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At least 775 records · Page 43Linked to original sources
PubMed2007

Stereodifferentiating drug-biomolecule interactions in the triplet excited state: studies on supramolecular carprofen/protein systems and on carprofen-tryptophan model dyads.

Stereoselective interaction between a chiral nonsteroidal antiinflammatory drug, namely carprofen (CP), and human serum albumin (HSA) was studied, and the results were compared with those obtained with model dyads. In the presence of albumin the same triplet-triplet transition was detected for both CP stereoisomers; however, time-resolved measurements revealed a remarkable stereodifferentiation in the CP/HSA interaction. For each stereoisomer, the decay dynamics evidenced the presence of two components with different lifetimes that can be correlated with complexation of CP to the two possible albumin binding sites (site I and site II). This assignment was confirmed by using ibuprofen, a site II displacer. Thus, the shorter lived components, for which stereodifferentiation was more important (tauR/tauS ca. 4), were ascribed to the CP triplet state in site I; the lifetime shortening can be attributed to electron-transfer quenching by the only tryptophan (Trp) of the protein. Laser flash photolysis of model dyads containing covalently linked CP and Trp revealed formation of the expected Trp radical cation, providing support for such a mechanism. Moreover, significant stereodifferentiation was observed between the (R)- and (S)-CP-Trp dyads. In the case of CP/HSA complexes, as well as in the model compounds, the stereodifferentiation detected in the decays is in good agreement with that observed in the formation of the only CP photoproduct, resulting from a photodehalogenation process. Moreover, stereodifferentiation was also found to occur for the photobinding of CP to the protein.

Anti-Inflammatory Agents, Non-Steroidal↗
PubMed2001

Photolysis of alpha-azidoacetophenones: trapping of triplet alkyl nitrenes in solution.

[reaction: see text] Selective excitation of the ketone chromophore in alpha-azidoacetophenones, 1, leads to intramolecular triplet energy transfer to the azido group, which forms the corresponding triplet alkyl nitrene, 2. Azides 1 also undergo alpha-cleavage to form benzoyl and methyl azido radicals in competition with nitrene formation. Thus the major photoproduct, 2-benzoylamino-1-phenylethanone, 3, comes from trapping of 2 with a benzoyl radical. This appears to be the first observation of bimolecular trapping of triplet alkyl nitrenes in solution.

Journal Article↗
PubMed2004

Triplet H-C-SiHCl(2): combined matrix-IR and CCSD(T) identification, and the role of the open-shell singlet state.

Triplet carbene H-C-SiHCl(2) (1), prepared by a photochemical [1,2]H-shift from 1,1-dichlorosilaethylene, was identified by matching experimental and [CCSD(T)/cc-pVTZ] infrared absorptions. Parts of the potential energy surface were explored utilizing CCSD(T)/cc-pVTZ and B3LYP/6-311+G computations. DFT reproduces the experimental features and CCSD(T) computations for the triplet surface but fails in the description of the open-shell singlet state of 1. We emphasize the notion of electropositive heterosubstitution for the generation of persistent ground-state triplet carbenes.

Journal Article↗
PubMed2000

Processing lexical ambiguities in word triplets: evidence of lexical-semantic deficits following dominant nonthalamic subcortical lesions.

Lexical-semantic function was investigated in 10 participants with lesions of the dominant nonthalamic subcortical (NS) region and a matched normal control group. Participants performed speeded lexical decisions on the 3rd member of auditorily presented word triplets. The 4 critical triplet conditions were concordant (coin-bank-money), discordant (river-bank-money), neutral (day-bank-money), and unrelated (river-day-money). When the interstimulus interval (ISI) between the words in the triplets was 100 ms, patients with NS lesions obtained priming that indicated nonselective lexical access; at 1,250-ms ISI, however, there was no significant priming effect. This pattern of results is consistent with the view that patients with NS lesions can automatically access lexical-semantic information but may be unable to sustain lexical activation through controlled or attentional forms of processing.

Aged↗
PubMed2001

Generation and characterization of a fairly stable triplet carbene.

