David I. Schuster

Biological applications of fullerenes

Thus far, development of applications of fullerenes in biology has been hampered by the poor water solubility of fullerenes. In spite of such concerns, fullerenes have proved useful for a wide variety of biological applications. As derivatized and underivatized fullerenes continue to become increasingly available, additional applications and further development of those discussed in this article will invariably follow.

Modulation of [3H]-dopamine binding by cholecystokinin octapeptide (CCK-8)

Cholecystokinin-octapeptide (CCK-8) is a putative neurotransmitter which has been demonstrated previously to occur in midbrain dopamine neurones. We observe that CCK-8 causes changes in both the affinity and density of binding sites for [3H]-dopamine in rat striatal homogenates, in vitro, upon incubation with the peptide at a concentration of 1 micromolar. A dose-response study of the competetion of CCK-8 with [3H]-dopamine binding indicates an IC50 for the peptide of 450 nM; desulfated CCK-8 and the related peptide caerulin are at least 4-fold less active than CCK-8. CCK-8 was also administered to rats in a separate study; the binding of [3H]-dopamine was evaluated to homogenates of striata and olfactory tubercles obtained from these animals, which had been treated with systemic injection at a dose of 20 micrograms/kg, daily, for four days. A decrease in the number of striatal binding sites for the radioligand was observed, with a concomitant increase in the number of binding sites in the olfactory tubercle. These data collectively suggest a possible regulatory role for CCK-8 in the ascending dopamine systems.

A superior synthesis of [6,6]-methanofullerenes: The reaction of sulfonium ylides with C60

The addition-elimination reaction of stabilized sulfonium ylides 1a–f to C60 provides a new high yield route to [6,6]-methanofullerenes. The rates of reaction differ with the nucleophilicity of the ylides.

Synthesis and photophysics of new types of fullerene-porphyrin dyads

Abstract The synthesis and photophysical properties of three types of porphyrin–fullerene (P-C 60 ) dyads are presented: (a) conformationally flexible dyads with polyether linkers, (b) rigid dyads with steroid linkers, and (c) a parachute-shaped dyad in which the flat porphyrin and spherical fullerene chromophores are in very close proximity. The course of events following photoexcitation of these dyads and (in some cases) corresponding porphyrin–zinc complexes has been probed through studies of fluorescence quenching, measurements of fluorescence lifetimes and quantum yields for singlet molecular oxygen formation, and transient absorption spectroscopy. These data provide insight into the dynamic competition between intramolecular energy transfer and electron transfer in these dyads, and, for the case of the parachute-shaped dyad, the kinetics of back-electron transfer.

Anti-human immunodeficiency virus activity and cytotoxicity of derivatized buckminsterfullerenes

Abstract Eleven new functional derivatives of C 60 have been tested for anti-HIV activity in peripheral blood mononuclear cells infected with HIV-1 LAV . Nine compounds displayed activity in the low micromolar range (EC 50 ), three of them lower than any fullerene derivative reported to date. With one exception, these compounds displayed no cytotoxicity up to 100 μM.

Photophysical properties of three hydrofullerenes

Abstract The ground state and lowest triplet state properties of three hydrofullerenes, C60H2, C60H4 and [1-methylsuccinate-4-methyl-cyclohexadiene-2,3]buckminsterfullerene, have been investigated in toluene at room temperature. Their visible ground state absorption spectra are compared to those of methanofullerenes and methanolfulleroids. The triplet properties, including the absorption spectrum between 300 and 800 nm, molar absorption coefficient, quantum yield of formation and quantum yield for the photosensitized production of O 2 ( 1 Δ g ) , are compared to those of other C60 derivatives.

Molecular modelling of fullerene–porphyrin dyads

Computational studies have been performed on a variety of C60–porphyrin dyads, a class of donor–acceptor materials which have been a subject of considerable attention in recent years. Molecular modelling studies were carried out to clarify the relationship between molecular topology and experimentally determined rates of intramolecular electron and energy transfer in these systems. The systems studied include doubly linked cyclophane-like C60–porphyrin dyads, where structural variations were made computationally on the porphyrin and linker portions, as well as dyads with flexible polyether and rigid steroid linkers. The molecular modelling studies involved building and minimising structures of the various fullerene–porphyrin dyads, followed by molecular dynamics to find the equilibrium and lowest energy conformations. The study confirmed that attractive van der Waals interactions between porphyrin and C60 moieties cause these dyads to adopt unusual conformations in which these groups are in close proximity, often in orientations which are not readily predictable from conventional structural representations. The implications of these computational data for the design of fullerene–porphyrin dyads with specific properties in the context of electron and energy transfer processes are discussed.

Chiral non-racemic C60 derivatives: A proposed sector rule for fullerene absolute configuration

Abstract A number of higher fullerenes as well as carbon nanotubes have been predicted to be chiral. Our lab has recently reported the resolution and CD spectra of several enantiomeric C 60 derivatives. It appears that chromatographic resolutions using Whelk-O columns are rather general. Using optically active enones and dienes, non-racemic C 60 adducts can also be formed. Recently we have discovered that fullerene CD spectra appear to arise from a UV band diagnostic for reaction at the CC bond between two six membered rings, the most common site for mono-addition. A Cotton effect is observed in the CD associated with this chromophore and allows proposal of a sector rule for fullerene chirality. Preparation of chiral (non-racemic) fullerene adducts of known absolute configuration allows assignment of the sign to each of the sectors. These CD bands are due to asymmetric perturbation of an intrinsically symmetric chromophore and provide a new tool for a priori determination of absolute configuration.

Alternative Demetalation Method for Cu(I)-Phenanthroline-Based Catenanes and Rotaxanes

A new and less hazardous procedure for demetalation of Cu(I)-phenanthroline-based interlocked molecules, using aqueous NH(4)OH rather than toxic KCN, has been developed. The conditions are compatible with materials containing nucleophile-sensitive appended groups such as C(60), and coordinating moieties such as zinc(II)-porphyrins.

Photochemistry of ketones in solution LVIII: Mechanism of photoreduction of benzophenone by benzhydrol

Abstract New studies of the mechanism of photoreduction of benzophenone by benzhydrol are reported. They demonstrate conclusively that the primary photochemical act involves hydrogen abstraction from benzhydrol by the n,π* uncomplexed triplet state of benzophenone to give a triplet radical pair which does not couple to a measurable extent within the solvent cage. Escape of the radicals from the cage is followed by a series of hydrogen transfer reactions with ground state ketone, leading to scrambling of label when either the ketone or the hydrol is labeled initially. The competition between coupling and disproportionation of Ph2ĊOH and (CH3)2ĊOH is contrasted and rationalized, as is the virtual absence of in-cage reaction in the benzophenone—benzhydrol system and the moderately efficient in-cage reactions in the benzophenone—isopropyl alcohol system. Thus, textbook descriptions of this prototype photoreduction should reflect the fact that the initially produced pair of radicals do not couple directly, but undergo a series of hydrogen transfers prior to coupling outside the initial solvent cage.