Results: 5

1.
Figure 1

Figure 1. From: Characterization of antimicrobial peptide activity by electrochemical impedance spectroscopy.

A surface-supported bilayer and its equivalent electrical circuit. The RsCs element corresponds to the support-electrolyte interface with the resistance Rs describing solid-to-liquid charge transfer due to redox reaction and Cs being the double layer and the space charge capacitance, while the RmCm (or RmCPE) element corresponds to the resistance and capacitance of the bilayer, and the series resistor Re corresponds to the resistance of the electrolyte (buffer). The values of all electrical components are experimentally determined by measuring electrochemical impedance of the surface-supported bilayer.

William K. Chang, et al. Biochim Biophys Acta. ;1778(10):2430-2436.
2.
Figure 4

Figure 4. From: Characterization of antimicrobial peptide activity by electrochemical impedance spectroscopy.

Resistance of DPhPC/PEG2K bilayer versus time at different peptide/lipid ratios and DC potentials. FSKRGY (top panel) and AGGKGF (bottom panel) was added at t = 10min at the peptide/lipid ratios and DC potentials indicated. The membrane resistance drops within one minute followed by minor equilibration over the next 3–10 minutes. The presence of FSKRGY results in ~5 times lower membrane resistance than the presence of AGGKGF. There is no measurable dependence on membrane potential or peptide concentration (in the range tested).

William K. Chang, et al. Biochim Biophys Acta. ;1778(10):2430-2436.
3.
Figure 5

Figure 5. From: Characterization of antimicrobial peptide activity by electrochemical impedance spectroscopy.

Membrane homogeneity n versus time at different peptide/lipid ratios and DC potentials. FSKRGY (top panel) and AGGKGF (bottom panel) was added at t = 10 min at the peptide/lipid ratios and DC bias potentials indicated. For all DC bias potentials and peptide concentrations, n decreased upon addition of FSKRGY. No such changes were observed upon addition of AGGKGF. In FSKRGY experiments, more positive membrane potentials resulted in more moderate changes; in addition, the changes were larger at higher peptide concentration. Value of n reached minimum within 1–2 minutes, followed by a partial relaxation with a time constant of about 8 min.

William K. Chang, et al. Biochim Biophys Acta. ;1778(10):2430-2436.
4.
Figure 3

Figure 3. From: Characterization of antimicrobial peptide activity by electrochemical impedance spectroscopy.

Modulus, |Z| and phase angle, Θ, of the electrochemical impedance Z of surface-supported bilayer versus frequency of applied oscillating voltage. DPhPC/PEG2k bilayer (top panel). The measured points (filled circles) were fitted to the response of the electrical circuit shown in Fig. 1 using RC element for the membrane and yielding Rs = 10 MΩcm2, Cs = 2.1 μF/cm2, Rm = 7.2 kΩcm2, Cm = 0.9 μF/cm2, Re = 40 Ωcm2. Bilayer after the addition of 1/100 P/L AGGKGF peptide (bottom panel). The measured points were fitted using CPE (see Fig. 1) yielding Rs = 10 MΩcm2, Cs = 2.18 μF/cm2, Rm = 4.9 kΩcm2, Q = 4.35×10−6/cm2, n = 0.84, Re = 42 Ωcm2. Note that the units of Q depend on n, see eq.(2). From these values we obtain Cm = 0.87 μF/cm2 at 5 kHz, see eq. (3).

William K. Chang, et al. Biochim Biophys Acta. ;1778(10):2430-2436.
5.
Figure 2

Figure 2. From: Characterization of antimicrobial peptide activity by electrochemical impedance spectroscopy.

AMP modes of action and corresponding changes in membrane parameters: resistance Rm and homogeneity n. Vertical dashed line in the graphs shows the moment of peptide injection. If the peptides create a barrel-stave pore (a), the membrane becomes more permeable to ions (Rm decreases), but the membrane thickness and homogeneity do not change. If the peptides associate in a toroidal pore (b), the membrane also becomes leakier and the membrane thickness decreases along the pore perimeter resulting in a slightly decreased homogeneity. If the peptides aggregate as in the carpet model (c), membrane homogeneity certainly decreases while membrane resistance may decrease (in case the peptides introduce a disorder in the lipids) or increase (in case the peptides have opposite, ordering effect [29]). In the detergent-like model (d) the peptides disrupt the membrane, resulting in a huge decrease or a complete disappearance of membrane resistance. The membrane can still provide an electrochemical barrier if the disrupted regions can be “repaired” by lateral lipid diffusion into those regions. One may expect a decrease in homogeneity (lower n) due to the existence of different membrane regions: intact, disrupted, repaired. Overall, this disruption/repair process would result in a decrease of lipid density and, as a consequence, decrease of membrane resistance Rm, and decrease of membrane thickness (higher Cm).

William K. Chang, et al. Biochim Biophys Acta. ;1778(10):2430-2436.

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