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PubMed2003

Assemblies of Alzheimer's peptides A beta 25-35 and A beta 31-35: reverse-turn conformation and side-chain interactions revealed by X-ray diffraction.

Alzheimer's beta amyloid protein (A beta) is a 39 to 43 amino acid peptide that is a major component in the neuritic plaques of Alzheimer's disease (AD). The assemblies constituted from residues 25-35 (A beta 25-35), which is a sequence homologous to the tachykinin or neurokinin class of neuropeptides, are neurotoxic. We used X-ray diffraction and electron microscopy to investigate the structure of the assemblies formed by A beta 25-35 peptides and of various length sequences therein, and of tachykinin-like analogues. Most solubilized peptides after subsequent drying produced diffraction patterns characteristic of beta-sheet structure. Moreover, the peptides A beta 31-35 (Ile-Ile-Gly-Leu-Met) and tachykinin analogue A beta(Phe(31))31-35 (Phe-Ile-Gly-Leu-Met) gave powder diffraction patterns to 2.8A Bragg spacing. The observed reflections were indexed by an orthogonal unit cell having dimensions of a=9.36 A, b=15.83 A, and c=20.10 A for the native A beta 31-35 peptide, and a=9.46 A, b=16.22 A, and c=11.06 A for the peptide having the Ile31Phe substitution. The initial model was a beta strand where the hydrogen bonding, chain, and intersheet directions were placed along the a, b, and c axes. An atomic model was fit to the electron density distribution, and subsequent refinement resulted in R factors of 0.27 and 0.26, respectively. Both peptides showed a reverse turn at Gly33 which results in intramolecular hydrogen bonding between the antiparallel chains. Based on previous reports that antagonists for the tachykinin substance P require a reverse turn, and that A beta is cytotoxic when it is oligomeric or fibrillar, we propose that the tachykinin-like A beta 31-35 domain is a turn exposed at the A beta oligomer surface where it could interact with the ligand-binding site of the tachykinin G-protein-coupled receptor.

Amino Acids↗
PubMed2002

[Crystallography of drug polymorphism: emergence of new resolution methods and prediction of crystalline structures].

For the pharmaceutical and agrochemical industries, reducing development time is currently a primary objective to accelerate the marketing of new products. In-depth solid-state characterization is an essential element in achieving this performance. For this purpose, it is now recognized that, in a department of solid-state physical characterization, theoretical approaches have to be integrated with experimental tools. Benefiting from the constant improvement in computer hardware and software in terms of computing powers and algorithm development, these approaches appear to be a complementary method that can help explain and rationalize experimental data. In recent years, with the help of both molecular modeling tools and crystallographic databases, original methods have been developed and successfully applied to systems of variable complexity. The X-ray diffraction crystal structure determination makes it possible to calibrate the force field and to adapt it more precisely to the compound studied, thereby reducing one of the sources of uncertainty. Although crystal structures are preferentially determined from single crystal data, it has been recently shown that powder diffraction data can also be used when single crystals are missing. Therefore, the different benefits of such an approach are the following: ab initio predictive study of crystalline polymorphism, molecular packing analysis and detection of cleavage planes, prediction of morphology in vacuum. In conjunction with crystallite preferred orientation observed in powder patterns, grinding effects on the crystal habit and solvent effects in modifying growth can be clearly understood. Selected industrial cases exemplify the operational application of this methodology.

Crystallization↗
PubMed1995

A report on the composition of mercurials used in traditional medicines in Oman.

A brick red powder, used by a 'native physician' (a practitioner of traditional medicine in Oman) to treat a female patient for vitiligo, has been analysed by X-ray powder diffraction and scanning electron microscopy (S.E.M.). It was found to be a mixture of cinnabar (HgS) and calomel (Hg2Cl2). Symptoms of acute mercury poisoning resulted from the patient repeatedly breathing in elemental mercury vapour after this brick red powder and elemental mercury encased in a lime, were thrown on an open fire in a closed room. The patient also presented with central nervous system toxicity which improved gradually after treatment with Dimercaprol was discontinued. The use of mercurials as traditional medicines in Oman is briefly reviewed; as is the variation in literature values for 'normal'/'abnormal'/toxic levels of mercury in human blood.

