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991.
Usha P. Andley Philip Sutherland Jack N. Liang B. Chakrabarti 《Photochemistry and photobiology》1984,40(3):343-349
The effect of 300 nm irradiation on the three lens crystallins, α-, β-, and γ-, was studied by using fluorescence and circular dichroism techniques. α-Crystallin showed a pronounced change in tertiary structure as manifested in fluorescence and circular dichroism measurements. This finding is in agreement with our earlier findings that the tryptophan residues of α-crystallin are more exposed than those of the other two crystallins. The results of studies using inhibitors specific for the different active species of oxygen suggest that H2 O2 -mediated damage is involved in the change of tertiary structure of the proteins. Analyses of circular dichroism spectra indicate that, upon irradiation, the secondary structure of α-crystallin remains virtually unaltered, and that the change in tertiary structure results primarily from photoinduced damage to the tryptophan residues. 相似文献
992.
A series of hydrophilic organic dendron ligands was designed and synthesized for stabilizing high-quality semiconductor and noble metal nanocrystals. The focal point of the dendron ligands is chosen to be a thiol group which is a universal coordinating site for compound semiconductor and noble metal nanocrystals. The methods for binding these dendron ligands onto the surface of the nanocrystals are simple and straightforward. The thin, about 1-2 nm, but closely packed and tangled ligand shell provides sufficient stability for the "dendron-protected nanocrystals" to withstand the rigors of the coupling chemistry and the standard separation/purification techniques. The chemistry presented can be immediately applied for the development of a new generation of biomedical labeling reagents based on high-quality semiconductor nanocrystals. It also provides an alternative path to apply noble metal nanocrystals for developing sensitive detection schemes for chemical and biochemical purposes. The concept may further provide an optimal solution for many other problems encountered in nanocrystal-related research and development, for which the stability of the nanocrystals is a critical issue. Furthermore, the experimental results confirmed that the photochemical stability of colloidal semiconductor and noble metal nanocrystals is the key for developing reliable and reproducible processing chemistry for these nanocrystals. 相似文献
993.
Marino ST Stachurska-Buczek D Huggins DA Krywult BM Sheehan CS Nguyen T Choi N Parsons JG Griffiths PG James IW Bray AM White JM Boyce RS 《Molecules (Basel, Switzerland)》2004,9(6):405-426
In the past decade there has been a significant growth in the sales of pharmaceutical drugs worldwide, but more importantly there has been a dramatic growth in the sales of single enantiomer drugs. The pharmaceutical industry has a rising demand for chiral intermediates and research reagents because of the continuing imperative to improve drug efficacy. This in turn impacts on researchers involved in preclinical discovery work. Besides traditional chiral pool and resolution of racemates as sources of chiral building blocks, many new synthetic methods including a great variety of catalytic reactions have been developed which facilitate the production of complex chiral drug candidates for clinical trials. The most ambitious technique is to synthesise homochiral compounds from non-chiral starting materials using chiral metal catalysts and related chemistry. Examples of the synthesis of chiral building blocks from achiral materials utilizing asymmetric hydrogenation and asymmetric epoxidation are presented. 相似文献
994.
Ren Amsturz Thomas Laube W. Bernd Schweizer Dieter Seebach Jack D. Dunitz 《Helvetica chimica acta》1984,67(1):224-236
Phenylthiomethyllithium/N,N,N′,N′-tetramethylethylenediamine (TMEDA) (1:1) is shown by X-ray analysis to be dimeric in the crystalline state. The structural unit contains a six-membered ring in which each Li is bonded to the terminal C atom of one thioanisole and to the S atom of the other, as well as to the two N atoms of a TMEDA. Methylthiomethyllithium/TMEDA (1:1) is also dimeric, but the structural unit contains a four-membered ring in which each Li is bonded to two C atoms and to the two TMEDA N atoms. An error has been detected in an earlier published electron-density difference map for 2-lithio-2-phenyl-1, 3-dithiane/TMEDA/THF (1:1:1). The conclusion drawn then that this complex exists as a tight ion-pair in the crystal is now retracted. 相似文献
995.
Nat Jack 《The Journal of the Operational Research Society》1991,42(9):759-766
In this paper an integral equation approach is given for evaluating the expected cost of repair replacement policies over finite time horizons. An asymptotic estimate of this expected cost is also obtained. The policy involving imperfect repair on failure with replacement after N failures is taken as an illustrative example and optimal policies N* are found for both infinite and finite time horizons of use. 相似文献
996.
Du H Haddon RC Krossing I Passmore J Rawson JM Schriver MJ 《Chemical communications (Cambridge, England)》2002,(17):1836-1837
The trifluoromethyl-substituted dithiadiazolyl and dithiazolyl radicals, F3CCNSSN (1) and F3CCSNSCCF3 (2) associate through pi*-pi* covalent and electrostatic S delta+...N delta- interactions in the solid state, but melt with a dramatic volume increase to generate paramagnetic liquids; these radicals exhibit thermal hysteresis, which arises through a meta-stable super-cooled liquid state, close to room temperature. 相似文献
997.
