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1.
The structure of α-chitin has been determined by X-ray diffraction, based on the intensity data from deproteinized lobster tendon. Least-squares refinement shows that adjacent chains have alternating sense (i.e. are antiparallel). In addition, there is a statistical distribution of side-chain orientations, such that all the hydroxyl groups form hydrogen bonds. The unit cell is orthorhombic with dimensions a = 0.474 ± 0.001 nm, b = 1.886 ± 0.002 nm and c = 1.032 ± 0.002 nm (fiber axis); the space group is P2 12 12 1 and the cell contains disaccharide sections of the two chains passing through the center and corner of the ab projection. The chains form hydrogen-bonded sheets linked by CO…HN bonds approximately parallel to the a axis, and each chain has an O-3′H…O.5 intramolecular hydrogen bond, similar to that in cellulose. Adjacent chains along the ab diagonal have different conformations for the CH 2OH groups: on one chain these groups form O.6H…O.6′ intermolecular hydrogen bonds to the CH 2OH group on the adjacent chain along the ab diagonal. The latter group is oriented to form an intramolecular O.6′H…O.7 bond to the carboxyl oxygen on the next residue. The results indicate that a statistical mixture of CH 2OH orientations is present, equivalent to half oxygens on each residue, each forming inter- and intramolecular hydrogen bonds. As a result the structure contains two types of amide groups, which differ in their hydrogen bonding, and account for the splitting of the amide I band in the infrared spectrum. The Inability of this chitin polymorph to swell on soaking in water is explained by the extensive intermolecular hydrogen bonding. 相似文献
2.
The crystal and molecular structure of a tri- O-ethylamylose polymorph, TEA 3, has been solved by stereochemical conformation and packing analysis, combined with X-ray fibre diffraction analysis. The unit cell is orthorhombic, space group P2 12 12 1, with a 15.36 (±0.03) Å, b 12.18 (±0.05) Å, and c (fibre repeat) 15.48 (±0.01) Å. The actual chain conformation is a 4 3 helix with the EtO-6 group in the tg position, as was found in the polymorph TEA 1. 相似文献
3.
The title compound, when recrystallised from water, is monoclinic, space group P2 1, with a = 5.774(4), b = 7.189(5), c = 12.69(1) Å, β = 106.66(5)°, and Z = 2. The crystal structure was determined from three-dimensional X-ray diffraction data taken on an automatic diffractometer with CuKα, and refined by least-squares techniques to R = 0.034 for 977 reflexions. The pyranose ring adopts the 4C1 conformation. The conformation about the exocyclic C-5-C-6 bond is gauche-trans [the torsion angles O-6-C-6-C-5-O-5 and O-6-C-6-C-5-C-4 are 64.2(8) and ?175.6(7)°, respectively], which is significantly different from the gauche-gauche geometry in d-glucose 6-(barium phosphate). The phosphate ester bond, P-O-6, is 1.584(3) Å. All of the oxygen-bonded hydrogen atoms are involved in intermolecular hydrogen-bonds. 相似文献
4.
The crystal structure of zinc citrate [Zn(II) (C 6H 5O 7) 2·4NH 4+] shows isolated zinc ions octahedrally coordinated to two equivalent citrates via a central hydroxyl, central carboxyl, and one terminal carboxyl from each citrate. The clusters are linked through hydrogen bonds to ammonium ions in the lattice. The structure is distinctly different from that of other divalent cation triply ionized citrate complexes, which are polymeric. Crystal data : space group P2 1/ C, a = 8.784(3) Å, b = 13.499(4) Å, c = 9.083(3) Å, β = 113.4°(1), V = 988(1) Å 3. Citrate has been identified as the low molecular weight ligand that complexes zinc in human milk; this may be of interest in relation to intestinal zinc absorption. 相似文献
5.
The crystal and molecular structure of [Y(OH 2) 8]Cl 3·(15-crown-5) has been determined by single- crystal X-ray diffraction. The complex crystallizes in the monoclinic space group P2 1/ n with Z = 4. Lattice parameters are a = 9.202(2), b = 17.247(3), c = 15.208(3) Å, and β = 92.39(2)°. The structure was solved by Patterson and Fourier techniques and refined by least-squares to a final conventional R value of 0.081. The Y(III) ion is eight coordinate, bonded to the oxygen atoms of the eight water molecules. Three of the water molecules are hydrogen bonded to crown ether molecules. The three chloride ions participate in hydrogen bonds with the remaining five water molecules. The YO(water) distances range from 2.322(6) to 2.432(7) Å and average 2.37(4) Å. The average O(water)···Cl and O(water)···O(crown) hydrogen bonded separations are 3.08(4) and 2.76(7) Å, respectively. 相似文献
6.
