Biomedical subjects
Publications and source records attributed to Shijun Zheng.
Extended bis(donor)-substituted squaraine chromophores exhibit very high two-photon cross-sections (as high as 33 000 GM) in the near-IR; these can be attributed to the combination of large transition dipoles with small detuning energies. The modulus of the third-order nonlinear optical susceptibility at 1.3 mum has been found to be 7.0 x 10-11 esu for one of these chromophores.
Potential-energy surfaces for unimolecular decomposition of B(N3)(3) have been studied to understand the possible mechanism for BN generation. The decomposition of B(N3)(3) takes place on either the singlet and triplet surface, and both processes are high exothermic and obey sequential mechanisms. For the singlet reaction, the rate-determining step corresponds to cleavage of the first azide bond and linear (1)NBNN instead of (1)BN was suggested as the dominant product at room temperature. For the triplet surface, a fragment process from (3)BN(7) to (3)BN(5) is predicted to be the rate-determining step; once this barrier is counteracted, the subsequent decomposition processes could easily occur to form final product (3)BN. In addition, the possible mechanism for generating BN film via B(N3)(3) was discussed based on MC-SCF calculation results. These findings might be helpful in understanding the controllable decomposition of B(N3)(3) as well as its application in generating BN films.
Organic materials with large third-order nonlinearities in the near-infrared spectral regime are critical in the development of photonic devices to be utilized in all-optical signal processing. We have developed polymethine materials, specifically bisdioxaborine-terminated polymethine dyes, which possess large ultrafast third-order nonlinearities and low nonlinear loss all in the near-infrared spectral regime. An extended bisdioxaborine polymethine anion exhibited the largest value of gamma (third-order microscopic nonlinearity) at 1.3 mum (|gamma| = 5.7 x 10-32 esu) and showed no characteristics of symmetry breaking, unlike other polymethines of similar lengths. A neat film of this molecule maintained relatively low linear loss in the near-infrared and showed a large third-order macroscopic nonlinearity at 1.3 mum (|chi(3)| = 3.6 x 10-10 esu), with a temporal response of less than 8 ps. Furthermore, the real part of chi(3) was nearly an order-of-magnitude larger than the imaginary component. Consequently, this material exhibited good figures of merit for all-optical signal processing throughout the entire telecommunications band.
Both the singlet(1A') and triplet(3A'') potential energy surfaces (PESs) of F+N(3) reactions are investigated using the complete-active-space self-consistent field (CASSCF) and the multireference configuration interaction (MRCI) methods with a proper active space. The minimum energy crossing point (MECP) at the intersection seam between the 1A' and 3A'' PESs is located and used to clarify the reaction mechanisms. Two triplet transition states are found, with one in the cis form and the other one in the trans form. Further kinetic calculations are performed with the canonical unified statistical (CUS) theory on the singlet PES and the improved canonical variational transition-state (ICVT) method on the triplet PES. The rate constants are also reported. At 298 K, the calculated rate constant is in reasonably good agreement with experimental values, and spin-orbit coupling effects lower it by 28 %. The spectroscopic constants derived from the fitted potential-energy curves for the singlet and triplet states of NF are in very good agreement with experimental values. Our calculations indicate that the adiabatic reaction on the singlet PES leading to NF(a(1)Delta)+N(2) is the major channel, whereas the nonadiabatic reaction through the MECP, which leads to NF(X(3)Sigma(-))+N(2), is a minor channel.
Density functional calculations have been carried out on a series of BCN hybrid fullerenes with certain substitution patterns in comparison with their parent compounds Cn (n = 30, 32, 36, 38, 40, 44, 48, 50, 52). The substitutional structures, energy gaps between the highest occupied molecular orbital and the lowest unoccupied molecular orbital, ionization potentials, electron affinities, as well as molecular electrostatic potentials have been systematically investigated. The following important points of BCN hybrid fullerenes are stressed: The present studied fullerenes, comprising tubular "belt" and polar "cap", could be divided into three types of structure; each has different indexes of tubular structure and terminal caps. The properties of BCN hybrid fullerenes depend on the type of "tubular belt + polar cap" structures, especially, the HOMO and LUMO characters and MEPs of BCN fullerene are strongly governed by their structure types.
We report the first structural data for bis(diarylamine) "bipolarons": we have isolated and crystallographically characterized salts of the dications obtained by two-electron oxidation of E-4,4'-bis[di(p-anisyl)amino]stilbene and E,E-2,5-bis{4-[di(p-anisyl)amino]styryl}-3,4-di(n-butoxy)thiophene, [1](2+) and [2](2+) respectively. ESR, NMR, and magnetometry suggest both species have singlet ground states. X-ray structures, together with (1)H NMR coupling constants for [2](2+), indicate geometries in which the bond lengths are shifted toward a quinoidal pattern relative to that in the neutral species, but not to a fully quinoidal extent. In particular, the bond-length alternations across the vinylene bridging groups approach zero. DFT calculations with closed-shell singlet configurations reproduce the observed structures well. Our results indicate that singlet species for which one might expect quinoidal geometries (with differences of ca. 0.1 A between formally single and double bonds) on the basis of a limiting valence-bond representation of the structure can, in fact, show structures with significantly different patterns of bond lengths.
