| Photoinduced electron transfer (PET) between electro-active groups covalently attached on de novo peptide systems has been investigated. Two series of oligopeptides and one series of polypeptides were employed in the study. Experimental procedures used in the investigation included UV-Vis absorption and fluorescence spectrometry and laser flash photolysis.; For the series N-4-(1-pyrenylbutyroyl) L-tryptophan ethyl ester (PyBt-Trp)/N-1-pyrenesulfonyl L-alanyl L-tryptophan ethyl ester (Pyr-Ala-Trp), an amino acid that serves as an electron donor, tryptophan, was connected to an electron acceptor moiety, pyrene, through varied chemical links. As the consequence of the interplay of the intrinsic thermodynamic driving force and the solvent reorganization energy, the forward electron transfer rate constant for PyBt-Trp is sensitive to the polarity of solvents, while the rate for Pyr-Ala-Trp is not. Both of the conjugates were shown to form 1:1 host-guest complexes with the cyclic oligosaccharide β-cyclodextrin, in which the pyrene moiety is encapsulated in the cavity of the β-cyclodextrin. Based on fluorescence data for β-cyclodextrin binding and on additional experiments, it was shown that the two conjugates can be used to study microenvironments (micro-polarity) such as those found in micelles, vesicles and proteins.; In another series, Pyr-(Ala)n-DMPD, the electron donor dimethyl-p-phenylenediamine (DMPD) was connected to the C-termini of various oligopeptides in which the amino acid alanine (Ala) serves as a spacer (n = 1, 2, 3); a pyrenesulfonyl (Pyr) group is appended to the opposite N-terminus. The electron transfer rate decreases with increasing donor-acceptor separation. However, the falloff in rate is less than that estimated using the Marcus theory for electron transfer and assuming a single pathway for electronic coupling of the terminal electroactive groups. Further study shows that the conformational flexibility of the alanine spacer may provide an alternative electronic coupling pathway through an intramolecular hydrogen bond.; A photochemical electron transfer study was also carried out on a series of five 24-residue polypeptides having a high propensity to form an α-helical structure and having an electron acceptor, pyrene moiety appended to the C-terminus. The rate of electron transfer rate to a remotely-located tryptophan residue decreases as the through-bond distances between the donor and acceptor increases. The data show that the α-helical structure of the polypeptide is critical for promoting efficient electronic coupling through a protein-like matrix that allows through-bond, through-space, and through-hydrogen-bond interactions. |