The interaction of hydrated chloride salts of Gd 3+ and Lu 3+ with 15-crown-5 in a 1:3 mixture of CH 3OH:CH 3CN produces crystalline [M(OH 2) 8]Cl 3· (15-crown-5) (M = Gd, Lu). The crystal and molecular structures of both complexes have been determined by single crystal X-ray diffraction. Both are isostructural with previously determined Y analog and crystallize in the monoclinic space group P2 1/ n with Z = 4. Lattice parameters are a = 9.247(4), b = 17.312(5), c = 15.191(6) Å, β = 92.19(3)°, Dcalc = 1.72 g cm −3 for M = Gd and a = 9.150(1), b = 17.171(1), c = 15.217(1) Å, β = 92.64(1)°, Dcalc = 1.80 g cm −3 for M = Lu. Each complex was refined by least-squares to final conventional R values of 0.052 (M = Gd, 2932 observed [ Fo⩾5σ( Fo) reflections) and 0.036 (M = Lu, 3313 observed reflections). The octaaquo M(III) ions exist as a distorted dodecahedron with average MOH 2 separations of 2.41(4) Å (M = Gd) and 2.35(4) Å (M = Lu). The crown ether molecule is hydrogen bonded to metal coordinated water molecules to form polymeric chains along b. The remaining water molecule hydrogen atoms participate in hydrogen bonds with the chloride ions essentially in the ac plane. Two resolvable disordered crown ether conformations are observed with occupancies of 60%/40% (M = Gd) and 75%/25% (M = Lu). 相似文献
7.
The crystal structure of a regenerated form of (1→3)-α-d-glucan, obtained by solid state deacetylation of the triacetate derivative, has been determined by combined X-ray diffraction analysis and stereochemical model refinement. The structure crystallizes in an orthorhombic unit cell with parameters and c (fibre repeat)=8.44 Å, and space group P2 12 12 1. The chain conformation is nearly completely extended and is very close to a 2/1 helix, even though the dimer residue is the crystallographic repeat unit. An intramolecular O(2) O(4)′ hydrogen bond stabilizes the conformation and extensive intermolecular hydrogen-bonding abilizes the packing. The resulting structure is sheet-like, with an alternating polarity of chain directions within the sheet. In its sheet-like character, extensive hydrogen-bonding, and insolubility in water, this polymorph of (1→3)-α-d-glucan resembles regenerated cellulose. The reliability of the structure analysis is indicated by the X-ray residual R=0.206. 相似文献
8.
Drosophila melanogastercasein kinase II (DmCKII) is composed of catalytic α and regulatory β subunits associated as an α 2β 2heterotetramer. Using the two-hybrid system, we have screened a Drosophilaembryo cDNA library for proteins that interact with DmCKII α. One of the cDNAs encodes a novel β-like polypeptide, which we designate β′. In situhybridization localizes the corresponding gene to 56F1-2, a site distinct from that of both the β gene and the Stellatefamily of β-like sequences. The predicted sequence of β′ is more closely related to the β subunit of Drosophilaand other metazoans than to the Stellate family of proteins, suggesting that it is a second regulatory subunit. In vitroreconstitution studies show that a GST-β′ fusion protein associates with the α subunit to generate a tetrameric complex with regulatory properties similar to those of the native α 2β 2holoenzyme. The data are consistent with the proposed role of the β′ subunit as an integral component of the holoenzyme. 相似文献
9.
