Theoretical Study On Intramolecular Electron Transfer Of The System That Contains A Rigid Bridge And Its PH-dependency

This work consists of two parts. The fundamental theory of electron transfer (ET) is presented in the first part that covers the theoretical foundation, development and corresponding models are included. Then the factors such as the electronic coupling matrix element, the standard Gibbs free energy difference and the reorganization energy which affect the rate constant of an ET reaction are discussed based on the classical nonadiabatic Marcus'theory, also the corresponding computational methods are shown. Since the electronic coupling matrix element is a very important variable in the calculation of rate constant of ET, several general methods such as two states variation method, partition technological method, Generalized Mulliken-Hush method and Koopman's theory method generally used for computation are introduced. An additional part is attached to discuss the relationship between the electronic coupling element and the distance and orientation angle between the electronic donor and acceptor. Theoretical calculations are carried out for different conformations of the system of cyclopeptide, c-TrpH-TyrOH, which consists of tryptophan(Trp) and tyrosine(Tyr). Finally,a curve is lined out to fit the above mentioned relationship. In the second part, the mechanisms of ET reaction in aqueous solvent with different values of pH are studied. Experimental measurements showed that the ET mostly proceeds along the intramolecular pathway for the system which involve Trp and Tyr, so our theoretical computation also mainly focuses on the intramolecular ET mechanism. The cyclopeptide, c-TrpH-TyrOH,contains a rigid six-member ring as a bridge, which decreases the ET rate dependency on geometries greatly and facilitates our theoretical investigations. The change of rate constant of ET with the existence of σbond on the both ends of the bridge has been discussed in the first part, so we will not make further discussion in this part. It is well known that the different reactants and products of ET reaction exist in different forms in aqueous solvent with different pH, and they will influence the mechanisms of ET reaction. According to our research, the corresponding reaction mechanisms in aqueous solvent can be presented as follows: Trp[NH + . ]? (B)n?Tyr[OH]→Trp[NH]?(B)n?Tyr[O.]+H+ (1) Trp[N . ]? (B)n ?TyrOH→Trp[NH]?(B)n?TyrO. (2) Trp[N . ]? (B)n?Tyr[O? ]H2OTrp[NH]?(B)n?Tyr[O.]+OH? (3) Equations (1), (2) and (3) show the mechanisms of ET reactions that take place in acid, neutral and alkaline solvents respectively. B denotes the bridge of ET. After theoretical calculation, the reaction mechanism (2) that takes place in the neutral solution is proved to be a proton-coupled electron transfer (PCET) reaction. The theoretical computation of the PCET mechanism is very difficult because of the complexity of PCET mechanism, so we mainly study the ET reaction which happens in acid and alkaline solvent. In acid solvent the consecutive mechanism of proton transfer (PT) followed by ET in the ET reaction has been excluded firstly. In order to study the consecutive mechanism of ET followed by PT in the ET reaction that happens in acid solvent, the following pure electron transfer mechanism is designed. NNNOONHNNOOOHH+. OH+. The theoretical calculation shows that even in the gas phase the rate for the ET reaction represented by the above equation is very slow and will decreasegreatly in aqueous solvent. So, this reaction mechanism also should be excluded. Therefore, only the process of a concerted transfer of ET and PT is taken into account. It can be concluded that the ET reaction that happens in acid solution is PCET reaction. Except for that, the ET reaction mechanism that takes place in the alkaline aqueous solvent is also studied in this part. A vital variable for the computation of the rate constant in solvent is the solvent reorganization energy, which is proved to be very in controlling the ET reaction. In order to make it clear, the theory of non-equilibrium solvation developed during the recent half-century are summarized as an introduction at first. Then, a new theory of non-equilibrium solvation that brought forward by our group is given for our further computation. The computational results show that the rate constant of the ET reaction of our theoretical calculation is in agreement with the experimental observations. At last, the newly developed theory for PCET reaction is simply introduced as an end.