Most molecules are held together by covalent bonds-electron pairs jointly shared by the two atoms that are linked by the bond. Free radicals, in contrast, have at least one unpaired electron. In the case of carbon-based radicals, the carbon atom at the radical centre no longer makes four bonds with other atoms as it would do in its normal, tetravalent state. The presence of unpaired electrons renders such radicals highly reactive, so they normally occur only as transient intermediates during chemical reactions. But the discovery by Gomberg in 1900 of triphenylmethyl, the first relatively stable free radical containing a central trivalent carbon atom, illustrated that radicals with suitable geometrical and electronic structures can be stable. Compounds containing a divalent carbon atom that uses only two of its four valence electrons for bonding are usually less stable than Gomberg-type radicals with trivalent carbon. Although the role of these so-called carbenes in chemical reactions has long been postulated, they were unambiguously identified only in the 1950s. More recently, stable carbenes have been prepared, but the singlet state of these molecules, with the two nonbonding valence electrons paired, means that they are not radicals. Carbenes in the second possible electronic state, the triplet state, are radicals: the two nonbonding electrons have parallel spins and occupy different orbitals. Here we report the preparation and characterization of a triplet carbene, whose half-life of 19 minutes at room temperature shows it to be significantly more stable than previously observed triplet carbenes.

Journal Article↗
PubMed2003

DNA triplet repeats mediate heterochromatin-protein-1-sensitive variegated gene silencing.

Gene repression is crucial to the maintenance of differentiated cell types in multicellular organisms, whereas aberrant silencing can lead to disease. The organization of DNA into chromatin and heterochromatin is implicated in gene silencing. In chromatin, DNA wraps around histones, creating nucleosomes. Further condensation of chromatin, associated with large blocks of repetitive DNA sequences, is known as heterochromatin. Position effect variegation (PEV) occurs when a gene is located abnormally close to heterochromatin, silencing the affected gene in a proportion of cells. Here we show that the relatively short triplet-repeat expansions found in myotonic dystrophy and Friedreich's ataxia confer variegation of expression on a linked transgene in mice. Silencing was correlated with a decrease in promoter accessibility and was enhanced by the classical PEV modifier heterochromatin protein 1 (HP1). Notably, triplet-repeat-associated variegation was not restricted to classical heterochromatic regions but occurred irrespective of chromosomal location. Because the phenomenon described here shares important features with PEV, the mechanisms underlying heterochromatin-mediated silencing might have a role in gene regulation at many sites throughout the mammalian genome and modulate the extent of gene silencing and hence severity in several triplet-repeat diseases.

Animals↗
PubMed2003

Bose-Einstein condensation of the triplet states in the magnetic insulator TlCuCl3.

Bose-Einstein condensation denotes the formation of a collective quantum ground state of identical particles with integer spin or intrinsic angular momentum. In magnetic insulators, the magnetic properties are due to the unpaired shell electrons that have half-integer spin. However, in some such compounds (KCuCl3 and TlCuCl3), two Cu2+ ions are antiferromagnetically coupled to form a dimer in a crystalline network: the dimer ground state is a spin singlet (total spin zero), separated by an energy gap from the excited triplet state (total spin one). In these dimer compounds, Bose-Einstein condensation becomes theoretically possible. At a critical external magnetic field, the energy of one of the Zeeman split triplet components (a type of boson) intersects the ground-state singlet, resulting in long-range magnetic order; this transition represents a quantum critical point at which Bose-Einstein condensation occurs. Here we report an experimental investigation of the excitation spectrum in such a field-induced magnetically ordered state, using inelastic neutron scattering measurements of TlCuCl3 single crystals. We verify unambiguously the theoretically predicted gapless Goldstone mode characteristic of the Bose-Einstein condensation of the triplet states.

Journal Article↗
PubMed2006

A spin triplet supercurrent through the half-metallic ferromagnet CrO2.

In general, conventional superconductivity should not occur in a ferromagnet, though it has been seen in iron under pressure. Moreover, theory predicts that the current is always carried by pairs of electrons in a spin singlet state, so conventional superconductivity decays very rapidly when in contact with a ferromagnet, which normally prohibits the existence of singlet pairs. It has been predicted that this rapid spatial decay would not occur if spin triplet superconductivity could be induced in the ferromagnet. Here we report a Josephson supercurrent through the strong ferromagnet CrO2, from which we infer that it is a spin triplet supercurrent. Our experimental set-up is different from those envisaged in the earlier predictions, but we conclude that the underlying physical explanation for our result is a conversion from spin singlet pairs to spin triplets at the interface. The supercurrent can be switched with the direction of the magnetization, analogous to spin valve transistors, and therefore could enable magnetization-controlled Josephson junctions.

Journal Article↗
PubMed2002

Sensitization of singlet oxygen via encounter complexes and via exciplexes of pipi* triplet excited sensitizers and oxygen.

