647 research outputs found
X-RAY STRUCTURAL STUDIES ON 3 ANALOGS OF THE ALPHA-ANOMER OF THE ANTITUMOR ANTIBIOTIC SHOWDOMYCIN - DIFFERENTIAL RING-PUCKERING EFFECTS OF HYDROXYL SUGAR SUBSTITUENTS IN LYXO AND ARABINO CONFIGURATIONS
The crystal structures of three a-analogues of the anititumour antibiotic showdomycin (1) have been determined at room temperature. They are 2-(a-D-2'- deoxyribofuranosyl)maleimide (2), 2-(a-D-arabinofuranosyl) maleimide (3) and 2-(a-D-lyxofuranosyl)- maleimide (4). The structures were refined to R factors of 0-039, 0-042 and 0-029 for 971, 1025 and 716 observed reflections. The conformational properties of the sugar rings are discussed in detail, in relation to the differing hydroxyl group substitutions. Compounds (2) and (3) have CY-endo, C4"-exo and CY-endo ring puckers, whereas (3) has C2"-endo, C l'-exo puckers. Compound (3) has an intramolecular hydrogen bond between the 05' and 02' hydroxyl groups. This and the sugar pucker difference are in accord with NMR chemical shift data for the O5' and O2' protons. Crystal data: compound (2), C9H11NOs, orthorhombic, P212~21, a = 5-916 (1), b = 8-191 (1), c = 19.691 (3) A, Z = 4; compound (3), C9HIINO6, orthorhombic, P212121, a = 6.785 (1), b = 8.006 (1), c = 17.564 (2) A, Z= 4; compound (4), C9HI1NO6, monoclinic, P2~, a = 8.681 (1), b = 5.135 (1), c= 11-364 (1) A, Z= 2
CRYSTAL AND MOLECULAR-STRUCTURE OF 2',3',5'-TRI-O-ACETYL-6-O-(MESITYLENESULPHONYL)GUANOSINE
The structure of a 6-O-mesitylenesulphonyl derivative of 2',3',5'-tri-O-acetylguanosine, C 24 H 29 N50 10 S, has
been determined by X-ray diffraction. Crystals are monoclinic, a = 26.370 (4), b = 8.200 (2), c =
17.991 (3) A, fl = 132.77 (4) o. The solution of the structure in space group C2 was not straightforward and is
described in detail. Refinement converged at R = 0.110 for 1102 observed reflections. The guanine base
displays some deviations from its usual geometry due to the loss of C(6)-O(6) double-bond character. The
ribose sugar is C(2')-endo puckered
Polymorphic G:G mismatches act as hotspots for inducing right-handed Z DNA by DNA intercalation
DNA mismatches are highly polymorphic and dynamic in nature, albeit poorly characterized structurally. We utilized the antitumour antibiotic CoII(Chro)2 (Chro = chromomycin A3) to stabilize the palindromic duplex d(TTGGCGAA) DNA with two G:G mismatches, allowing X-ray crystallography-based monitoring of mismatch polymorphism. For the first time, the unusual geometry of several G:G mismatches including syn-syn, water mediated anti-syn and syn-syn-like conformations can be simultaneously observed in the crystal structure. The G:G mismatch sites of the d(TTGGCGAA) duplex can also act as a hotspot for the formation of alternative DNA structures with a GC/GA-5' intercalation site for binding by the GC-selective intercalator actinomycin D (ActiD). Direct intercalation of two ActiD molecules to G:G mismatch sites causes DNA rearrangements, resulting in backbone distortion to form right-handed Z-DNA structures with a single-step sharp kink. Our study provides insights on intercalators-mismatch DNA interactions and a rationale for mismatch interrogation and detection via DNA intercalation
Diffraction in low-energy electron scattering from DNA: bridging gas phase and solid state theory
Using high-quality gas phase electron scattering calculations and multiple
scattering theory, we attempt to gain insights on the radiation damage to DNA
induced by secondary low-energy electrons in the condensed phase, and to bridge
the existing gap with the gas phase theory and experiments. The origin of
different resonant features (arising from single molecules or diffraction) is
discussed and the calculations are compared to existing experiments in thin
films.Comment: 40 pages preprint, 12 figures, submitted to J. Chem. Phy
Targeting the ALS/FTD-associated A-DNA kink with anthracene-based metal complex causes DNA backbone straightening and groove contraction
