Date of Award

12-2012

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Chemistry

Advisor

Dominy, Brian N

Committee Member

Stuart , Steven J

Committee Member

McNeill , Jason D

Committee Member

Alexov , Emil G

Abstract

In the current dissertation, studies related to solvation energy of protein structures using implicit as well explicit solvation methods have been discussed. Special focus is given to explore effect of salt on the fold stability of proteins and enzymes. Salt plays a crucial role in the functioning of all proteins, enzymes and nucleic acids. Change in salt concentration of the medium has large impact on stability and activity of these biological macromolecules. Therefore exploring mechanism of salt effect on them and development of an efficient model to calculate the salt effect has fundamental as well as practical importance in the field of sciences.
In chapter two the development of an implicit solvation model to calculate salt effect on the fold stability of proteins and enzymes is shown. In combination of standard Poisson-Boltzmann formalism to calculate polar solvation energy, newly developed microscopic surface tension parameter as a function of ionic strength is used in the non-polar component of solvation free energy. The model was tested on series of Cold shock proteins whose stability as a function of NaCl concentration was calculated previously through experiments. Then the model was successfully used to explain the basis of experimentally observed increased stability of HIV-1 protease in the presence of high concentration of NaCl. Further, the same model also showed ability to capture salt specific Hofmeister effect on Cold shock proteins by using salt specific surface tension parameter.
In the third chapter, similar studies were extended through molecular dynamics simulations of explicit solvated aqueous systems of protein and salt. Effect of salt on the translation and rotational motion of bulk water as well as water in different layers from protein surface was closely monitored. Self hydration of salt ions was seen to follow their rank in Hofmeister series. Alternatively effect of salt on rotational motion of water in different layers from protein surface showed that rank of an ion in Hofmeister series have no significant correlation with its effect on water structure making or breaking properties. The largest impact of salt on restricted motion of water was seen on the layer of water which is on the brink of being hydration water and bulk water. This is the same layer where water is been exchanged continually between hydrated water and bulk water. With these results, it can be articulated that effect of salt on the exchange rate of water between hydration shell and bulk may also be behind the origin of Hofmeister effect on protein.
After looking at the salt effect through explicit as well as implicit solvation methods, in chapter four we will compare generalized Born with a simple switching (GBSW) implicit solvent and explicit solvent using TIP3P water model effect of solvent viscosity on peptide dynamics. We compared both solvents to see if absence of solvent viscosity and equilibration of solvent's degrees of freedom makes implicit solvent faster in sampling same conformational phase space than explicit solvent. To reach same equilibrium and sample phase space GBSW proved to be faster by factor of 10 than explicit solvent. An additional modified explicit solvent which thermodynamically identical to the original but higher in viscosity was studied too. The results confirmed that equilibrium properties of peptide calculated through implicit or explicit solvent matches and the efficiency of implicit solvent to sample similar phase space comes from inherent lack of friction and viscosity.

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