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Biophysical studies of membrane proteins/peptides. Interaction with ...

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INTRODUCTION: BIOMEMBRANES<br />

<strong>of</strong> the distribution functions <strong>of</strong> phosphate groups in each monolayer. In this way, the<br />

bilayer thickness <strong>of</strong> fluid DOPC is 37 Å. Another important concept is the hydrophobic<br />

thickness <strong>of</strong> a bilayer, i.e. the length across the bilayer normal which is occupied by the<br />

hydrocarbon core <strong>of</strong> lipids. Wiener and White (1992) showed that the transbilayer<br />

carbonyl-to-carbonyl distance accurately reported the thickness <strong>of</strong> the hydrocarbon core.<br />

For fluid DOPC, this value is <strong>of</strong> 27-28 Å (Lewis and Engelman, 1983; Nagle and<br />

Tristram-Nagle, 2000). Interestingly, the presence <strong>of</strong> the double bond in DOPC, which<br />

carries 18 carbons in the acyl-chains, decreases the hydrophobic thickness up to almost<br />

the same value <strong>of</strong> DMPC, which carries only 14 carbons but presents no unsaturation.<br />

As a general rule, the hydrophobic thickness <strong>of</strong> a phospholipid bilayer increases 1,75 Å<br />

per additional carbon in the phospholipid acyl-chain (Lewis and Engelman, 1983).<br />

Figure 1.6 - Crystallographic structure <strong>of</strong> fluid DOPC bilayers along the bilayer normal axis (Wiener and<br />

White, 1992).<br />

Apart from the phospholipid structure, other factors can influence significantly the<br />

thickness <strong>of</strong> a bilayer. Increases in temperature and hydration lead to decreases in the<br />

thickness <strong>of</strong> the bilayer. Cholesterol also has an important impact as it acts by<br />

thickening the <strong>membrane</strong> (Mouritsen, 2005).<br />

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