Solvation dynamics lipid bilayer

The emission of Trp 19 in melittin shifts to the red side peaking at 341 nm (Fig. 18), and the probe location slightly moves away from the lipid interface toward the channel center. Consistently, we observed a larger fraction of the ultrafast solvation component (35%) and a smaller contribution of slow ordered-water motion (38%). Melittin consists of 26 amino acid residues (Fig. 9), and the first 20 residues are predominantly hydrophobic, whereas the other 6 near the carboxyl terminus are hydrophilic under physiological conditions. This amphipathic property makes melittin easily bound to membranes, and extensive studies from both experiments [156-161] and MD simulations [162-166] have shown the formation of an 7-helix at the lipid interface. Self-assembly of 7-helical melittin monomers is believed to be important in its lytic activity of membranes [167-169]. Our observed hydration dynamics are consistent with previous studies, which support the view that melittin forms an 7-helix and inserts into the lipid bilayers and leaves the hydrophilic C-terminus protruding into the water channel. The orientational relaxation shows a completely restricted motion of Trp 19, and the anisotropy is constant in 1.5 ns (Fig. 20b), which is consistent with Trp 19 located close to the interface around the headgroups and rigid well-ordered water molecules. 

We have seen that hydration forces act between lipid bilayers in aqueous and non-aqueous solvents. The molecular nature of the interaetion is beginning to be understood, and involves solvation effects along with dynamic, and structural properties of the lipid bilayer. The relative importanee of these factors is still a controversial topic. However, because of the complexity of membrane systems, one cannot expect a force, which can be described by a simple mathematical for-... 

This chapter has given an overview of the structure and dynamics of lipid and water molecules in membrane systems, viewed with atomic resolution by molecular dynamics simulations of fully hydrated phospholipid bilayers. The calculations have permitted a detailed picture of the solvation of the lipid polar groups to be developed, and this picture has been used to elucidate the molecular origins of the dipole potential. The solvation structure has been discussed in terms of a somewhat arbitrary, but useful, definition of bound and bulk water molecules. 

Recently, a molecular dynamics study of the phospholipid DLPE was reported by Damodaran et al. using a united atom model. The model was built from the crystal structure of DLPE reported by Elder et al. The fully hydrated DLPE bilayer has an interlamellar water layer of 5 A. The bilayer was solvated by 553 SPCE waters ( 11 water molecules/lipid) in the head group region. This lipid has a gel-to-liquid-crystalline transition temperature of... 

One substantial difference between polyalanine and melittin is their effect on the membrane. Shen et al. found that the presence of the transmembrane polyalanine helix barely altered the structure and dynamics of the DMPC bilayer. Only the NMR order parameters were slightly lower, indicating that the protein did not Induce the formation of a more ordered layer of lipids. Moreover, a detailed analysis of the solvation of the helix showed that no well-defined annulus of lipids remained tethered to the protein. A similar conclusion was reached in... 

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