A photosensitizer dinuclear ruthenium complex: intramolecular energy transfer to a covalently linked fullerene acceptor.

A fullerene derivative (5) in which a dinuclear ruthenium complex is covalently linked to a fulleropyrrolidine (FP) through a rigid spacer has been prepared through azomethine ylide cycloaddition to C60. Electrochemical and photophysical studies revealed that ground-state electronic interactions between the bimetallic ruthenium chromophore and the FP moiety are small. The absorption spectrum of 5 displays a metal-to-ligand charge transfer (MLCT) transition at about 620 nm in CH2Cl2 which is shifted by nearly 160 nm relative to that of a previously reported mononuclear dyad (8). The photophysical investigations have also shown that both in dichloromethane and acetonitrile the photoexcited MLCT state of dyad 5 transforms into the fullerene triplet excited state with a quantum yield of 0.19 and that, contrary to mononuclear dyad 8, electron transfer, if any under the applied conditions, is negligible relative to energy transfer.

Selectivity in proton transfer, hydrogen bonding, and solvation.

The site of protonation (or deprotonation) of polyfunctional bases and acids can be determined through the comparison of experimental NMR properties (chemical shift and relaxation rates) with the corresponding data calculated by quantum chemical methods. The results can be interpreted in terms of the competition between intrinsic base strengths and solvation. Qualitatively similar criteria are found to hold for hydrogen bonding. The selective enrichment in a cosolvent in the solvation shell of a solute dissolved in a solvent mixture (preferential solvation) can be determined through the analysis of intermolecular cross-peak intensities in NOESY spectra.

Detecting hydrogen bonding by NMR relaxation of the acceptor nuclei

The formation of hydrogen bonds (HB) between phenol or N-methyltrifluoroacetamide and several acceptors (pyridine, carbonyl compounds, nitriles, amides) in CCl4 or CHCl3 been investigated through the analysis of NMR relaxation times (T1) of the heteronuclei (14N and 17O) directly involved in the HB interaction. Thus, a comparison is made between such T1 values (corrected for changes in molecular dynamics and motional anisotropy) and electric field gradients calculated by ab initio methods for the acceptor molecules, both isolated and in 1:1 or 2:1 hydrogen-bonded complexes. When other effects are properly accounted for, there is a good agreement between theoretical and experimental electric field gradient (efg) changes. The noticeable difference found between CCl4 or CHCl3 as solvents is discussed in relation to the presence of phenol oligomers, and the non-negligible HB donor power of CHCl3.

Trans-cis amide bond isomerization in fulleroprolines.

The 1H NMR study of fulleroproline derivative Ac-Fpr-OtBu and its Pro analogue Ac-L-Pro-OtBu over a range of temperatures in toluene-d8 solution has enabled the comparison of their equilibrium and activation parameters for the trans/cis interconversion around the amide partial double bond.

Beta-turn induction by C60-based fulleroproline: synthesis and conformational characterization of Fpr/Pro small peptides.

We have recently described the preparation of Fpr (C60-based fulleroproline). In this paper the synthesis and a conformational characterization of heterochiral di- and tripeptides containing this new alpha-amino acid are reported. A folded structure, induced by the -L-Fpr-D-Ala-sequence in chloroform solution and detected by Fourier transform infrared absorption and 1H nuclear magnetic resonance, has been compared with the known propensity of the cognate -L-Pro-D-Ala-sequence to adopt a beta II-turn conformation, which has also been confirmed in this work. The beta II-turn structure is retained in the crystal state by the Pro-peptides, as shown by the X-ray diffraction structures of Ibu-L-Pro-D-Ala-NHtBu and Z-L-Pro-D-Ala-L-Ala-OtBu.

A bioactive fullerene peptide.

The highly hydrophobic C60 (buckminsterfullerene) was water solubilized by covalently linking the synthon 1,2-dihydro-1,2-methanofullerene [60]-61-carboxylic acid to the alpha-amino group of the hydrophilic 4-8 sequence of peptide T, known to display potent human monocyte chemotaxis. The resulting compound, characterized by a variety of analytical techniques, including a UV spectrum in aqueous solution, exhibits remarkable chemotactic potency, comparable to that of the parent pentapeptide. Furthermore, this fullerene-peptide conjugate inhibits, albeit weakly, HIV-1 protease.