chirality.

Design of Peptides for Membrane Insertion: The Critical Role of Charge Separation

A physical understanding of membrane permeation and translocation by small, positively charged molecules can illuminate cell penetrating peptide mechanisms of entry and inform drug design. We have previously investigated the permeation of the doubly charged peptide WKW and proposed a defect-assisted permeation mechanism where a small molecule with +2 charge can achieve a metastable state spanning the bilayer by forming a membrane defect with charges stabilized by phospholipid phosphate groups. Here, we investigate the membrane permeation of two doubly charged peptides, WWK and WWWK, with charges separated by different lengths. Through complementary experiments and molecular dynamics simulations, we show that membrane permeation was an order of magnitude more favorable when charges were separated by an ∼2-3 Å greater distance on WWWK compared to WWK. These results agree with the previously proposed defect-assisted permeation mechanism, where a greater distance between positive charges would require a less extreme membrane defect to stabilize the membrane-spanning metastable state. We discuss the implications of these results in understanding the membrane permeation of cell-penetrating peptides and other small, positively charged membrane permeants.

Peptide Permeation across a Phosphocholine Membrane: An Atomically Detailed Mechanism Determined through Simulations and Supported by Experimentation

Cell-penetrating peptides (CPPs) facilitate translocation across biological membranes and are of significant biological and medical interest. Several CPPs can permeate into specific cells and organelles. We examine the incorporation and translocation of a novel anticancer CPP in a dioleoylphosphatidylcholine (DOPC) lipid bilayer membrane. The peptide, NAF-1^44-67, is a short fragment of a transmembrane protein, consisting of hydrophobic N-terminal and charged C-terminal segments. Experiments using fluorescently labeled NAF-1^44-67 in ∼100 nm DOPC vesicles and atomically detailed simulations conducted with Milestoning support a model in which a significant barrier for peptide-membrane entry is found at the interface between the aqueous solution and membrane. The initial step is the insertion of the N-terminal segment and the hydrophobic helix into the membrane, passing the hydrophilic head groups. Both experiments and simulations suggest that the free energy difference in the first step of the permeation mechanism in which the hydrophobic helix crosses the phospholipid head groups is -0.4 kcal mol^-1 slightly favoring motion into the membrane. Milestoning calculations of the mean first passage time and the committor function underscore the existence of an early polar barrier followed by a diffusive barrierless motion in the lipid tail region. Permeation events are coupled to membrane fluctuations that are examined in detail. Our study opens the way to investigate in atomistic resolution the molecular mechanism, kinetics, and thermodynamics of CPP permeation to diverse membranes.

Novel Arginine End-Tagging Antimicrobial Peptides to Combat Multidrug-Resistant Bacteria

The emergence of multidrug-resistant microorganisms has been termed one of the most common global health threats, emphasizing the discovery of new antibacterial agents. To address this issue, we engineered peptides harboring "RWWWR" as a central motif plus arginine (R) end-tagging and then tested them in vitro and in vivo. Our results demonstrate that Pep 6, one of the engineered peptides, shows great potential in combating Escherichia coli bacteremia and the Staphylococcus aureus skin burn infection model, which induces a 62-90% reduction in bacterial burden. Remarkably, after long serial passages of S. aureus and E. coli for 30 days, Pep 6 is still highly efficient in killing pathogens, compared with 64- and 128-fold increase in minimal inhibitory concentrations (MICs) for vancomycin and polymyxin B, respectively. We also found that Pep 6 exhibited robust biofilm-inhibiting activity and eliminated 61.33% of the mature methicillin-resistant Staphylococcus aureus (MRSA) biofilm with concentration in the MIC level. These results suggest that the RWWWR motif and binding of arginine end-tagging could be harnessed as a new agent for combating multidrug-resistant bacteria.

