Abstract
Gramicidin A (gA) forms several convertible conformations in different environments. In this study, we investigated the effect of calcium halides on the molecular state and antimicrobial activity of gramicidin A. The molecular state of gramicidin A is highly affected by the concentration of calcium salt and the type of halide anion. Gramicidin A can exist in two states that can be characterized by circular dichroism (CD), mass, nuclear magnetic resonance (NMR) and fluorescence spectroscopy. In State 1, the main molecular state of gramicidin A is as a dimer, and the addition of calcium salt can convert a mixture of four species into a single species, which is possibly a left-handed parallel double helix. In State 2, the addition of calcium halides drives gramicidin A dissociation and denaturation from a structured dimer into a rapid equilibrium of structured/unstructured monomer. We found that the abilities of dissociation and denaturation were highly dependent on the type of halide anion. The dissociation ability of calcium halides may play a vital role in the antimicrobial activity, as the structured monomeric form had the highest antimicrobial activity. Herein, our study demonstrated that the molecular state was correlated with the antimicrobial activity.
Highlights
Gramicidin is an antimicrobial peptide that is a byproduct of Bacillus brevis during sporulation [1]
We have previously demonstrated that the antimicrobial activity of gramicidin A may be associated with the formation of free radicals via the disruption of NADH/NAD+ synthesis [28]
The molecular state and conformation induced by calcium halide can further influence the antimicrobial activity of gramicidin A
Summary
Gramicidin is an antimicrobial peptide that is a byproduct of Bacillus brevis during sporulation [1]. Gramicidin adopts a number of different conformations in different environmental conditions [4,5,6,7] In organic solvents such as methanol, gramicidin forms a β-sheet-like double-strand helical structure, in which two monomers are interwound as DNA double helices [4,6,8]. The complex of gramicidin A/monovalent cation exists in a left-handed antiparallel double-helical conformation in methanol [15,16] and has a distinct CD spectrum compared to that of the ion-free form [17]. The rate of conversion from the double-stranded helical dimer into the channel dimer is dependent on environmental factors, such as temperature and ionic strength [21,22,23]. The molecular state and conformation induced by calcium halide can further influence the antimicrobial activity of gramicidin A
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