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Title: Neutron reflection study of the biosurfactant surfactin
Author: Shen, Hsin-Hui
ISNI:       0000 0001 3402 4172
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2008
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Three different deuterated surfactins were produced from the Bacillus sub/iUs strain, one perdeuterated, one with the four leucines perdeuterated, and one with everything except the four leucines perdeuterated. The neutron reflectivity profiles ofthese three samples in each of null reflecting water and D20 with a seventh profile of the protonated surfactin in D20 were measured under different pH values and ionic concentrations. This combination of different isotopic compositions made it possible to deduce the distribution of each type of iabelled fragment in the surfactin. The consistency of the three null water profiles showed how well defined the deuteration was in the three deuterated samples. The ball-like structure found for the surfactin at surfaces makes it more like a hydrophobic nanoparticle whose solubility in water is only maintained by the double charge. This is probably what makes it so surface active at such low concentrations and what contributes to its formation of very compact surface layers that are much thinner than observed for most conventional amphiphiles. The adsorbed structures of surfactin at all standard solid/liquid interfaces (silica/water, hydrophobic OTS/water, sapphire/water) at different pH conditions have also been investigated by neutron reflection. Surfactin is quite peculiar; it is basically a charged, or partially charged, hydrophobic ball and the general pattern ofadsorption are quite different from normal. anionic surfactants. The overall thickness of the surfactin is about 1.3 nm. The adsorption varies with different substrates and is dominated by hydrophobic and electrostatic interactions. Contrast variation plays an important role in increasing the sensitivity to bilayers and was utilized extensively in the study of the interaction of surfactin with phospholipids on the silicon/silica surface. The d31-DPPC-surfactin bilayer in D20 is a particularly favourable contrast among the four possibilities (h-DPPCID20, d31-DPPCID20, d62DPPC/ CmSi, and d7s-DPPC /CmSi) for the study of this system. It was found that surfactin disrupts and solubilizes phopsholipid bilayers. This happens when the concentration of surfactin in the bilayer is above a certain threshold and when surfactin micelles are present in the bulk solution. The solubilization rate is related to the bilayer coverage. More detailed studies have shown that the surfactin is eXclusively located in the outer leaflet of the bilayer when it starts to penetrate and in this, and some other features, the adsorption into bilayers resembles that of conventional non-ionic/anionic surfactant mixtures.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available