No abstract

The effect of a transmembrane potential on ion channel formation by zervamicin II (ZER-II) was studied in a vesicular model system. The dissipation of diffusion potential caused by addition of ZER-II to small phosphatidylcholine vesicles was monitored using fluorescent (Safranine T) and optical (Oxonol YI) probes. Cis-positive potentials facilitated channel formation, while at cis-negative potentials, ion fluxes were inhibited. A potential-independent behavior of ZER-II was observed at high peptide concentrations, most likely due to its membrane modifying property.z 1999 Federation of European Biochemical Societies.

Permeabilization of the phospholipid membrane, induced by the antibiotic peptides zervamicin IIB (ZER), ampullosporin A (AMP) and antiamoebin I (ANT) was investigated in a vesicular model system. Membrane-perturbing properties of these 15/16 residue peptides were examined by measuring the K(+) transport across phosphatidyl choline (PC) membrane and by dissipation of the transmembrane potential. The membrane activities are found to decrease in the order ZER>AMP>>ANT, which correlates with the sequence of their binding affinities. To follow the insertion of the N-terminal Trp residue of ZER and AMP, the environmental sensitivity of its fluorescence was explored as well as the fluorescence quenching by water-soluble (iodide) and membrane-bound (5- and 16-doxyl stearic acids) quenchers. In contrast to AMP, the binding affinity of ZER as well as the depth of its Trp penetration is strongly influenced by the thickness of the membrane (diC(16:1)PC, diC(18:1)PC, C(16:0)/C(18:1)PC, diC(20:1)PC). In thin membranes, ZER shows a higher tendency to transmembrane alignment. In thick membranes, the in-plane surface association of these peptaibols results in a deeper insertion of the Trp residue of AMP which is in agreement with model calculations on the localization of both peptide molecules at the hydrophilic-hydrophobic interface. The observed differences between the membrane affinities/activities of the studied peptaibols are discussed in relation to their hydrophobic and amphipathic properties.

EPR spectra of ubiquinone-0 (UQ 0 ) and UQ 10 anion radicals selectively labeled with 13 C at the 4-CdO position are reported. The environmental effect on the 4-13 C splitting constant was studied in mixed solvents in which the molar fraction of the protic component was changed over a wide range. The quinones were electrochemically reduced, with cyclic voltammetry and concomitant optical spectroscopy being performed as controls. The value and the sign of the 4-13 C hyperfine splitting constant strongly depend on the H-bonding properties (proticity) of the solvent, due to the formation of mono-and disolvates through hydrogen bonding of the carbonyl oxygens in protic solvents. The formation of disolvates is less favorable by a factor of 80 than formation of the monosolvated anion. The results are discussed in light of recent EPR, FTIR, and NMR experiments on the primary 13 C-labeled acceptor quinone Q A in bacterial photosynthetic reaction centers. We propose that in the neutral state both carbonyls of Q A are not or only weakly hydrogen bonded to the protein and that formation of anionic Q A •-results in a much stronger H-bond for the 4-carbonyl only. The large shift of the 4-CdO IR signal reported for UQ in ViVo is in our view mostly due to a change in bond order induced by binding of UQ to the protein, perhaps through a change in sp 2 hybridization at the 4-C position.