Electron transfer mechanism and potential applications of α-helical peptides

Understanding long range electron transfer (ET) in proteins is of fundamental interest to elucidate the complex nature of many biological processes. The mechanistic discussion is highly debated in the literature and the factors that control this process are still not clear. Because of the structural complexity and dynamic nature, it is very difficult to correctly evaluate long range ET in proteins. The study of simple model peptides having specific secondary structures is useful for a systematic and accurate evaluation. The polypeptide matrix in the photosynthetic reaction centre is rich in helices and this particular structural motif is believed to play an important role in ET in nature. In this thesis, ET study through some synthetic α-helical model peptides is described. The model peptides studied herein contain the redox-active ferrocene at one end and the thiol-functionalised Cys residue at the other. Films of these peptides were formed on the surface of gold electrodes via the Au-S bond, and by employing cyclic voltammetry, the rate of ET between the pendant ferrocene and the gold electrode through the peptide spacer has been evaluated. My study indicates that ET in α-helical peptides is a function of molecular dynamics and occurs via a tunnelling mechanism. These findings are significant and expected to offer new directions in the highly controversial discussion on ET in proteins. This thesis also describes investigations in two important areas of applications of the α-helices. The first is “photocurrent generation upon laser excitation of light-harvesting chromophore-functionalised peptides” which mimics the natural photosynthetic centre. This important area of research can promote development of nano-scaled photovoltaic devices. Surprisingly, following the conventional experimental protocols, a photocurrent was observed in the absence of a chromophore and even by the irradiation of a bare gold electrode with laser light. It is suggested that an important consequence of laser irradiation has been overlooked in several publications and the so-called photocurrent phenomenon may be a consequence of laser heating. “Peptide-protected nanoparticles” is another area of research receiving significant attention these days due to its potential relevance in biomedical applications. However, peptides are highly flexible and their structure can change depending on the nature of the environment. Since the reactivity of a peptide is related to its secondary structure, any conformational change could seriously alter the overall activity of the peptide-protected nanoparticles. In this thesis, the structural investigation of an α-helical peptide was carried out and it was found that the radius of curvature of nanoparticles has a profound effect on the structure of the adsorbate peptides and thereby, may affect the overall activity of the peptide-protected nanoparticles