Adult↗
PubMed2004

Preparation and topotactical oxidation of ScVO3 with bixbyte structure: a low-temperature route to stabilize the new defect fluorite ScVO(3.5) metastable phase.

ScVO3 has been prepared by controlled reduction of a ScVO4 precursor under an H2/N2 flow at 1250 degrees C. The crystal structure of this material has been studied at room temperature by Rietveld refinement of high-resolution neutron powder diffraction (NPD) data. Sc3+ and V3+ are distributed at random over the metal sites of a C-M2O3 bixbyite-type structure, space group Ia3, a = 9.6182(1) Angstroms. The thermal analysis of ScVO3 in an air flow shows two subsequent oxidation processes, with a final reversal to ScVO4 above 600 degrees C. An intermediate phase of composition ScVO(3.5), containing V4+ cations, can be isolated by isothermal annealing at 350 degrees C in air. This metastable phase has been identified by X-ray diffraction (XRD) as a fluorite-type oxide (space group Fm3m, a = 4.947(2) Angstroms), also showing a random distribution of Sc and V cations over the metal positions. The Rietveld refinement of the ScVO(3.5) structure from powder XRD data in a fluorite structural model yields abnormally high thermal factors for the oxygen atoms, suggesting oxygen mobility in this metastable material.

Journal Article↗
PubMed1978

A study of the mineral phase of cementifying fibroma.

Physico-chemical study of a case of cementifying fibroma in the mandible of 78-year-old woman was carried out by X-ray diffraction analysis and thermogravimetric and differential-thermal analyses (TGA-DTA). The center of the tumor consisted of masses of acellular cementum-like tissues; while the periphery consisted of cementicle-like tissues. X-ray powder diffraction patterns showed that the mineral phase of these tissues was apatitic in nature. X-ray line broadening analysis revealed that the crystallinity of apatite of the cementum-like tissue was higher than that of apatite of cementicle-like tissue, and lower than that of bone apatite obtained from the same patient. Furthermore, TGA-DTA studies showed that the mineral (ash) value of cementum-like tissue was slightly higher than that of bone.

Aged↗
PubMed1985

Polymorphism in metoclopramide hydrochloride and metoclopramide.

Metoclopramide hydrochloride (MCPHCl.H2O) and metoclopramide base (MCP) have been studied by DSC, thermomicroscopy, X-ray diffraction and infrared spectroscopy. MCPHCl.H2O does not readily lose water of crystallization either from the solid state or from the melt, but depending on the conditions, dehydration can give rise to two anhydrous polymorphs, MCPHCl/Form I (mp 187 degrees C) and MCPHCL/Form II (mp 155 degrees C). Form I crystallizes from the melt of Form II and not by a reversible solid-solid transition. The anhydrous hydrochloride therefore shows monotropic polymorphism where Form I is the stable polymorph and Form II, a metastable polymorph. Thermal analysis of MCP shows that the base exists as two enantiotropic polymorphs. The transition of the form stable at room temperature (MCP/Form I) to the form stable at high temperatures (MCP/Form II mp 147 degrees C) occurs extremely rapidly at 125 degrees C but the reverse process requires one month at room temperature (approximately equal to 22 degrees C). It is therefore possible to compare the X-ray diffraction powder patterns and infrared spectra of MCP Forms I and II.

Calorimetry, Differential Scanning↗
PubMed2004

Characterization of dehydration behavior of untreated and pulverized creatine monohydrate powders.