Both excited singlet states, 1sigma(g)+ and 1delta(g), and the triplet ground state, 3sigma(g)-, of molecular oxygen are competitively formed during the quenching by O2 of triplet (T1) excited sensitizers of sufficient energy. The corresponding overall rate constants kT(1sigma), kT(1delta), and kT(3sigma) as well as the T1 state energies E(T) and the oxidation potentials E(ox), have been determined for a series of six fluorene derivatives. Graduated and in part strong charge transfer (CT) effects on kT(1sigma), kT(1delta), and kT(3sigma) are observed. These and literature data strongly indicate that quenching occurs in two different channels each capable of producing O2(1sigma(g)-), O2(1delta(g)), and O2(3sigma(g)-). One proceeds via internal conversion (IC) of excited 1,3(T1 x 3sigma) complexes with no CT character (nCT), which cannot be distinguished from encounter complexes, the other via IC of 1,3(T1 x 3sigma) exciplexes with partial CT character (pCT). The contributions of nCT and pCT deactivation channels to the overall formation of O2(1sigma(g)+), O2(1delta(g)). and O2(3sigma(g)-) depend on E(T) and E(ox). The rate constants of the nCT channel are controlled by the excess energies of the respective IC processes by an energy gap law. The rate constants of the pCT channel depend on the change of free energy deltaG(CET) for complete electron transfer from T1 excited sensitizer to O2. Equations are presented which show the functional form of the dependence of the oxygen quenching rate constants on E(T) and E(ox). Particular emphasis is laid on the question of whether these relations could generally be valid for pipi* triplet sensitizers.

Journal Article↗
PubMed2003

Cationic arylation through photo(sensitised) decomposition of diazonium salts. Chemoselectivity of triplet phenyl cations.

The photodediazoniation of some 4-X-phenyldiazonium tetrafluoborates in MeCN leads to the singlet phenyl cations (X = H, tert-butyl, NMe2, CN), which add to the solvent yielding the corresponding acetanilides. Triplet sensitisation, however, leads to the triplet phenyl cation, which is reduced in neat solvent and is trapped by pi nucleophiles (allyltrimethylsilane and benzene), resulting in an ionic analogue of the Meerwein or Gomberg arylations. With the 4-nitro derivative intersystem crossing prevails over dediazoniation from the singlet and with the 4-cyano competes with it, so that in those cases the triplet phenyl cation is formed also upon direct irradiation.

Journal Article↗
PubMed2005

The balance between charge transfer and non-charge transfer pathways in the sensitization of singlet oxygen by pi pi* triplet states.

A charge transfer (CT) channel and a non-CT deactivation channel, both leading to formation of O(2)((1)Sigma (g)(+)), O(2)((1) Delta(g)) and O(2)((3)Sigma(g)(-)), compete in the quenching of triplet states by O(2). Recent studies by our group demonstrated that these channels are described by rather simple and general quantitative relations. In the present paper we use the detailed kinetic data on the quenching by O(2) of pi pi* triplet sensitizers of three homologous aromatic series in CCl(4) to derive a parameter, which describes the balance between CT and non-CT deactivation. This quantity, p(CT), is the relative contribution of CT mediated deactivation and is easily calculated for a sensitizer of known triplet energy from its quenching rate constant. The parameter p(CT) quantitatively describes the balance between both deactivation channels without requiring any knowledge of oxidation potentials. It is shown how the variation of p(CT) influences the efficiencies and the rate constants of O(2)((1)Sigma(g)(+)), O(2)((1)Delta(g)) and O(2)((3)Sigma(g)(-)) formation in the quenching process.

Carbon Tetrachloride↗
PubMed2006

Triplet exciplexes as energy transfer photosensitisers.

Experimental evidence is provided for the occurrence of triplet-triplet energy transfer from benzoylthiophene-indole exciplexes to naphthalenes with a remarkable stereodifferentiation; chiral recognition is also observed in the decay of the generated naphthalene triplets.

Journal Article↗
PubMed2006

Competing through-space and through-bond, intramolecular triplet-energy transfer in a supposedly rigid ruthenium(II) tris(2,2'-bipyridine)--fullerene molecular dyad.