Corals and other biomineralizing organisms use proteins and other molecules to form different crystalline polymorphs and biomineral structures. In corals, it’s been suggested that proteins such as Coral Acid Rich Proteins (CARPs) play a major role in the polymorph selection of their calcium carbonate (CaCO 3) aragonite exoskeleton. To date, four CARPs (1–4) have been characterized: each with a different amino acid composition and different temporal and spatial expression patterns during coral developmental stages. Interestingly, CARP3 is able to alter crystallization pathways in vitro, yet its function in this process remains enigmatic. To better understand the CARP3 function, we performed two independent in vitro CaCO 3 polymorph selection experiments using purified recombinant CARP3 at different concentrations and at low or zero Mg 2+ concentration. Our results show that, in the absence of Mg 2+, CARP3 selects for the vaterite polymorph and inhibits calcite. However, in the presence of a low concentration of Mg 2+ and CARP3 both Mg-calcite and vaterite are formed, with the relative amount of Mg-calcite increasing with CARP3 concentration. In all conditions, CARP3 did not select for the aragonite polymorph, which is the polymorph associated to CARP3 in vivo, even in the presence of Mg 2+ (Mg:Ca molar ratio equal to 1). These results further emphasize the importance of Mg:Ca molar ratios similar to that in seawater (Mg:Ca equal to 5) and the activity of the biological system in a aragonite polymorph selection in coral skeleton formation. 相似文献
10.
Some theoretical studies have predicted that the conformational freedom of the α-aminoisobutyric acid (H-Aib-OH) residue is restricted to the α-helical region of the Ramachandran map. In order to obtain conformational experimental data, two model peptide derivatives, MeCO-Aib-NHMe 1 and Bu tCO-LPro-Aib-NHMe 2 , have been investigated. The Aib dipeptide 1 crystallizes in the monoclinic system ( a = 12.71 Å, b = 10.19 Å, c = 7.29 Å, β = 110.02°, Cc space group) and its crystal structure was elucidated by x-ray diffraction analysis. The azimuthal angles depicting the molecular conformation (? = ?55.5°, ψ = ?39.3°) fall in the α-helical region of the Ramachandran map and molecules are hydrogen-bonded in a three-dimensional network. In CCl 4 solution, ir spectroscopy provides evidence for the occurrence of the so-called 5 and C 7 conformers stabilized by the intramolecular i → i and i + 2 → i hydrogen bonds, respectively. The tripeptide 2 was studied in various solvents [CCl 4, CD 2Cl 2, CDCl 3, (CD 3) 2SO, and D 2O] by ir and pmr spectroscopies. It was shown to accommodate predominantly the βII folded state stabilized by the i + 3 → i hydrogen bond. All these experimental findings indicate that the Aib residue displays the same conformational behavior as the other natural chiral amino acid residues. 相似文献
11.
The melting behavior of a variety of saturated long chain compounds is shown to be related to hydrocarbon chain length by the equation TN = C0 + T∞N where T is the absolute melting temperature, and N is the number of long chain carbon atoms. The constants C0 and T∞ are determined graphically or analytically from TN vs. N data. The linear relationship, derived from fundamental thermodynamic principles, is empirically satisfied. For each homologous series considered, coefficients of the equation provide a rational means for correlation and comparison with other polymorphs and indicate the relative importance of chain length, chain parity (even or odd), and headgroup polarity to melting behavior. 相似文献
13.
Two zinc complexes—trichloroadeninium zinc(II)(Form 11), C 5H 6N 5Cl 3Zn [structure(I)] and a similar complex of Arprinocid, (6-amino-9-(2-chloro-6-fluorobenzyl)purine], C 12H 10N 5FCl 4Zn [structure(II)]—have been prepared Structure(I) crystallizes in the space group P2 1/ c with a = 8.223(1)Å, b = 6.755(1) Å, c = 18.698(3) Å, β = 96.10(2)°,and Z = 4. Structure(II) crystallizes in the space group P2 1/ c with a = 8.209(2) Å, b = 6.421(8) Å, c = 31.794(8) Å, β = 90.76(2)°, and Z = 4. Both of these structures were solved by the heavy atom method using diffractometric data and refined to R = 0.028 [structure(I)] and 0.038 [structure(II)]. Zinc with a distorted tetrahedral coordination having three chlorines and N(7) as ligators, protonation of the adenine moiety at N(1), dissymmetry of exocyclic angles at N(7), and an interligand hydrogen bond (“indirect chelation”) involving one of the three chlorines, coordinated to zinc and a proton of the exocylic amino group are the striking features common to both structures. Similar types of indirect chelation as observed in the different complexes of purines have been discussed. The zinc ion deviates from the imidazole plane by 0.412 Å in structure(I) and 0.524 Å in Structure(II). The imidazol and pyrimidine planes fold about the C(4)-C(5) bond by 2.4° in strctur(I) and 3.8° in structure(II). In structure(I), inversion related molecules are paired through N(9)-H…N(3) hydrogen bonds. N-H…Cl hydrogen bonds and C(8)-H…Cl interactions have been observed in both structures. 相似文献
14.