Creatine, which is well known as an important substance for muscular activity, is synthesized from amino acids such as glycine, arginine and ornithine in liver and kidney. It then accumulates in skeletal muscle as creatine phosphoric acid. The aim of this study was to understand the dehydration behavior of untreated and pulverized creatine monohydrate at various temperatures. The removal of crystal water was investigated by using differential scanning calorimetry (DSC), X-ray powder diffraction and scanning electron microscopy (SEM). The X-ray diffraction pattern of untreated and pulverized creatine monohydrate agreed with reported data for creatine monohydrate. However, the diffraction peaks of the (100), (200) and (300) planes of pulverized creatine monohydrate were much stronger than those of untreated creatine monohydrate. On the other hand, the diffraction peaks of the (012) and (013) planes of untreated creatine monohydrate were much stronger than those of pulverized creatine monohydrate. The dehydration of untreated and pulverized creatine monohydrate was investigated at various storage temperatures, and the results indicated that untreated and pulverized creatine monohydrate were transformed into the anhydrate at more than 30 degrees C. After dehydration, the particles of untreated and pulverized creatine anhydrate had many cracks. The dehydration kinetics of untreated and pulverized creatine monohydrate were analyzed by the Hancock-Sharp equation on the basis of the isothermal DSC data. The dehydrations of untreated and pulverized creatine monohydrate both followed a zero-order mechanism (Polany-Winger equation). However, the transition rate constant, calculated from the slope of the straight line, was about 2.2-7.7 times higher for pulverized creatine monohydrate than for untreated creatine monohydrate. The Arrhenius plots (natural logarithm of the dehydration rate constant versus the reciprocal of absolute temperature) of the isothermal DSC data for untreated and pulverized creatine monohydrate were linear. The activation energies of dehydration in the 40-60 degrees C range for untreated and pulverized creatine monohydrate were 15.02 and 10.1 kJ/mol, respectively. Dehydration of untreated creatine monohydrate had a pronounced effect on the particle size of the powder. Compared with pulverized creatine monohydrate, the particle size of untreated creatine monohydrate was significantly decreased by dehydration.

Calorimetry, Differential Scanning↗
PubMed2003

[Preparation and pharmaceutical properties of salcatonin dry powder inhalations].

AIM: To prepare salcatonin dry powder inhalations (sCT-DPIs) A (mixture of mannitol and L-leucine) and B (mixture of manntiol and lactose) by spray-drying and then to study their main pharmaceutical properties. METHODS: Dumping rate of sCT-DPIs capsules and deposited fraction of sCT at effective part were determined according to Chinese Pharmacopiea 2000. Particle morphology under different relative humidity (RH) was observed by scanning electronics microphotograph, particle size and its distribution were determined by Malvern Mastersizer and the transition of morphorous state for carriers before and after spray-drying was investigated by differential thermal analysis (DTA) and X-ray powder diffraction (XRPD). RESULTS: Dumping rates of sCT-DPIs A and B capsules were both above 10% and deposited fraction of sCT at effective part was above 90% for both A and B, which were all in agreement with the standard of Chinese Pharmacopiea 2000. Powder particle of sCT-DPIs A was round and existed one by one after keeping one month under RH 0, 23% and 52%, but aggregation can be observed under RH 75%; many particles which were also round agglomerated in sCT-DPIs B even under zero RH; mean particle size of sCT-DPIs A was 1.67 microns, which was much smaller than that of sCT-DPIs B; In sCT-DPIs A particle with empty core which was lighter than the same size particle with concreted core was observed. It was shown by DTA that melted heat of L-leucine in sCT-DPIs composed of mannitol and L-leucine lowered much more than that of L-leucine exisited alone after spray-drying. It was confirmed by XRPD that diffraction intensity of carriers in sCT-DPIs decreased more than that of carriers before spray-drying. CONCLUSION: Round particle can be made when mannitol was added to carriers and ultra low density carriers can be formed when L-leucine was added. It was suggested by SEM that DPIs should be kept under certain RH. Particle size and distribution of sCT-DPIs all accorded with demand of DPIs. Complex spray-drying carriers formed amorphous state easier than single carrier.

Administration, Inhalation↗
PubMed2006

Solid-state characterization of two polymorphic forms of R-albuterol sulfate.