998.
Turek AM Krishnamoorthy G Sears DF Garcia I Dmitrenko O Saltiel J 《The journal of physical chemistry. A》2005,109(2):293-303
all-trans-1,6-diphenyl-1,3,5-hexatriene (DPH) fluorescence in solution consists of emissions from the S1 (2(1)A(g)) and S2 (1(1)B(u)) states of the s-trans,s-trans conformer (s-t-DPH) and emission from the S1 state of the s-cis,s-trans conformer (s-c-DPH). The contribution of s-c-DPH fluorescence increases upon excitation at longer wavelengths, and both minor emissions, s-c-DPH and 1(1)B(u) s-t-DPH fluorescence, contribute more at higher temperatures (Ts). Resolution of a spectrothermal matrix of DPH fluorescence spectra by principal component analysis with self-modeling (PCA-SM) is hampered by T-dependent changes in the spectra of the individual components. We avoided differential polarizability-dependent spectral shifts by measuring the spectra in n-alkanes (Cn, C8 to C16 with n even) at T values selected to keep the index of refraction constant, hence under isopolarizability conditions. Compensation of the spectra for T-induced broadening allowed resolution of the spectral matrix into its three components. The optimum van't Hoff plot gives Delta H = 2.83 kcal/mol for s-c-DPH/s-t-DPH equilibration, somewhat smaller than the 3.4 kcal/mol calculated value, and the optimum Boltzmann distribution law plot gives Delta E(ab) = 4.09 kcal/mol for 1(1)B(u)/2(1)A(g) equilibration. The 1(1)B(u) fluorescence spectrum bears mirror-image symmetry with the DPH absorption spectrum, and the energy gap, 1431 cm(-1), is consistent with the 1615 cm(-1) difference between the lowest energy bands in the 1(1)B(u) and 2(1)A(g) fluorescence spectra. The results give V(ab) = 198 +/- 12 cm(-1) for the vibronic matrix coupling element between the 2(1)A(g) and 1(1)B(u) states. Fluorescence quantum yields and lifetimes under isopolarizability conditions reveal an increase in the effective radiative rate constant of s-t-DPH with increasing T. 相似文献
999.
Jack W. Jordan Kayleigh L. Y. Fung Stephen T. Skowron Christopher S. Allen Johannes Biskupek Graham N. Newton Ute Kaiser Andrei N. Khlobystov 《Chemical science》2021,12(21):7377
We induce and study reactions of polyoxometalate (POM) molecules, [PW12O40]3− (Keggin) and [P2W18O62]6− (Wells–Dawson), at the single-molecule level. Several identical carbon nanotubes aligned side by side within a bundle provided a platform for spatiotemporally resolved imaging of ca. 100 molecules encapsulated within the nanotubes by transmission electron microscopy (TEM). Due to the entrapment of POM molecules their proximity to one another is effectively controlled, limiting molecular motion in two dimensions but leaving the third dimension available for intermolecular reactions between pairs of neighbouring molecules. By coupling the information gained from high resolution structural and kinetics experiments via the variation of key imaging parameters in the TEM, we shed light on the reaction mechanism. The dissociation of W–O bonds, a key initial step of POM reactions, is revealed to be reversible by the kinetic analysis, followed by an irreversible bonding of POM molecules to their nearest neighbours, leading to a continuous tungsten oxide nanowire, which subsequently transforms into amorphous tungsten-rich clusters due to progressive loss of oxygen atoms. The overall intermolecular reaction can therefore be described as a step-wise reductive polycondensation of POM molecules, via an intermediate state of an oxide nanowire. Kinetic analysis enabled by controlled variation of the electron flux in TEM revealed the reaction to be highly flux-dependent, which leads to reaction rates too fast to follow under the standard TEM imaging conditions. Although this presents a challenge for traditional structural characterisation of POM molecules, we harness this effect by controlling the conditions around the molecules and tuning the imaging parameters in TEM, which combined with theoretical modelling and image simulation, can shed light on the atomistic mechanisms of the reactions of POMs. This approach, based on the direct space and real time chemical reaction analysis by TEM, adds a new method to the arsenal of single-molecule kinetics techniques.We induce and study reactions of polyoxometalate (POM) molecules, [PW12O40]3− (Keggin) and [P2W18O62]6− (Wells–Dawson), at the single-molecule level, utilising TEM as an analytical tool, and nanotubes as test tubes. 相似文献
1000.
The total synthesis of spectinabilin and its biomimetic conversion to SNF4435C and SNF4435D 总被引:1,自引:0,他引:1
[reaction: see text] A short synthesis of (+/-)-spectinabilin via a trans-selective Suzuki coupling and subsequent Negishi-type methylation, and its biomimetic conversion to (+/-)-SNF4435C and (+/-)-SNF4435D is described. 相似文献