The β structure of seven periodic copolypeptides of l-alanine and glycine 2 namely: A 2G, AG, A 2G 2, A 3G 3, A 2G 3, AG 2 and AG 3 has been studied by infrared, X-ray and electron diffraction techniques. The sheets are made of anti-parallel chains and intersheet spacing is observed to depend both on residue sequence and composition. Samples with glycine molar fractions varying from 0.33 to 0.6 are found to be good models for group II of silk fibroins. The structure of Bombyx mori silk fibroin is discussed in the light of these new data. 相似文献
15.
β-Helical poly(L -glutamic acid) in a gel state was found to be easily converted to the antiparallel β form by heating. Two β forms were obtained, depending on the temperature of heating. Temperatures between 40° and 85°C produced a β form with a spacing between pleated sheets (d 001) of 9.03 Å, termed β 1. If the heating was carried out at temperatures higher than 85°C, the β 1 form underwent another conformational transition reducing the d 001 value from 9.03 to 7.83 Å (termed β 2) without any prominent change in the fiber repeat distance (i.e., the polypeptide backbone conformation). The time course of these two transitions was followed by measuring the infrared spectra of the samples, and it was concluded that the α → β 1 transition in its initial stage obeys a pseudo-first order rate process with activation enthalpy and entropy of 54 kcal/mol and 92 eu, respectively. On the other hand, the typical sigmoidal conversion curves observed for the transition between the two types of β forms (β 1 → β 2) indicate that this transition proceeds via a socalled “nucleation and growth” process. The kinetic theory of phase transitions developed by Avrami can be applied with success to explain this transition. The infrared spectra, in the region from 1800 to 200 cm ?1, were measured for these two β forms and the results showed that the conformation of the side chains and the mode of the hydrogen bonding between the side-chain carboxyl groups undergo appreciable change during the transition. The heat-induced conformational transition of poly(L -Glu 78 L -Val 22) was also studied. The copolymer was transformed from the α-helical conformation directly to the β 2 form. The reason for this was thought to be due to the fact that the L -valine residues and the L -glutamyl residues near the L -valine residues have a strong tendency to take the more compact β 2 form. 相似文献
16.
When slowly evaporated, the reaction of NdCl 3· nH 2O with 15-crown-5 in a 3:1 mixture of acetonitrile:methanol produces two crystalline hydrates. The decahydrate, [Nd(OH 2) 9]Cl 3·15-crown-5·H 2O, is orthorhombic, P2 12 12 1, with (at −150 °C) a = 10.571(4), b = 15.220(7), c = 15.686(7) Å, and Dcalc = 1.71 g cm −3 for Z = 4. These crystals are stable to the moisture in air. Each Nd is nine-coordinate with tricapped trigonal prismatic geometry. The nine coordinated water molecules are hydrogen bonded to two symmetry related crown ethers, all three chloride ions, and the tenth water molecule. The crown has a total of six hydrogen bonds, four on one side (two to a single oxygen atom) and two on the other. This ether exhibits conformational disorder. The hexahydrate, [NdCl 2(OH 2) 6]Cl·15-crown-5 is deliquescent, dissolving in air and recrystallizing as [NdCl 2(OH 2) 6]Cl. Crystals of this complex are monoclinic, P2 1/ n, with (at 20 °C) a = 9.821(3), b = 16.978(9), c = 12.849(8) Å, β = 94.06(5)°, and Dcalc = 1.80 g cm −3 for Z = 4. The Nd atom exists in a distorted dodecahedral geometry with one chlorine in an A site and one in a B site. The coordinated chlorine atoms accept hydrogen bonds producing polymeric zigzag hydrogen bonded chains along c. The third noncoordinated chloride ion accepts four hydrogen bonds, three from one formula unit and one from a second formula unit related by a unit translation along a. The crown ethers accept five hydrogen bonds, two on one side, and three on the other, thus separating the zigzag chains along b. 相似文献
17.