A ditopic ruthenium(II) tris(2,2'-bipyridyl)-based fullerene conjugate has been synthesized so as to separate the photoactive terminals by way of a short ethynylene spacer group that is expected to act as a rigid rod. Intramolecular triplet-energy transfer from the metal complex to the fullerene is quantitative at all temperatures and there is no indication for competing electron transfer. Temperature dependence studies indicate two pathways for triplet-energy transfer. An activationless route dominates at low temperature and is attributed to through-bond electron exchange that takes place via super-exchange interactions. The triplet energy of the bridging unit lies well above that of the metal complex. An activated process is switched-on at high temperatures and is believed to involve through-space electron exchange within closed conformations. Molecular dynamics simulations predict that, in addition to an extended conformation, the linker can distort in such a way that the terminals come into orbital contact. In fact, the resultant closed conformation possesses an idealised geometry for fast electron exchange.

Journal Article↗
PubMed1982

Fluorescent triplet probes for measuring the rotational diffusion of membrane proteins.

We have previously described a method for measuring the rotational diffusion of membrane proteins by using fluorescent triplet probes [Johnson & Garland (1981) FEBS Lett. 135, 252-256]. We now describe the criteria by which the suitability of such probes may be judged. In general, the greatest sensitivity is achievable with probes where the ratio of the quantum yields for prompt fluorescene (phi f) and triplet formation (phi t) are high, as with Rhodamine (phi f/phi t congruent to 10(3)). However, considerations of heat generation at the sample membrane, of time resolution of fast rotations and of irreversible bleaching of the fluorescent probe also apply. The immediate environment of a probe molecule at a membrane protein must also be important in determining the performance of a given probe. Nevertheless, we describe guidelines for evaluating the likely usefulness of fluorescent triplet probes in measurements of membrane protein rotation.

Diffusion↗
PubMed2001

Gemcitabine, Ifosfamide and Navelbine (GIN): activity and safety of a non-platinum-based triplet in advanced non-small-cell lung cancer (NSCLC).

To evaluate activity and toxicity of a non platinum-based triplet including Gemcitabine, Ifosfamide and Navelbine (GIN) in advanced NSCLC. Stage IIIB/IV NSCLC patients with WHO PS < 2 and bidimensionally measurable disease entered the study. Gemcitabine 1000 mg/sqm day 1 and 1000-800 mg/sqm day 4, Ifosfamide 3 g/sqm day 1 (with Mesna), Navelbine 25 mg/sqm day 1 and 25-20 mg/sqm day 4 were administered intravenously every 3 weeks. Objective responses (ORs) were evaluated every 2 courses: a maximum of 6 courses were administered in responding patients. According to Simon's optimal two-stage design more than 18 ORs out of 54 patients were required to establish the activity of this regimen. Fifty patients entered the study. Main characteristics of the 48 evaluated patients were: median age 63 years, ECOG performance status 0 = 65%, stage IV disease 79% and non-squamous histology 71%. The total number of courses administered was 200, median per patient 4 (range 1-6). Toxicities were evaluated according to WHO criteria: neutropenia grade 3-4 occurred in 47% of the courses; thrombocytopenia grade 3-4 in 6.6%; anaemia grade 3 in 3.5%. Twelve episodes of febrile neutropenia were reported and three patients required hospital admission. No toxic death was reported. Non-haematological toxicity, including skin rash, alopecia and fatigue, were generally. Twenty-five ORs (1 complete response and 24 partial responses) were obtained for a response rate of 52% (95% CI: 37.4-66.5%). One-year survival was 46.5%. This non-platinum-based outpatient triplet showed promising activity against NSCLC with myelosuppression, in particular neutropenia, being dose-limiting. The GIN regimen may represent a valuable alternative to standard platinum-based doublets and triplets in the treatment of advanced NSCLC and further studies with this platinum-free combination are warranted.

Adult↗
PubMed2005

Theoretical study of photoinduced singlet and triplet excited states of 4-dimethylaminobenzonitrile and its derivatives.

Singlet and triplet low-lying states of the 4-dimethylaminobenzonitrile and its derivatives have been studied by the density functional theory and ab initio methodologies. Calculations reveal that the existence of the methyl groups in the phenyl ring and the amino twisting significantly modify properties of their excited states. A twisted singlet intramolecular charge-transfer state can be accessed through decay of the second planar singlet excited state with charge-transfer character along the amino twisting coordinate or by an intramolecular charge-transfer reaction involved with a locally first excited singlet state. Plausible charge-transfer triplet states and intersystem crossing processes among singlet and triplet states have been explored by spin-orbit coupling calculations. The intersystem crossing process was predicted to be the dominant deactivation channel of the photoexcited 4-dimethylaminobenzonitrile.

