Defense Date

1982

Document Type

Thesis

Degree Name

Master of Science

Department

Chemistry

First Advisor

Fred M. Hawkridge

Abstract

The application of electrochemical techniques to biological systems has become an attractive method for characterizing the redox and electron transfer behavior of biomacromolecules. This study focuses on the role of soluble spinach ferredoxin (Fd) in photosynthesis where it functions as an electron carrier to membrane bound species in the plant chloroplast. An electrochemical model is used where in Fd now serves as a soluble redox species which undergoes oxidation and reduction via electron transfer at an electrode. This type of model is significant in that both the membrane and electrode reactive sites are characterized by charged bilayer interfacial regions which significantly influence electron transfer rates.

The formal heterogeneous electron transfer rate constant, kf0:h' and the eletrochemical transfer coefficient, a, were determined using transient and steady state electrochemical techniques. Transient oxidative and reductive kinetic data were obtained spectroelectrochemically via single potential step chronoabsorptometry (SPSC) at three different surfaces: methyl viologen modified gold (MVMG) minigrid electrodes, tin oxide (Sn02) semiconductor electrodes and at metallized plastic gold optically transparent electrodes (MPOTE). The steady state measurements were made via hydrodynamic voltammetry at a MVMG rotating ring - disk electrode (RRDE).

Only the modified gold surfaces resulted in behavior suitable for the determination of heterogeneous electron transfer kinetic parameters. The transient kinetics of Fd were initially evaluated using an irreversible Butler-Volmer kinetic model descriptive of species exhibiting slow rates of electron transfer at an electrode surface. This yielded an average kf0;h = 6.5 (± 1.3) x 10- 5cm/sec and a = 0.60 (±0.16) for reductive experiments performed at four

different modified gold surfaces . Recently the theory for SPSC was extended to quasi-reversible systems, which undergo moderate rates of electron transfer. Since Fd exhibits quasi-reversible behavior at the MVMG grid, the transient reductive data were reanalyzed using this more exact model. The values obtained using the quasi-reversible model were kf,0’h= 1.16 (±0.5) x 10-4 cm/sec and α = 0.42 ( ±0.06 ). The corresponding reductive steady state results at the

modified RRDE yielded kf,0’h = 5.9 ( ±0.05 ) x 10-4 cm/sec and α=0.476 (± 0.001). Oxidative experiments were only successful with transient experiments at the MVMG minigrid surface where in kf,0’h =3.39 (± 0.01) x 1 0- 5 cm/sec and α = 1.17 (0.02). Absorbance data also indicates protein adsorption as a prestep to electron transfer. This type of phenomenon is common to biological molecules reacting at electrodes. The behavior of Fd at these surfaces and the kinetic results are discussed in terms of mechanistic implications.The formal heterogeneous electron transfer rate constant, kf0h and the eletrochemical transfer coefficient, a, were determined using transient and steady state electrochemical techniques. Transient oxidative and reductive kinetic data were obtained spectroelectrochemically via single potential step chronoabsorptometry (SPSC) at three different surfaces: methyl viologen modified gold (MVMG) minigrid electrodes, tin oxide (Sn02) semiconductor electrodes and at metallized plastic gold optically transparent electrodes (MPOTE). The steady state measurements were made via hydrodynamic voltammetry at a MVMG rotating ring - disk electrode (RRDE).

Only the modified gold surfaces resulted in behavior suitable for the determination of heterogeneous electron transfer kinetic parameters. The transient kinetics of Fd were initially evaluated using an irreversible Butler-Volmer kinetic model descriptive of species exhibiting slow rates of electron transfer at an electrode surface. This yielded an average kf0h = 6.5 (± 1.3) x 10- 5cm/sec and a = 0 . 60 (±0.16) for reductive experiments performed at four

different modified gold surfaces . Recently the theory for SPSC was extended to quasi-reversible systems, which undergo moderate rates of electron transfer. Since Fd exhibits quasi-reversible behavior at the MVMG grid, the transient reductive data were reanalyzed using this more exact model. The values obtained using the quasi-reversible model were kf0h= 1.16 (±0 . 5) x 10-4 cm/sec and α = 0.42 ( ±0.06 ). The corresponding reductive steady state results at the

modified RRDE yielded kf0h = 5.9 ( ±0.05 ) x 10-4 cm/sec and α=0.476 (± 0.001). Oxidative experiments were only successful with transient experiments at the MVMG minigrid surface where in kf0h =3.39 (± 0.01) x 1 0- 5 cm/sec and α = 1.17 (0.02). Absorbance data also indicates protein adsorption as a prestep to electron transfer. This type of phenomenon is common to biological molecules reacting at electrodes. The behavior of Fd at these surfaces and the kinetic results are discussed in terms of mechanistic implications.

Rights

© The Author

Is Part Of

VCU University Archives

Is Part Of

VCU Theses and Dissertations

Date of Submission

9-13-2016

Included in

Chemistry Commons

Share

COinS