CPF-C1 analog with effective antimicrobial and antibiofilm activities against Staphylococcus aureus including MRSA

The evolution of Staphylococcus aureus (S. aureus) with the ability to acquire and develop resistance to antibiotics has been described as a distinct strain emergence event. Methicillin-resistant S. aureus (MRSA) is responsible for most global S. aureus bacteremia cases. Bacterial biofilms are one of the primary reasons for drug resistance. Biofilms formed by S. aureus are the most common cause of biofilm-associated infections, which increase the difficulty of treatment. Antimicrobial peptides (AMPs) represent promising candidates for the future treatment of antibiotic-resistant bacterial and biofilm-associated infections. In this study, we designed and synthesized a series of analogs to increase the druggability of the natural antimicrobial peptide CPF-C1. Among the analogs, CPF-2 showed high antimicrobial activity against MRSA and multidrug-resistant S. aureus isolated from clinics. In the serum and physiological salt environment, CPF-2 also exhibited effective antimicrobial activity. Importantly, CPF-2 did not determine resistance and showed no hemolytic activity at the active concentration. Concerning the mechanism of action, CPF-2 produced a rapid bactericidal effect by interrupting the bacterial membranes. Even more surprisingly, CPF-2 showed an excellent ability to prevent and eradicate biofilms caused by S. aureus and MRSA not only in vitro but also in vivo. Our results suggested that CPF-2 has potential as a lead compound to treat infections caused by S. aureus and MRSA, including the associated biofilms.

Characterization of an antibacterial dodecapeptide from pig as a potential food preservative and its antibacterial mechanism

A membrane-active dodecapeptide was generated from porcine antimicrobial peptide with promising antimicrobial activity for application in the food industry as a potential bio-preservative to prevent microbial spoilage.

Defect-Assisted Permeation Through a Phospholipid Membrane: Experimental and Computational Study of the Peptide WKW

We investigate membrane permeation by the peptide WKW that is amidated at its C-terminus and therefore carries a positive charge of +2. To facilitate an efficient calculation, we introduce a novel set of simple coarse variables that measure permeation depth and membrane distortion. The phospholipid head groups shift toward the center of the membrane, following the permeating peptide, and create a defect that assists permeation. The Milestoning algorithm was used in the new coarse space to compute the free-energy profile and the mean first passage time. The barrier was lower than expected from a simple continuum estimate. This behavior is consistent with the known behavior of positively charged cell-penetrating peptides, and is explained by a detailed mechanism of defect formation and propagation revealed by the simulations.

Design of Trp-Rich Dodecapeptides with Broad-Spectrum Antimicrobial Potency and Membrane-Disruptive Mechanism

There has recently been great concern regarding antibiotics due to potential drug resistance and the impact of antibiotics on the environment. Antimicrobial peptides are believed to have potential as novel antimicrobial agents to address the problems of antibiotics. Herein, we report a set of Trp-rich dodecapeptides derived from PMAP-36 that are based on the peptide folding principle and the amino acid characteristics. An effective peptide design template, (WXYX)₃, where X represents Arg or Lys and Y represents hydrophobic or neutral amino acid, was summarized with the distribution of Trp at H-bond formation sites along the α-helical structure. The template peptide 6 (3W-2), with low amphipathicity, displayed strong antimicrobial activity against laboratory strains and clinical isolates while showing no cytotoxicity. Furthermore, 6 was able to suppress the emergence of antimicrobial resistance. Membrane permeabilization assays and microscope observations revealed the potent membrane-disruptive mechanism of 6. Overall, this study diminishes the randomness in peptide design and provides a strategy for generating effective antibiotic alternatives to overcome antibiotic resistance.

Partition of Positively and Negatively Charged Tryptophan Ions in Membranes with Inverted Phospholipid Heads: Simulations and Experiments

A joint experimental and computational study illustrates that the partitioning of positively and negatively charged tryptophan in a phospholipid bilayer is significantly altered by a reversal in the head group dipole arrangement. Experiments were conducted using tryptophan as a fluorescent reporter of its local environment. Based on the experimental design in a recent publication ( Anderson , C. M. ; Cardenas , A. ; Elber , R. ; Webb , L. J. J. Phys. Chem. B 2018 , 123 , 170 - 179 ), we were able to determine that the arrangement of the head group dipole altered the degree of partitioning of charged tryptophan in the lipid bilayer. In parallel, atomically detailed simulations were performed for the two membrane systems. The simulation results are in accord with the experimental findings and support a simple molecular partition mechanism of electrostatic interactions with the head groups, glycerol linkers, and interfacial water dipoles.

Interaction of monomeric and self-assembled aromatic amino acids with model membranes: self-reproduction phenomena

The spontaneous formation of amyloid structures of proteins is responsible for several major human neurodegenerative diseases.