Rugged energy landscape for water motion

The cellular membrane contains many other molecules. As already mentioned, it includes trans-membrane proteins that transport important nutrients and chemicals to the interior of the cell across the membrane. The membrane also includes other lipids, such as cholesterol etc., that serve to strengthen the phospholipid bilayers. 

In this chapter we are concerned with the stmcture and dynamics of water around lipid bilayers. This area has seen a good deal of studies in recent times. Both experiments and simulation studies have been carried out to understand the correlation between water and lipid motions. 

2 Hydration of different constituents phospholipids and buried proteins 

Next to the hydrated region is an intermediate region that is only partially hydrated. This boundaiy layer is approximately 0.3 nm thick. Within this short distance, the water concentration drops from 2 M on the headgroup side to nearly zero on the tail (core) side [1]. However, in some cases water is knovra to access pretty deep into the buried region. 

Water molecules at the surface of the bilayers can exist in two states. They can be strongly hydrogen-bonded to the charged or polar head groups or they can be in the layer but not bonded. We again use the terms bound and free to denote these two types of water molecules. Of course, no water molecule remains bound for ever and there is a dynamic exchange between the two groups as mentioned earlier in Chapter 6. Thus, what we essentially mean by bound water is a quasi-bound water molecule, and bound is used to describe the instantaneous state of the system. 

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