The Statistical Mechanics Studies Of Type I Antifreeze Proteins

With the development of scientific technology and test conditions, people gradually isolated different types of antifreeze proteins from the organisms of ocean fishes, terrestrial insects, plants, bacteria and fungi, and the gene sequences and structures of these antifreeze proteins were also determined. These proteins have not only a wealth of structural information, but also the special anti-freezing properties, so that it makes these antifreeze proteins have broad applicational prospects, and thus their study is also concerned in many fields, from molecular biology to biological physics to chemical field, from oceanography to fisheries ecology and then to the field of biological evolution. In recent years, it has also attracted the interest of the business community. In this paper, based on the statistical mechanics and statistical thermodynamics theory, the statistical mechanics model is established, and it is applicable to explain the thermal hysteresis activity of the type I antifreeze protein(AFPI). By using the solution lattice model and non-considering the interaction between molecules, the thermodynamic properties of linear protein molecular solution are discussed in detail; based on this, the thermodynamic properties of AFPI solution are studied further when the protein-water interaction is also considered. The interaction energy between AFPI and ice crystal is calculated theoretically. Finally, the adsorption orientation of AFPIs on the ice surface is considered in the statistical mechanics model. Some main results of the thesis are generalized as following:[1] By using statistical thermodynamics theory, the statistical mechanics model is established, and it is applicable to explain the thermal hysteresis activity of AFPI. Thermal hysteresis temperatures of some mutants of AFPIs as functions of antifreeze proteins concentration are calculated. In the calculating process, it gives the partition functions and free energies of the ice crystal with AFPIs adsorption, and without AFPI adsorption, respectively. Mean while, it is considered the adsorption fashion of AFPI on the ice surface is same as that of ideal gas on surface. The function which expresses the relation of thermal hysteresis temperature and coverage rate is given, and the thermal hysteresis temperatures of some mutants of AFPI are calculated and compared with the experimental data. The results show that the theoretical results are in agreement with the experimental data, and our statistical mechanics model can explain the thermal hysteresis activity of AFPI successfully; the increase of AFPI chemical potential will result in the increase of thermal hysteresis temperature, and the interaction energy between AFPI and ice also influences the thermal hysteresis temperature obviously. [2] With the preliminary approximation of non-considering the influence of interactions between molecules, statistical thermodynamics theory and solution lattice model are applied to discuss the thermodynamic properties of linear protein solution. The results show that the Gibbs function of the solution decreases with the protein concentration, and with the continuing increase of protein concentration, the Gibbs function will be increased, and the minimum of Gibbs function moves to the smaller concentration when the protein molecular chain is longer. The Gibbs function decreases with the increasing of temperature. The chemical potential of protein increases with the increase of its concentration, and it decreases with the increase of temperature when the concentration is small, and it increases with the temperature when the concentration exceeds a centern value. Finally, as an application of the theory, the chemical potential of AFPI is calculated. Comparing the result with that of ideal solution, it can be found that, the solution is can not be discussed simply as an ideal solution even though the solution is dilute.[3] Considering the interaction between protein and water molecules, the thermodynamic properties of AFPI solution are discussed in detail. The results show the Gibbs function of the solution with concentration decreases firstly, and it increases quickly as the concentration still increases; the chemical potential of AFPI in the solution increases with the increase of AFPI concentration, and the chemical potential of water in the solution decreases with the increasing of AFPI concentration, and these result in the fact that the Gibbs function of the solution varies with the AFPI concentration, and this variety first decreases and then increases. Mean while, it gives the result that the depression of the solution freezing point caused by the addition of AFPI is a colligative effect, and the depression of freezing point is calculated. It is found that this temperature depression is very small, and usually can be omitted when discussing the properties of the solution and the thermal hysteresis activity of antifreeze proteins, and this means that the antifreeze activity of AFPs is a non-colligative effect.[4] The formal statistical mechanics model gives the conclusion that the interaction energy between antifreeze proteins and ice plays the important part on the thermal hysteresis temperature, and then the interaction energy between AFPI and ice are calculated theoretically. In the calculation process, it is considered that only some amino acid groups at certain locations can interact with ice, and this interaction is Van der Waals interaction, and at the mean while, the repulsive interaction between same neighboring groups is also considered in the calculation. As an example, the protein-ice interaction energies of some mutants of AFPIs are calculated. The results show the interaction energy between AFPI and ice relates to the number of methyl groups on the alanines (or threonines), and it is also influenced by the distance between AFPI and ice. The more the number of methyl groups, the larger the interaction energy is; the larger the distance between AFPI and ice, the smaller the interaction energy is.[5] With the further improvement of the statistical mechanics model, the thermal hysteresis of AFPI is discussed when the adsorption orientation is included. The results show the thermal hysteresis temperature is smaller than that of non-considering adsorption orientation, and this is because the coverage rate of the former is smaller than the latter. Both of the chemical potential of AFPI and the AFPI-ice interaction energy influence the coverage rate, and then affect the thermal hysteresis. The statistical mechanics model including the adsorption orientation can express the thermal hysteresis of AFPI better, and the adsorption orientation can not be neglected.