Lipid bilayers thermodynamic properties

The previous result is an important one. It indicates that there can be yet another fruitful route to describe lipid bilayers. The idea is to consider the conformational properties of a probe molecule, and then replace all the other molecules by an external potential field (see Figure 11). This external potential may be called the mean-field or self-consistent potential, as it represents the mean behaviour of all molecules self-consistently. There are mean-field theories in many branches of science, for example (quantum) physics, physical chemistry, etc. Very often mean-field theories simplify the system to such an extent that structural as well as thermodynamic properties can be found analytically. This means that there is no need to use a computer. However, the lipid membrane problem is so complicated that the help of the computer is still needed. The method has been refined over the years to a detailed and complex framework, whose results correspond closely with those of MD simulations. The computer time needed for these calculations is however an order of 105 times less (this estimate is certainly too small when SCF calculations are compared with massive MD simulations in which up to 1000 lipids are considered). Indeed, the calculations can be done on a desktop PC with typical... 

The presence of proteins in the lipid bilayer influences greatly the thermodynamic properties of the membrane. The lipids in the area of immediate protein influence are in gel-state and their molecular movements are restricted. 

The study of the thermodynamic properties of lipid bilayers is a satisfactory approach to Lmderstand the biological membranes. 

The precise arrangement and degree of ordering of the lipid molecules in the final structure is not known for certain. However, it seems highly probable that the bilayer nature of the assembly is a consequence of the thermodynamics of free-energy changes at the metal-lipid surface and at the lipid-aqueous solution interface [4,13]. Our measurements of the electrical properties of supported lipid bilayers described here are consistent with those of conventional BLMs and closely related systems. 

Spectroscopy will also be vital if and when we can search the atmospheres of potentially habitable planets for the presence of molecules that indicate the existence of life, a topic discussed in Chap. 5. The discussion of intermolecular forces, especially hydrogen bonding, in Sect. 1.4 serves as an introductiOTi to Chap. 6, which is devoted to the role in biochemical systems of a molecule, water, whose universality on Earth might blind us to its remarkable properties. Quite a lot of this introductory chapter has been devoted to thermodynamics. The role and importance of thermodynamics when we consider what conditions might lead to and sustain life are particularly brought out in some of the later chapters of this book. The forces between dissolved species and their solvent and between molecules that are at the boundary of solubility and therefore can form micelles and lipid bilayers were introduced in Sect. 1.4. These species and their properties re-emerge in Chap. 9 as does the topic of enzyme catalysis introduced in Sect. 1.6. 

The entire thermodynamic system of the membrane and TM protein must be considered to understand how the protein and bilayer achieve their native state. We have summarized four of the mechanisms, hydrophobic matching, tilt angles, and specific protein/lipid and protein/protein interactions that are important in determining the stability (Fig. 5). Other important factors, such as the stability of lipid/lipid interactions, have been left out of our protein-centric view. We describe a hydrophobic mismatch as an unfavorable interaction that can be relieved by the other three processes, but we would expect all these properties of the system to interact. We could easily describe the same equilibria by saying that a strain in curvature is relieved by a hydrophobic mismatch or that strong protein/protein packing interactions might help relieve the hydrophobic mismatch or curvature stress. The complex interplay between all these interactions is at the heart of what determines membrane protein stability and will no doubt be difficult to quantify. 

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