Interactions Between Small Anionic Peptides and A Model Membrane in the Presence of Essential Metal Ions
The anionic small peptides H-DDDDDDD-OH (D7) and H-GDDDDDD-OH (GD6) have previously been shown to inhibit the growth of both gram-negative and gram-positive bacteria in the presence of zinc ions. However, a detailed description of the microbial killing mechanism remains unclear. This study focuses on changes in the structural characteristics of D7 and GD6 at neutral pH value in the presence of the model membrane dimyristoylphosphatidylcholine (DMPC) and three essential metal ions, calcium, magnesium, and zinc. Techniques employed included: Infrared (IR), Raman, and visible spectroscopies, Nuclear Magnetic Resonance (NMR) and transmission electronic microscopy (TEM). Fourier transform infrared spectroscopy with second derivative resolution enhancement and curve-fitting methods were applied for quantitative analysis of conformational transitions. The D7 and GD6 peptides were found to consist of a mixture of turns, random coils and β-sheet structures. In the presence of DMPC or upon metal addition, both peptides primarily adopt β-sheet structures, but in the presence of Zn2+ some &agr;-helix configuration was induced. Further investigation revealed that bonding occurs through the oxygen atom of the amide group and through the phosphorous atom of the phosphate group. It is important to note that the bonding of Zn2+ to both peptides is different from that of Ca2+ and Mg 2+ and the order of strength appears to be Zn2+-O>Zn 2+-Mg2+>Zn-Ca2+. Low-frequency Raman spectra of D7 and GD6 in the presence of metal ions confirmed the results obtained using FTIR. Moreover, according to the magnitude of the shifts, the bonding of metals was electrostatic, with the bonding of Zn2+ to both peptides being the strongest. Proton NMR spectra revealed that the Nitrogen atom of the amide group was involved in the bonding. The bonding follows the trend of FTIR and Raman including the interaction of D7 with DMPC, where in the presence of Zn it was also very strong. Simultaneous IR and turbidity measurements revealed that both peptides formed aggregates in the presence of the metal ions. The NTU values were significantly higher for D7, GD6, D7-Zn2+ and GD6-Zn2+. Thus, there was a correlation between the NTU values and β-sheets formation. TEM images provided visual conformation for all investigations. These peptide-metal ion systems constitutes a good example of a metal-induced perturbation of peptide structure.
"Interactions Between Small Anionic Peptides and A Model Membrane in the Presence of Essential Metal Ions"
ETD Collection for Tennessee State University.