An Electrospray Ionization Mass Spectrometry Study on the “In Vacuo” Hetero-Oligomers Formed by the Antimicrobial Peptides, Surfactin and Gramicidin S

• Published in: Journal of the American Society for Mass Spectrometry Vol. 28; no. 8; pp. 1623 - 1637
• Main Authors: Rautenbach, Marina, Vlok, N. Maré, Eyéghé-Bickong, Hans A., van der Merwe, Marthinus J., Stander, Marietjie A.
• Format: Journal Article
• Language: English
• Published: New York Springer US 01-08-2017; Springer Nature B.V
• Subjects: Analytical Chemistry; Antiinfectives and antibacterials; Antimicrobial agents; Bacteria; Bioinformatics; Biotechnology; CALCIUM IONS; Chemistry; Chemistry and Materials Science; CONCENTRATION RATIO; DIMERS; DISSOCIATION; Electrospraying; EQUILIBRIUM; INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND TECHNOLOGY; INTERACTIONS; ION MOBILITY; Ionic mobility; IONIZATION; Ions; Mass spectrometry; MASS SPECTROSCOPY; Oligomerization; Oligomers; Organic Chemistry; PEPTIDES; Proteomics; Research Article; Scientific imaging; SODIUM IONS; Spectroscopy; Surfactin; TRAVELLING WAVES; Surfactin; Antagonism; Gramicidin S; Electrospray ionization mass spectrometry; Molecular interaction; Lipopeptide; Antimicrobial peptide
• ISSN: 1044-0305; 1879-1123
• DOI: 10.1007/s13361-017-1685-0

It was previously observed that the lipopeptide surfactants in surfactin (Srf) have an antagonistic action towards the highly potent antimicrobial cyclodecapeptide, gramicidin S (GS). This study reports on some of the molecular aspects of the antagonism as investigated through complementary electrospray ionization mass spectrometry techniques. We were able to detect stable 1:1 and 2:1 hetero-oligomers in a mixture of surfactin and gramicidin S. The noncovalent interaction between GS and Srf, with the proposed equilibrium: GS~Srf↔GS+Srf correlated to apparent
K
d
values of 6–9 μM in gas-phase and 1 μM in aqueous solution. The apparent
K
d
values decreased with a longer incubation time and indicated a slow oligomerization equilibrium. Furthermore, the low μM
K
d
app
values of GS~Srf↔GS+Srf fell within the biological concentration range and related to the 2- to 3-fold increase in [GS] needed for bacterial growth inhibition in the presence of Srf. Competition studies indicated that neither Na
+
nor Ca
2+
had a major effect on the stability of preformed heterodimers and that GS in fact out-competed Ca
2+
and Na
+
from Srf. Traveling wave ion mobility mass spectrometry revealed near symmetrical peaks of the heterodimers correlating to a compact dimer conformation that depend on specific interactions. Collision-induced dissociation studies indicated that the peptide interaction is most probably between one Orn residue in GS and the Asp residue, but not the Glu residue in Srf. We propose that flanking hydrophobic residues in both peptides stabilize the antagonistic and inactive peptide hetero-oligomers and shield the specific polar interactions in an aqueous environment.
Graphical Abstract
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