Journal Article↗
PubMed2005

Application of equation-of-motion coupled-cluster methods to low-lying singlet and triplet electronic states of HBO and BOH.

The equilibrium structures and physical properties of the X (1)sigma(+) linear electronic states, linear excited singlet and triplet electronic states of hydroboron monoxide (HBO) (A (1)sigma(-), B (1)delta, a (3)sigma(+), and b (3)delta) and boron hydroxide (BOH) (A (1)sigma(+), B (1)Pi, and b (3)Pi), and their bent counterparts (HBO a (3)A('), b (3)A("), A (1)A("), B (1)A(') and BOH X (1)A('), b (3)A('), c (3)A("), A (1)A('), B (1)A('), C (1)A(")) are investigated using excited electronic state ab initio equation-of-motion coupled-cluster (EOM-CC) methods. A new implementation of open-shell EOM-CC including iterative partial triple excitations (EOM-CC3) was tested. Coupled-cluster wave functions with single and double excitations (CCSD), single, double, and iterative partial triple excitations (CC3), and single, double, and full triple excitations (CCSDT) are employed with the correlation-consistent quadruple and quintuple zeta basis sets. The linear HBO X (1)sigma(+) state is predicted to lie 48.3 kcal mol(-1) (2.09 eV) lower in energy than the BOH X (1)sigma(+) linear stationary point at the CCSDT level of theory. The CCSDT BOH barrier to linearity is predicted to lie 3.7 kcal mol(-1) (0.16 eV). With a harmonic zero-point vibrational energy correction, the HBO X (1)sigma(+)-BOH X (1)A(') energy difference is 45.2 kcal mol(-1) (1.96 eV). The lowest triplet excited electronic state of HBO, a (3)A('), has a predicted excitation energy (T(e)) of 115 kcal mol(-1) (4.97 eV) from the HBO ground state minimum, while the lowest-bound BOH excited electronic state, b (3)A('), has a T(e) of 70.2 kcal mol(-1) (3.04 eV) with respect to BOH X (1)A('). The T(e) values predicted for the lowest singlet excited states are A (1)A(")<--X (1)sigma(+)=139 kcal mol(-1) (6.01 eV) for HBO and A (1)A(')<--X (1)A(')=102 kcal mol(-1) (4.42 eV) for BOH. Also for BOH, the triplet vertical transition energies are b (3)A(')<--X (1)A(')=71.4 kcal mol(-1) (3.10 eV) and c (3)A(")<--X (1)A(')=87.2 kcal mol(-1) (3.78 eV).

Journal Article↗
PubMed2005

Ab initio calculation of the NH(3sigma-)-NH(3sigma-) interaction potentials in the quintet, triplet, and singlet states.

We present the ab initio potential-energy surfaces of the NH-NH complex that correlate with two NH molecules in their 3sigma- electronic ground state. Three distinct potential-energy surfaces, split by exchange interactions, correspond to the coupling of the S(A) = 1 and S(B) = 1 electronic spins of the monomers to dimer states with S = 0, 1, and 2. Exploratory calculations on the quintet (S = 2), triplet (S = 1), and singlet (S = 0) states and their exchange splittings were performed with the valence bond self-consistent-field method that explicitly accounts for the nonorthogonality of the orbitals on different monomers. The potential surface of the quintet state, which can be described by a single Slater determinant reference function, was calculated at the coupled cluster level with single and double excitations and noniterative treatment of the triples. The triplet and singlet states require multiconfiguration reference wave functions and the exchange splittings between the three potential surfaces were calculated with the complete active space self-consistent-field method supplemented with perturbative configuration interaction calculations of second and third orders. Full potential-energy surfaces were computed as a function of the four intermolecular Jacobi coordinates, with an aug-cc-pVTZ basis on the N and H atoms and bond functions at the midpoint of the intermolecular vector R. An analytical representation of these potentials was given by expanding their dependence on the molecular orientations in coupled spherical harmonics, and representing the dependence of the expansion coefficients on the intermolecular distance R by the reproducing kernel Hilbert space method. The quintet surface has a van der Waals minimum of depth D(e) = 675 cm(-1) at R(e) = 6.6a0 for a linear geometry with the two NH electric dipoles aligned. The singlet and triplet surfaces show similar, slightly deeper, van der Waals wells, but when R is decreased the weakly bound NH dimer with S = 0 and S = 1 converts into the chemically bound N2H2 diimide (also called diazene) molecule with only a small energy barrier to overcome.

Journal Article↗