R-albuterol (levalbuterol) is a drug used for asthma therapy and some formulations of it are in solid dosage forms. The aim of this work was to describe and characterize two polymorphic modifications of R-albuterol sulfate by means of typical structure-sensitive analytical techniques such as X-ray powder diffraction, FT-IR spectroscopy, visual and microscopic inspection, and DSC. Substantial differences were observed between the solid-state properties of the crystals, confirming the existence of at least two polymorphic forms for R-albuterol sulfate: Form I and Form II.

Albuterol↗
PubMed2004

Determination of the physical state of norethindrone acetate containing transdermal drug delivery systems by isothermal microcalorimetry, X-ray diffraction, and optical microscopy.

Transdermal drug delivery systems (TDDS) enable a controlled drug delivery to the skin. The low permeability of the stratum corneum necessitates a high drug concentration of the polymeric matrix and often requires supersaturation. This, however, promotes crystallisation of supersaturated systems. Isothermal microcalorimetry at 25 degrees C, polarisation light microscopy, and X-ray powder diffraction (XRPD) were used to characterise the crystal growth of norethindrone acetate (NEA). The solubility of NEA in the patches determined by these methods is about 4%. The crystallisation process could be measured reliably and with a high accuracy by microcalorimetry and microscopy. XRPD was considerably less sensitive but was the only method allowing a semi-quantitative determination of the amounts of crystals formed. The drug-associated heat measured by microcalorimetry increased proportionally with increasing NEA concentration in the concentration range of 4-10% demonstrating a constant crystallisation rate. At a higher supersaturation, such as 12% drug content, the crystallisation process was accelerated. The application of Johnson-Mehl-Avrami kinetics for the analysis of nucleation and crystal growth of the punched patches indicated a site-saturated nucleation mechanism and a one-dimensional crystal growth. The crystallisation enthalpy of NEA was -22.8+/-2.6 kJ/mol. The most specific method to observe the crystal formation is polarisation light microscopy. However, the microscopic analysis requires much longer storage times than microcalorimetry to detect crystallisation.

Administration, Cutaneous↗
PubMed2001

From V8Ga36.9Zn4.1 and Cr8Ga29.8Zn11.2 to Mn8Ga27.4Zn13.6: a remarkable onset of Zn-cluster formation in an intermetallic framework.

The series of isotypic compounds V8Ga41 --> V8Ga36.9Zn4.1 --> Cr8Ga29.5Zn11.2 --> Mn8Ga27.4Zn13.6 with the V8Ga41 structure type (space group R3, Z = 3) was prepared and structurally characterised by X-ray diffraction experiments (V8Ga41: a 13.9351(5), 14.8828(12); V8Ga36.9Zn4.1: a = 13.9244(7), c = 14.8660(9): Cr8Ga29.8Zn11.2: 13.7153(5), c = 14.6872(9); Mn8Ga27.4Zn13.6: a = 13.6033(6), c = 14.6058(16)). The site occupancies of the ternary compounds were refined from neutron powder-diffraction data and exposed a startling segregation of Zn and Ga, which finally resulted in the formation of separated Zn13 cluster entities-corresponding to almost ideal centred cuboctahedra or small pieces of fcc metal-in the Mn compound, which has the highest Zn content in the series. The homogeneity ranges of the underlying phases T8Ga41 xZnx were determined to be 0 < x < 4.1(3), 8.7(3) < x < 11.2(3) and 13.6(4) < x < 16.5(3) for T = V, Cr and Mn, respectively. The different ranges of composition of the phases reflect the requirement of an optimum electron concentration for a stable V8Ga41-type structure, which is in the narrow range between 159 and 165 electrons per formula unit. First-principles electronic-structure calculations could explain this fact by the occurrence of a pseudo gap in the density of states at which the Fermi level is put for this particular electron concentration. Furthermore the nature of the Zn/Ga segregation was revealed: T-Zn interactions were found to be considerably weaker than those for T-Ga. This places the Zn atoms as far as possible from the T atoms, thus leading to the formation of cuboctahedral Zn13 entities.

