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Biomedical subjects

Andreas Hirsch

Publications and source records attributed to Andreas Hirsch.

18 recordsLinked to original sources
PubMed2003

Electrostatic complexation and photoinduced electron transfer between Zn-cytochrome c and [olyanionic fullerene dendrimers.

Two dendritic fullerene (DF) monoadducts, 2 and 3, which can carry up to 9 and 18 negative charges, respectively, were examined with respect to electrostatic complexation with Cytochrome c (Cytc). To facilitate comprehensive photophysical investigations, the zinc analogue of Cytc (ZnCytc) was prepared according to a novel, modified procedure. The association of ZnCytc and DF, and consequential photoinduced electron transfer within ZnCytc-DF from the photoexcited protein to the fullerene, was proven by fluorescence spectroscopy and transient absorption spectroscopy. These findings were also supported by circular dichroism as well as by extensive molecular dynamics (MD) simulations.

Anions↗
PubMed2003

Functionalization of single-walled carbon nanotubes with (R-)oxycarbonyl nitrenes.

Sidewall functionalization of single-walled carbon nanotubes (SWCNTs) via the addition of (R-)oxycarbonyl nitrenes allows for the covalent binding of a variety of different groups such as alkyl chains, aromatic groups, dendrimers, crown ethers, and oligoethylene glycol units. Such additions lead to a considerable increase in the solubility in organic solvents such as 1,1,2,2-tetrachloroethane (TCE), dimethyl sulfoxide (DMSO), and 1,2-dichlorobenzene (ODCB). The highest solubilities of 1.2 mg/mL were found for SWCNT adducts with nitrenes containing crown ether of oligoethylene glycol moieties in DMSO and TCE, respectively. The presence of chelating donor groups within the addends allowed for the complexation of Cu(2+) and Cd(2+). Atomic force microscopy (AFM) and transmission electron microscopy (TEM) revealed that the functionalized tubes form thin bundles with typical diameters of 10 nm. The presence of thin bundles in solution is supported by (1)H NMR spectroscopy. The elemental composition of the functionalized SWCNT was determined by X-ray photoelectron spectroscopy (XPS). The use of Raman and electron absorption spectroscopy (UV/Vis-nIR) showed that the electronic properties of the SWCNTs are mostly retained after functionalization, indicating a low degree of addition within this series of SWCNT derivatives.

Journal Article↗
PubMed2003

Synthesis of the C59N+ carbocation. A monomeric azafullerene isoelectronic to C60.

A heterofullerene isoelectronic to C60 is reported. The azafullerenium cation C59N+ can be isolated in good yield as a carborane salt via the two-electron oxidation of the C-C bond of (C59N)2 dimer. [C59N][Ag(CB11H6Cl6)2] has been characterized by electronic, IR, Raman, and 13C NMR spectroscopies, MALDI spectrometry, DFT calculations, and X-ray crystallography.

Aza Compounds↗
PubMed2003

Structures and magnetic properties of mono-doped fullerenes C59Xn and C59X(6mn)m (X=Bm, N+, P+, As+, Si): isoelectronic analogues of C60 and C60(6m).

Structures of mono-doped fullerenes, C59Xn and C59X(6mn)m (X=Bm, N+, P+, As+, Si), the isoelectronic analogues to C60 and C606m with 60 and 66 pi-electrons, have been investigated at the B3LYP/6-31G* level of density functional theory. On the basis of the computed nucleus independent chemical shifts (NICS) at the cage center and also at the center of individual rings as magnetic criteria, heterofullerenes with 60 pi-electrons are as aromatic as the parent C60, while those with 66 pi-electrons are much less aromatic than C606m. The very distinct endohedral chemical shifts of the 66 pi-electron systems may be useful to identify the heterofullerenes through their endohedral 3He NMR chemical shifts.

Arsenic↗
PubMed2002

A highly regioselective approach to multiple adducts of C60 governed by strain minimization of macrocyclic malonate addends.

New macrocyclic malonates 2-5 have been prepared by reaction of malonyl dichloride with alkanediols. Reactions of these cyclo-[n]-alkylmalonates with C60 are highly regioselective. The macrocycles containing identical alkyl spacers selectively form bis- and trisadducts of C60 with rotational symmetry. The addition pattern of the regioselectively formed oligoadducts is determined by the size of the alkyl spacer within the macrocyclic malonate. A variety of bis-, tris-, tetra-, and hexaadducts have been synthesized to show the scope of this approach. "Exotic" addition patterns such as trans-4,trans-4,trans-4, which has been synthesized and completely characterized for the first time, are also accessible by this method. The regioselectivity is ruled by the even distribution of the strain within the macrocyclic malonates containing spacer alkane chains of identical lengths: addition patterns with rotational symmetry provide exactly identical distances of the malonate oxygen atoms and are thus exclusively formed by this method. In contrast, when macrocycles with two different alkyl spacer lengths are used, such as 9 and 10, the reaction exclusively yields C(s)-symmetric bisadducts.

Journal Article↗
PubMed2002

Evidence of pronounced electronic coupling in a directly bonded fullerene--ferrocene dyad.

