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The achiral, nitroxyl-contg. a-amino acid TOAC (TOAC = 2,2,6,6- tetramethylpiperidine-1-oxyl-4-amino-4-carboxylic acid), in combination with the chiral a-amino acid Ca-Me valine [(aMe)Val], was used to prep. short peptides (from di- to hexa-) that induced the enantioselective oxidn. of racemic 1-phenylethanol to acetophenone. The best catalyst was an Na-acylated dipeptide alkylamide with the -TOAC-(aMe)Val- sequence folded in a stable, intramolecularly hydrogen-bonded b-turn conformation with large, lipophilic (hydrophobic) N- and C-terminal blocking groups. We rationalized our findings by proposing models for the diastereomeric intermediates between (R)-[and (S)]-1-phenylethanol and the catalyst Fmoc- TOAC-L-(aMe)Val-NHiPr, based on the X-ray diffraction structure of the latter.

The intramolecular dissociative electron transfer (ET) across donor–bridge–acceptor (D–B–A) systems consisting of a series of trans ring-substituted 4-benzoyloxy-1-methylcyclohexyl bromides in N,Ndimethylformamide has been studied by cyclic voltammetry. X-ray diffraction crystallography and 1H NMR spectroscopy showed that the investigated D–B–A molecules have the trans(cyclohexane) axial(benzoyloxy)-axial(bromide) conformation. As previously found with the corresponding cis equatorial, axial conformers (Antonello and Maran, 1998 [25]), electroreduction entails initial formation of a benzoate radical anion (donor D) followed by intramolecular dissociative ET to the C–Br bond (acceptor A) through the 1,4-cyclohexanediyl spacer (bridge B). The intramolecular ETs are exergonic with driving force in the range from -0.5 to -1.2 eV. The electrode process follows the same mechanism previously established for the cis series of isomers, but the ET rate constants obtained with the trans axial, axial isomers are larger by 1.1 orders of magnitude. X-ray crystallography structures show that the rate increase cannot be ascribed to a simple decrease of the geometrical distance between the electron-exchanging centers and, therefore, the results witness a remarkable stereochemical effect on the ET rate. Application of the German–Kuznetsov theory of nonadiabatic dissociative ET (German and Kuznetsov, 1994 [35]) shows that the rate increase is caused by a more favorable coupling etween the electronic wave functions describing the reagent and product states. The data and trends are discussed in comparison with other nonadiabatic intramolecular dissociative ET processes

Monodisperse Au25L18(0) (L = S(CH2)2Ph) and [n-Oct4N+][Au25L18(-)] clusters were synthesized in tetrahydrofuran. An original strategy was then devised to oxidize them: in the presence of bis(pentafluorobenzoyl) peroxide, the neutral or the negatively charged clusters react as efficient electron donors in a dissociative electron-transfer (ET) process, in the former case yielding [Au25L18+][C6F5CO2-]. As opposed to other reported redox methods, this dissociative ET approach is irreversible, easily controllable, and clean, particularly for NMR purposes, as no hydrogen atoms are introduced. By using this approach, the -1, 0, and +1 charge states of Au25L18 could be fully characterized by 1H and 13C NMR spectroscopy, using one- and two-dimensional techniques, in various solvents, and as a function of temperature. For all charge states, the NMR results and analysis nicely match recent structural findings about the presence of two different ligand populations in the capping monolayer, each resonance of the two ligand families displaying distinct NMR patterns. The radical nature of Au25L18(0) is particularly evident in the 1H and 13C NMR patterns of the inner ligands. The NMR behavior of radical Au25L18(0) was also simulated by DFT calculations, and the interplay between theory and experiments revealed a fundamental paramagnetic contribution coming from Fermi contact shifts. Interestingly, the NMR patterns of Au25L18(-) and Au25L18(+) were found to be quite similar, pointing to the latter cluster form as a diamagnetic species.

To study the relationship between rate and driving force of intramolecular dissociative electron transfer, a series of donor-spacer-acceptor systems (D-Sp-A) has been devised and synthesized. Cis-1-aminocyclohexane-4-carboxylic acid and a perester functional group were kept constant as the spacer and the acceptor, respectively, whereas unsubstituted or variously substituted phthalimide groups were selected as the donors. X-ray crystallography and ab inition conformational calculations pointed to D-Sp-A molecules having the cis-(cyclohexane) equatorial(phthalimido) - axial(perester) conformation and the same D/A orientation. The electrochemical analysis provided clear evidence of a concerted dissociative electron-trasfer mechanism. The intramolecular rate constants were determined and compared with the corresponding intermolecular values, the latter being obtained by using model molecules.

In this study, we employed thiolated peptides of the conformationally constrained, strongly helicogenic alpha-aminoisobutyric acid (Aib) residue to prepare self-assembled monolayers (SAMs) on gold surfaces. Electrochemistry and infrared reflection absorption spectroscopy support the formation of very well packed Aib-peptide SAMs. The immobilized peptides retain their helical structure, and the resulting SAMs are stabilized by a network of intermolecular H bonds involving the NH groups adjacent to the Au surface. Binary SAMs containing a synthetically defined glycosylated mannose-functionalized Aib-peptide as the second component display similar features, thereby providing reproducible substrates suitable for the controlled display of bioactive carbohydrate ligands. The efficiency of such Aib-based SAMs as a biomolecular recognition platform was evidenced by examining the mannose-concanavalin A interaction via surface plasmon resonance biosensing.