Journal Article↗
PubMed2006

Kinetics and equilibria of lysozyme precipitation and crystallization in concentrated ammonium sulfate solutions.

The kinetics and thermodynamics of lysozyme precipitation in ammonium sulfate solutions at pH 4 and 8 and room temperature were studied. X-ray powder diffraction (XRD) was used to characterize the structure of lysozyme precipitates. It was found that, if sufficient time was allowed, microcrystals developed following an induction period after initial lysozyme precipitation, even up to ionic strengths of 8 m and at acidic pH, where lysozyme is refractory to crystallization in ammonium sulfate. The full set of precipitation and crystallization data allowed construction of a phase diagram of lysozyme, showing the ammonium sulfate dependence. It suggests that precipitation may reflect a frustrated metastable liquid-liquid phase separation, which would allow this process to be understood within the framework of the generic phase diagram for proteins. The results also demonstrate that XRD, more frequently used for characterizing inorganic and organic polycrystalline materials, is useful both in characterizing the presence of crystals in the dense phase and in verifying the crystal form of proteins.

Ammonium Sulfate↗
PubMed2000

Stability of polymorphic forms of ranitidine hydrochloride.

Ranitidine-HCl can exist in two different polymorphic forms: form I (m.p. 134-140 degrees C) and form II (m.p. 140-144 degrees C). In the present study the stability of form I of ranitidine-HCl to a selection of powder pretreatments, to reflect conditions which might occur in manufacturing procedures, and also to a limited range of storage conditions was investigated. The original samples of form I and form II used were characterised by X-ray powder diffraction (XRPD), hot stage microscopy (HSM) and differential scanning calorimetry (DSC). A quantitative XRPD method for determining the fraction of form II in the presence of form I was used. XRPD data were analysed using regression techniques and artificial neural networks (ANN). The quantitative XRPD technique was then used to monitor the relative proportion of form II in each treated sample. Pretreatments of form I included (i) mixing with form II or with common excipients (ii) compression and grinding (iii) contact with solvents (followed by drying) before storage. Storage conditions involved three temperatures (20 degrees C, 30 degrees C, 42 degrees C) and three relative humidities (45% RH; 55% RH; 75% RH). Samples were stored for a period of 6 months. A limited factorial design was used. No increase in the form II:form I ratio was observed in the following pretreatment processes: introduction of form II nuclei into form I; introduction of excipients to form I; compression of form I powder at 5 and 15 tons; normal mixing and grinding processes; addition of isopropanol (IPA) or water/IPA mix followed by drying. In the pretreatment process where water was added to form I powder (with most or all of the powder dissolving), drying of the liquefied mass led to a mix of form I and form II. On storage at room temperature (20-30 degrees C), low relative humidity (45-55% RH), and in an air-tight container there was no increase in the form II:form I ratio. Storage of form I/form II mixes, particularly at high humidity, resulted in a preferential loss of form II (compared to form I). Loss was greater at 30 degrees C/75% RH than at 20 degrees C/75% RH. Form II was also preferentially lost under low humidity conditions created by a saturated solution of potassium carbonate (45% RH) at the elevated temperature of 42 degrees C. This environment was shown to be acidic.

Calorimetry, Differential Scanning↗
PubMed2001

Formation of gallium dimers in the intermetallic compounds R(5)Ga(3) (R = Sc, Y, Ho, Er, Tm, Lu). Deformation of the Mn(5)Si(3)-type structure.