A new donor-acceptor dyad (7) involving a ferrocene moiety as donor and an azafullerene as acceptor has been synthesized by treating bisazafullerenyl (1) with ferrocenium hexafluorophosphate. This compound represents the first example of a fullerene-based dyad where two electroactive groups are connected by only a single sigma-bond. The cyclic voltammetry of 7, in comparison to the corresponding reference systems, clearly reveals strong electronic coupling between the ferrocene and the azafullerene moiety in the ground state. For example, the Fc-based, reversible, one electron oxidation wave is significantly positively shifted by 183 mV with respect to that of the parent ferrocene. This indicates the existence of intramolecular charge transfer (ICT) from the donating Fc to the accepting azafullerenyl group. Photophysical studies on 7 were carried out by means of emission and transient absorption spectroscopy. An instantaneous deactivation of the fullerene singlet excited-state results in the formation of the charge-separated (C59N.-)-(Fc.+) radical pair. From the charge-transfer dynamics with a lower limit of > or = 5 x 10(10) s-1, we infer strong electronic coupling (V) between the azafullerene and the ferrocene moiety of the order of 60 cm-1 in benzonitrile.

Journal Article↗
PubMed2002

T(h)-symmetrical hexakisadducts of C(60) with a densely packed pi-donor shell can act as energy- or electron-transducing systems.

For the first time several T(h)-symmetrical hexakisadducts of C(60) bearing up to six electro- and photoactive o-phenylene diamine or 9,10-dialkoxyanthracene moieties were synthesized and subjected to photoinduced electron/energy-transfer studies. Both donors form a densely packed pi-donor shell surrounding the fullerene core. In these novel core-shell ensembles (7 and 19), either an efficient energy transfer from the dialkoxyanthracene periphery, or an electron transfer from the o-phenylene diamine periphery transduces the flow of excited-state energy or electrons, respectively, to the fullerene moiety, which resides in the central core. Due to the relatively high reduction potential of the fullerene core, which is anodically shifted by approximately equal to 0.7 V, compared with that of pristine C(60), the outcome of these intramolecular reactions depends mainly on the donor ability of the peripheral system. Interestingly, the charge-separated state in the o-phenylene diamine heptad (7; tau=2380 ns in benzonitrile) is stabilized by a factor of 20 relative to the corresponding o-phenylene diamine dyad (6; tau=120 ns in benzonitrile), an effect that points unequivocally to the optimized storage of charges in this highly functionalized fullerene ensemble.

Journal Article↗
PubMed2002

Organic functionalization of carbon nanotubes.

A very general and versatile method for functionalizing different types of carbon nanotubes is described, using the 1,3-dipolar cycloaddition of azomethine ylides. Approximately one organic group per 100 carbon atoms of the nanotube is introduced, to yield remakably soluble bundles of nanotubes, as seen in transmission electron micrographs. The solubilization of the nanotubes generates a novel, interesting class of materials, which combines the properties of the nanotubes and the organic moiety, thus offering new opportunities for applications in materials science, including the preparation of nanocomposites.

Journal Article↗
PubMed2002

End-cap stabilized oligoynes: model compounds for the linear sp carbon allotrope carbyne.

Three series of differently 3,5-disubstituted alpha,omega-diphenylpolyynes Ar-(C[triple bond]C)n-Ar (n=2, 4, 6, 8, 10) were synthesized under optimized Cadiot-Chodkiewicz conditions, isolated and completely characterized. These compounds can be considered as model substances for the hypothetical one-dimensional carbon allotrope carbyne Cx. The longest sp-carbon chain contains 20 atoms and is therefore the longest, purely organic polyyne studied with NMR techniques. Extinction coefficients over 600,000 M(-1) cm(-1) represent the highest measured quantitative values for that compound class so far. Comparisons with previous investigations and electrochemical studies allow the assignment of absorption for both wavelength regions structuring the UV/Vis spectra. Based on the trends in the spectroscopic behaviour of those molecules with increasing chain length, electronic as well as the NMR properties of carbyne are predicted, in line with our previously reported results. The observed stability properties promise the synthesis of even longer polyynes.

Journal Article↗
PubMed2002

Cytotoxicity and photocytotoxicity of a dendritic C(60) mono-adduct and a malonic acid C(60) tris-adduct on Jurkat cells.

The cytotoxic and photocytotoxic effects of two water-soluble fullerene derivatives, a dendritic C(60) mono-adduct and the malonic acid C(60) tris-adduct were tested on Jurkat cells. Cell growth and vitality were determined by a cell counting and staining technique. After 2 weeks cultivation in the presence of the fullerene derivatives, it was found that only the dendritic mono-adduct inhibits cell growth (within 2 weeks the cell number decreased to 19%), whereas the tris-malonic acid adduct has little effect. The growth inhibition is reversible; cultivating the same cells further in the absence of fullerene, the cell number increased to 106.4%. Other experiments showed that these fullerene derivatives become toxic when irradiated with UVA or UVB light. The cell death is mainly caused by membrane damage and it is UV dose-dependent. Tris-malonic acid fullerene was found to be more phototoxic than the dendritic derivative. This result is in contrast to the singlet oxygen quantum yields determined for the two compounds. We propose that the two fullerene derivatives may interact with the cell membrane in different ways thus causing the observed effects. Further experiments will be done to determine the location and concentration of the two compounds in and on the cells.

Cell Division↗
PubMed2002

Molecular design of strong single-wall carbon nanotube/polyelectrolyte multilayer composites.

The mechanical failure of hybrid materials made from polymers and single-wall carbon nanotubes (SWNT) is primarily attributed to poor matrix-SWNT connectivity and severe phase segregation. Both problems can be successfully mitigated when the SWNT composite is made following the protocol of layer-by-layer assembly. This deposition technique prevents phase segregation of the polymer/SWNT binary system, and after subsequent crosslinking, the nanometre-scale uniform composite with SWNT loading as high as 50 wt% can be obtained. The free-standing SWNT/polyelectrolyte membranes delaminated from the substrate were found to be exceptionally strong with a tensile strength approaching that of hard ceramics. Because of the lightweight nature of SWNT composites, the prepared free-standing membranes can serve as components for a variety of long-lifetime devices.

Acrylic Resins↗