The use of a small molecule compound to reduce toxic repeat RNA transcripts or their translated aberrant proteins to target repeat-expanded RNA/DNA with a G4C2 motif is a promising strategy to treat C9orf72-linked disorders. In this study, the crystal structures of DNA and RNA-DNA hybrid duplexes with the -GGGCCG- region as a G4C2 repeat motif were solved. Unusual groove widening and sharper bending of the G4C2 DNA duplex A-DNA conformation with B-form characteristics inside was observed. The G4C2 RNA-DNA hybrid duplex adopts a more typical rigid A form structure. Detailed structural analysis revealed that the G4C2 repeat motif of the DNA duplex exhibits a hydration shell and greater flexibility and serves as a 'hot-spot' for binding of the anthracene-based nickel complex, NiII(Chro)2 (Chro = Chromomycin A3). In addition to the original GGCC recognition site, NiII(Chro)2 has extended specificity and binds the flanked G:C base pairs of the GGCC core, resulting in minor groove contraction and straightening of the DNA backbone. We have also shown that Chro-metal complexes inhibit neuronal toxicity and suppresses locomotor deficits in a Drosophila model of C9orf72-associated ALS. The approach represents a new direction for drug discovery against ALS and FTD diseases by targeting G4C2 repeat motif DNA
Phase transition of a single star polymer: a Wang-Landau sampling study
Star polymer is a typical nonlinear macromolecule possessing special
thermodynamic behaviors for the existence of a jointing point. The
thermodynamic transitions of a single star polymer are systematically studied
with bond fluctuation model using Wang-Landau sampling technique. A new
analysis method applying the shape factor is proposed to determine coil-globule
(CG) and liquid-crystal (LC) transitions, which shows higher efficiency and
precision than canonical specific heat function. It is found that the LC
transition of star polymer at lower temperature obeys the identical scaling law
as linear polymer. With the increase of the arm density of star polymer,
however, the CG transition point, corresponding to {\theta} temperature, shifts
towards the LC transition and the reason comes from the high density arms of
star polymer, which requires the lower temperature for attracting force to
overcome the volume excluding effects of chain. This work clearly demonstrates
that the distinction of linear and star polymers in structures only affects CG
transition and has no influence on LC transition.Comment: 30 pages, 10 figures, submit to JC
Studies on 1,3-diaryltriazene analogues of berenil: molecules with potential GC base-pair selectivity
Partial core power transformer
This thesis describes the design, construction, and testing of a 15kVA, 11kV/230V partial
core power transformer (PCPT) for continuous operation. While applications for the partial
core transformer have been developed for many years, the concept of constructing a partial
core transformer, from conventional copper windings, as a power transformer has not been
investigated, specifically to have a continuous operation. In this thesis, this concept has been
investigated and tested.
The first part of the research involved creating a computer program to model the physical
dimensions and the electrical performance of a partial core transformer, based on the existing
partial core transformer models.
Also, since the hot-spot temperature is the key factor for limiting the power rating of the
PCPT, the second part of the research investigates a thermal model to simulate the change of
the hot-spot temperature for the designed PCPT. The cooling fluid of the PCPT applied in
this project was BIOTEMP®. The original thermal model used was from the IEEE Guide for
Loading Mineral-Oil-Immersed transformer. However, some changes to the original thermal
model had to be made since the original model does not include BIOTEMP® as a type of
cooling fluid. The constructed partial core transformer was tested to determine its hot-spot
temperature when it is immersed by BIOTEMP®, and the results compared with the thermal
model.
The third part of the research involved using both the electrical model and the thermal model
to design a PCPT. The PCPT was tested to obtain the actual electrical and the thermal
performance for the PCPT.
The overall performance of the PCPT was very close to the model estimation. However,
cooling of the PCPT was not sufficient to allow the PCPT to operate at the design rated load
for continuous operation. Therefore, the PCPT was down rated from 15kVA to maintain the
hot-spot temperature at 100°C for continuous operation. The actual rating of the PCPT is 80%
of the original power rating, which is 12kVA
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