Herein, we report the synthesis and characterization of a third polymorph of trans-[Co(2,3,2-tet)(NO 2) 2]NO 3, III, crystallizing in space group (No. 2) obtained during an attempt to reproduce the synthesis of a previously reported polymorph, I (for more details of polymorphs I and II, see Introduction and references cited therein). The cations of polymorphs I and II differ primarily by the angles that the planes of the two -NO 2 ligands make with one another; the former being considerably larger than that in II. Polymorph III resembles II in that the torsional angular differences between the trans-nitro ligands are also small, but differ notably from that in I.The structure of the compound [(5-Me-(dpt)Co(NO 2) 3], was determined also. The space group is P2 1/ n, with two molecules in the asymmetric unit, whose occupancies are 65% and 35% for molecules IVa and IVb, respectively. Again, the two differ by the torsional angles of the nitro ligands, specially two of them whose angular orientations are vastly different. Molecules IVa and IVb are compared with a previously obtained polymorph V of this same compound reported earlier. Once more, V is closely related in stereochemistry to IVb, but differs markedly from IVa in nitro torsional angles.In all cases, the Co(amine) fragments are closely super-imposable and the differences in nitro torsional angles are the result of the availability of several amine hydrogens of the basal plane with which to make intra-molecular hydrogen bonds. Clearly, these hydrogen bonds must be of very similar strength and the barriers to rotation of the -NO 2 ligands must have energies similar to the energetics of the hydrogen bonds causing the torsional motions. 相似文献
18.
Single crystal electron diffraction patterns are obtained from thin microcrystals of two polymorphic forms of a pure ketonic wax secreted by the wooly alder aphid, Prociphilus tessellatus. One crystalline form gives intensity data which agree well with the commonly observed O⊥ methylene subcell. The unit cell constants for the projection are and . The other polymorph, which is found less often, is a rectangular supercell with pseudohexagonal symmetry in the intensity-weighted hk0 reciprocal net. The supercell parameters in projection are , and with the inner order hexad of intense spots occurring at 4.20 Å. 相似文献
19.
The hydrogen ion changes resulting from the photolysis of the rod visual pigment, rhodopsin, have been investigated. Low temperature was used to isolate the metarhodopsin I 478 to II 380 reaction of rhodopsin and indicator dye was used to simultaneously measure the hydrogen ion changes of the rhodopsin solution.The results indicate that illuminated rhodopsin takes up a proton during the metarhodopsin I 478 to II 380 reaction and releases protons at later intermediate stages. The results are consistent with data indicating p K changes of rhodopsin as the basis for the R 2 phase of the early receptor potential and hydrogen ion changes of the medium or p K changes of rhodopsin as having effects on the late receptor potential. 相似文献
20.
Otoliths, calcium carbonate (CaCO3) ear stones of fish, contain a wealth of information about fish life and environmental history yet the CaCO3 polymorph form the otolith is made of is a critical, but seldom considered, piece of information during otolith analysis. Otolith trace element chemistry data increasingly informs management decisions, but recent work has shown that CaCO3 polymorphs—aragonite, vaterite, and calcite—can bear on incorporation of trace elements in a non-trivial way. Most fishes are thought to have otoliths of the aragonite CaCO3 form, but this construct is potentially outdated with many recent literature reports showing otherwise. Our study used previously unpublished neutron diffraction data and reports from published literature to address three objectives: (1) summarize the relative effects of otolith CaCO3 polymorphism on otolith microchemistry, (2) summarize reports of otolith polymorphs to gain a better understanding of the extent of non-aragonite otoliths among fishes, (3) outline future research needed to align interpretations of microchemistry with our current understanding of otolith polymorph diversity. We found that while aragonite otoliths are the most common, so are exceptions. For example, the ostensibly rare (among species) CaCO3 form vaterite was reported in at least some otoliths of 40% of the species surveyed. Our work suggests that examination of the CaCO3 polymorph composition of otoliths should become more common particularly in studies where results will or may be used to inform management decisions. Future research should work to attribute controls on otolith CaCO3 polymorph expression using a combination of -omics and material characterization approaches to enrich the life history and environmental information output from otoliths and increase our understanding of the assumptions made in otolith trace element chemistry studies. 相似文献
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