Investigation of the phase behavior of bicellar mixtures by differential scanning calorimetry and variable pressure ²H NMR

Uddin, Mohammed Nasir (2010) Investigation of the phase behavior of bicellar mixtures by differential scanning calorimetry and variable pressure ²H NMR. Masters thesis, Memorial University of Newfoundland.

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Abstract

Depending on temperature, composition and hydration, mixtures of long and short chain lipids, namely, 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 1,2-dihexanoyl-sn-glycero-3-phosphocholine (DHPC) can assemble into structures in which the DMPC lipids are aggregated in planar bilayer regions while the DHPC lipids are in the edges of planar bilayers. Such mixture is referred as 'bicellar' and can be used as model membrane to study membrane associated proteins and peptides. We have studied the phase behavior of DMPC/DHPC mixtures of molar ratios 3:1 and 4.4:1 with one or the other lipid deuterated, using differential scanning calorimetry (DSC) and variable pressure ²H NMR. Application of pressure shifts the isotropic-nematic and the nematic-lamellar transition temperatures for both mixtures, by roughly the same rate as the DMPC gel-liquid crystal transition. DSC measurements show that DMPC-normalized transition enthalpy is similar to that of pure DMPC. In the nematic temperature range, DMPC chain order does not change much and DSC shows samples absorb heat over that temperature range. Taken together, these observations suggest that isotropic-nematic-lamellar transitions are strongly coupled to the melting of ordered DMPC in the bicelles and the chain melting is not only the process absorbing heat at the transition. It thus appears that some of the heat input goes to changing the hydration of the DHPC headgroups. Pressure reduces the magnetic orientability in DMPC-d₅₄ of 3:1 mixture compared to 4.4:1 mixture in nematic phase temperature range. This may be due to particle size effects. In the nematic and lamellar phase, DHPC-d₂₂ chains become more ordered with increasing temperature. This reflects fast exchange of DHPC-d₂₂ between highly curved edge regions and planar DMPC-rich regions, and the fraction of time spent by a DHPC molecule in the DMPC-rich environment increases with increasing temperature. This work provides insight into how interactions between the long and short chain lipids in the bicellar mixture drive the observed changes in morphology.

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