[reaction: see text] The unusually stable perchloro-2,5,8-triazaphenalenyl radical 1 and its twisted dechlorinated dimer 2 were synthesized and characterized by ESR spectroscopy and X-ray crystallography. The X-ray structure of dimer 2 shows that the double bond connecting the two triazaphenalene systems is strongly twisted. Dimer 2 has a dramatic color shift from the solid state to solution, which may be due to a change of the twisting angle between both states.
The reactions of INCO to IOCN, INCS to ISCN, and INCSe to ISeCN have been studied at the MP2/6-311++G(2df)//B3LYP/6-311++G(2df) level. Geometries of reactants, transition states, and products have been optimized, and geometries of transition states are reported for the first time. The reasons that INCO, ISCN, and ISeCN are easily detected instead of IOCN, INCS, and INCSe have been explained successfully. The breakage and formation of chemical bonds along the reaction paths have been discussed by the topological analysis method of electronic density. The calculated results show that there are two kinds of structure transition states (STS) in the reactions studied.
Pure thionyl azide SO(N(3))(2), which is the only gaseous reaction product, has been generated in a vacuum by the heterogeneous reaction of SOCl(2) vapor with AgN(3) at room temperature at a SOCl(2) vapor pressure of 1 x 10(-3) Torr. Evidence for the formation of SO(N(3))(2) is given by on line photoelectron spectroscopy (PES) combined with outer valence Green's function (OVGF) calculations with the 6-311++G(2df) basis set. The good agreement between the PES experiment and the OVGF calculation shows that SO(N(3))(2) has C(1) symmetry. The first ionization energy of SO(N(3))(2) is 10.18 eV.
Development of type I diabetes requires coordinated expression of myriad genes responsible for the initiation and progression of the disease. Expression of these genes are regulated by a small number of transcription factors including the Rel/NF-kappaB family. To determine the roles of the Rel/NF-kappaB family in type I diabetes, we studied multiple low-dose streptozotocin-induced diabetes in mice deficient in either c-Rel or NF-kappaB1. We found that mice deficient in each of these NF-kappaB subunits were resistant to streptozotocin-induced diabetes. However, the mechanisms of the disease resistance may differ in different cases. Deficiency in c-Rel selectively reduced Th1, but not Th2 responses, whereas NF-kappaB1 deficiency had little effect on T cell responses to anti-CD3 stimulation. Death of dendritic cells was accelerated in the absence of NF-kappaB1, whereas death of macrophages and granulocytes was affected primarily by c-Rel deficiency. Furthermore, Stat-1 expression was significantly reduced in macrophages deficient in NF-kappaB1, but not c-Rel. These results indicate that both c-Rel and NF-kappaB1are essential for the development of type I diabetes and that strategies targeting each of these subunits would be effective in preventing the disease.
The 2-azaphenalenyl radical 2 has been synthesized and characterized by ESR spectroscopy. Variable-temperature ESR measurements were carried out on both the phenalenyl (1) and the 2-azaphenalenyl (2) radicals. The phenalenyl radical 1 has the known propensity to dimerize at temperatures below 20 degrees C, but unexpectedly less so than originally reported. The first experimental measurement of bond dissociation enthalpy for the dimerization of the phenalenyl radical 1 was obtained in CCl(4) (11.34 +/- 0.11 kcal/mol) and toluene (9.8 +/- 0.7 kcal/mol). The 2-azaphenalenyl radical 2 does not show a propensity to dimerize over the measurable temperature range (220-330 K), but does so in the presence of Cu(hfac)(2) (hfac = hexafluoroacetylacetonate). The latter complex was characterized by X-ray crystallography.
Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) selectively induces apoptosis of tumor cells but not most normal cells. Its roles in normal nontransformed tissues are not clear. To explore the potential roles of TRAIL in type 1 diabetes, we examined the consequences of TRAIL blockade or TRAIL deficiency in two animal models of autoimmune diabetes. In the first model, NOD mice received an injection of a soluble TRAIL receptor to block TRAIL function. This significantly accelerated the diabetes and increased the degree of autoimmune inflammation in both pancreatic islets and salivary glands. The GAD65-specific immune responses were also significantly enhanced in animals that received the soluble TRAIL receptor. In the second model, we treated normal and TRAIL-deficient C57BL/6 mice with multiple low-dose streptozotocin to induce diabetes. We found that both the incidence and the degree of islet inflammation were significantly enhanced in TRAIL-deficient animals. On the basis of these observations, we conclude that TRAIL deficiency accelerates autoimmune diabetes and enhances autoimmune responses.
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