Padmaja Juvvadi

Orientation of Cecropin A Helices in Phospholipid Bilayers Determined by Solid-State NMR Spectroscopy

The orientation of the insect antibiotic peptide cecropin A (CecA) in the phospholipid bilayer membrane was determined using (15)N solid-state NMR spectroscopy. Two peptide samples, each specifically labeled with (15)N at Val(11) or Ala(27), were synthesized by solid phase techniques. The peptides were incorporated into phospholipid bilayers, prepared from a mixture of dimyristoylphosphatidylcholine and dimyristoylphosphatidylglycerol, and oriented on glass slides. The (15)N chemical shift solid-state NMR spectra from these uniaxially oriented samples display a single (15)N chemical shift frequency for each labeled residue. Both frequencies are near the upfield end of the (15)N chemical shift powder pattern, as expected for an alpha-helix with its long axis in the plane of the membrane and the NH bonds perpendicular to the direction of the magnetic field. These results support a mechanism of action in which CecA binds to and covers the membrane surface, thereby causing a general destabilization and leakiness of the lipid bilayer membrane. The data are discussed in relation to a proposed mechanism of membrane lysis and bacterial killing via an ion channel activity of CecA.

7Li and 23Na NMR studies of transmembrane cation transport mediated by ionophore lasalocid A

Ion transport across phospholipid vesicles was studied by 7Li and 23Na‐NMR using an aqueous anionic paramagnetic shift reagent, dysprosium nitrilotriacetate [Dy(NTA)2]3−, mediated by ionophores, lasalocid A and A23187. The intra‐ and extracellular 7Li and 23Na‐NMR signals were well separated (20 Hz) at mM concentration of the shift reagent. The observed data on the rate constant for lithium transport across DPPC vesicles at various concentrations of the ionophores indicated that lasalocid A is a more efficient carrier for lithium ion compared with the sodium ion transport by this ionophore, while A23187 was not specific to either of the ions (Li or Na). ©1998 European Peptide Society and John Wiley & Sons, Ltd.

Synthesis and Study of the Antimicrobial Action of Cecropin A-Melittin Hybrids with Aib Substitution

Cecropins [1] and melittins [2] are a group of potent antimicrobial peptides. The cecropin melittin analogs are highly antimicrobial [3], had antitumor activity [4] and cecropins are potent fungicides [5]. To understand the role of substitution of hydrophobic amino acids with hydrophilic amino acids and Aib, different substituted analogs were separately synthesized and their antimicrobial activity studied.