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''Entry by Angelo Mao, AP 225, Fall 2010
''

'''Title:''' Membrane Interactions of Novicidin, a Novel Antimicrobial Peptide: Phosphatidylglycerol Promotes Bilayer Insertion

'''Authors:''' Dorosz J, et al

'''Journal:''' J. Phys. Chem. B

'''Volume:''' Vol 114(34)

'''Pages:''' 11053-11060


==Summary==
Novicidin is a novel antimicrobial peptide that is an improvement on certain preexisting antimicrobial peptides (such as melittin) in that it more effectively targets bacteria and causes less hemolysis. Through various experiments, the researchers propose that an explanation for this difference is the relative proportion of hydrophobic to hydrophilic elements in novicidin composition, as well as the locations of these elements, which lowers the free energy of novicidin interaction and embedding in negative (bacterial) membranes in comparison to zwitterionic (human) membranes.

''soft matter keywords:'' phosphilipid, membrane, hydrophobic interactions, hydrophilic interactions, free energy

==Background==

[[Image:dorosz1.gif|thumb|center|350px|Figure 1. Schematic of novicidin and interaction with negative parts of the membrane.]]

[[Image:dorosz2.gif|thumb|center|270px|Figure 2. Helical wheel projection of novicidin.]]

Novicidin is a peptide derived from a previous antimicrobial candidate, ovispirin, which unfortunately was highly cytotoxic. As shown in figure 1, novicidin is a generally positively charged peptide. The helical wheel projection of novicidin indicates where the positively charged amino acids, lysine and arginine (in gray), are localized. The hydrophobic residues (green and yellow) are situated on the other side of the wheel, leading to a hydrophobic moment (arrow) in the novicidin molecule. From this, one would expect that novicidin would be able to embed itself in phospholipid membranes with its positive charges facing outside and its hydrophobic elements facing the hydrophobic core of the membrane.

==Inserting a negative group into the membrane==

[[Image:dorosz3.gif|thumb|center|270px|Figure 3. Isothermal adsorption of novicidin to membrane layers with different proportions of negative groups.]]

[[Image:dorosz4.gif|thumb|center|270px|Figure 4. Fluorescence dose−response curves of lipid/PDA vesicles.]]

The researchers created phospholipid monolayers with varying amounts of a negatively charged phospholipid called dimyristoylphosphatidylglycerol (DMPG). The line (iv) in figure 3 indicates the result from the membrane that is entirely composed of DMPG, while the line (i) indicates the result from membrane without any DMPG. Adsorption of novicidin into the membrane increased with DMPG composition, starting from a baseline of near zero adsorption. The researchers performed a similar set of experiments with vesicles of varying amounts of phospholipid/polydiacetylene (PDA) and DMPG. In this other set of experiments, the results of which are shown in figure 4, fluorescent materials leaking from the vesicles were quantified. The results showed positive correlation between DMPG composition and fluorescence intensity. The implication is that novicidin incorporation is not only correlative with the present of negatively charged components in the membrane, but that this incorporation is also correlative to membrane disruption.