Indolicidin is a cationic antimicrobial peptide, rich in proline and tryptophan residues. It is the shortest known naturally-occuring antimicrobial peptide, toxic to both prokaryotes and eukaryotes. The killing mechanism is not fully understood yet. The interactions between indolicidin with model liposomes of various compositions were examined using Raman, Fourier Transformation infrared (FTIR), UV visible, fluorescence, and atomic force microscopy (AFM) techniques. FTIR with second derivative resolution enhancement methods and curve-fitting procedures were applied for quantitative analysis of indolicidin conformation variations. The quantitative analysis indicated that indolicidin adopted mainly an a-helix structure. Upon membrane addition, a major reduction of the  a-helix structure was observed, with the POPG:DLPC system inducing a mixture of �-sheet and turn structures. POPG:DPPC, DPPG:DPPC, and DPPG:DLPC systems induced some coils and turn structures. The strongest interaction of indolicidin was found with the POPG:DLPC system. Absorption spectra indicated a strong interaction between indolicidin and the POPG:DLPC system, in agreement with FTIR and Raman data. Thermodynamic parameters involved in the complex formation (?G, ?H, ?S) were calculated from the UV visible data. Fluorescence emission spectra of indolicidin with the POPG:DLPC system showed fluorescence enhancement and blue shift. The formation constants of the complexes were also calculated from the fluorescence data using the modified Benesi-Hildebrand equation. The absorption and fluorescence emission data indicated that the indolicidin-POPG:DLPC system was the most stable. AFM images of indolicidin alone showed aggregate-like structures at high concentration. AFM images of indolicidin-liposome systems showed large aggregates of micelle-like globules. Analysis of the AFM height images showed that indolicidin interacts with liposomes via different mechanisms. The results gathered by different biophysical techniques provide new insights on the activity mechanism of indolicidin.

Spectroscopic studies on interaction of indolicidin with mixed phospholipid monolayers

247th National Meeting (2014)

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