The R(5)Ga(3) (R = Sc, Y, Ho, Er, Tm, Lu) phases were prepared by high-temperature solid-state techniques. The structure of monoclinic Sc(5)Ga(3) was determined by single-crystal X-ray diffraction means (C2/m, No. 12, Z = 4, a = 8.0793(5) A, b = 14.003(1) A, c = 5.9297(3) A, beta = 90.994(5) degrees ), and those of the isotypic R(5)Ga(3), R = Y, Ho, Er, Tm, Lu, were determined by Guinier powder diffraction. The new Sc(5)Ga(3) structure is a deformation of the hexagonal Mn(5)Si(3) type (P6(3)/mcm) and contains two types of gallium dimers with d(Ga-Ga) = 2.91 and 3.14 A. The closely spaced Sc1 chains in the parent Mn(5)Si(3) type transform to zigzag chains in concert with displacements of the uniformly spaced gallium atoms to form dimers within distorted confacial square antiprisms of Sc. Matrix effects appear important in the different Ga(2) bond lengths. Electronic calculations reveal that the transformation from the hypothetical Mn(5)Si(3) to the Sc(5)Ga(3) type is aided by antibonding Ga-Ga interactions between the dimers that are pushed above E(F) and Ga-Ga and Ga-Sc bonding states just below E(F) that are stabilized. Sc(5)Ga(3) is appropriately metallic. Except for R = Sc, Lu, the arc-melted R(5)Ga(3) compounds above slowly transform on annealing at 1150 degrees C and below into tetragonal Ba(5)Si(3)-type structures.

Journal Article↗
PubMed2001

Time-resolved in situ neutron diffraction studies of gas hydrate: transformation of structure II (sII) to structure I (sI).

We report the in situ observation from diffraction data of the conversion of a gas hydrate with the structure II (sII) lattice to one with the structure I (sI) lattice. Initially, the in situ formation, dissociation, and reactivity of argon gas clathrate hydrate was investigated by time-of-flight neutron powder diffraction at temperatures ranging from 230 to 263 K and pressures up to 5000 psi (34.5 MPa). These samples were prepared from deuterated ice crystals and transformed to hydrate by pressurizing the system with argon gas. Complete transformation from D(2)O ice to sII Ar hydrate was observed as the sample temperature was slowly increased through the D(2)O ice melting point. The transformation of sII argon hydrate to sI hydrate was achieved by removing excess Ar gas and exposing the hydrate to liquid CO(2) by pressurizing the Ar hydrate with CO(2). Results suggest the sI hydrate formed from CO(2) exchange in argon sII hydrate is a mixed Ar/CO(2) hydrate. The proposed exchange mechanism is consistent with clathrate hydrate being an equilibrium system in which guest molecules are exchanging between encapsulated molecules in the solid hydrate and free molecules in the surrounding gas or liquid phase.

Journal Article↗
PubMed2005

Crystal structure of MCM-70: A microporous material with high framework density.

The crystal structure of the borosilicate MCM-70 (siliceous framework formula Si12O24) was determined from synchrotron powder diffraction data with the program FOCUS. The framework crystallizes in space group Pmn2(1), where a = 13.663, b = 4.779, c = 8.723 A, and forms 1D ellipsoidal 10-ring channels with the following dimensions: 5.0 x 3.1 A. Rietveld refinement of the model against synchrotron powder data from solvated material gives Rwp = 0.15, R(F2) = 0.11. In addition to the four tetrahedral sites and seven framework oxygens, one potassium position is found during this refinement. Because of the unreasonable geometry of five putative extraframework oxygen sites, another synchrotron pattern was obtained from a dehydrated specimen. A refinement in space group P1n1 (removing the mirror operation of Pmn2(1)), where a = 13.670, b = 4.781, c = 8.687 A, and beta = 90.24 degrees , verified that the previous framework geometry is preserved as well as the potassium position. One extraframework oxygen was located that would yield a reasonable K-O distance. The existence of potassium is verified by electron energy dispersive spectroscopic measurements as well as quantitative elemental analysis. (There are approximately 2.35 K sites per 12 Si in the unit cell.) It is likely that the constricted channels occlude KOH. 11B and 29Si MAS NMR measurements indicate a framework SiO2/B2O3 of approximately 40:1, which is consistent with a wavelength dispersive spectroscopic measurement. The silicate framework density is 2.10 gm/cm3. The resulting framework density for T sites, 21.1, is unusually high for a zeolite, just below the value for paracelsian (21.4) or scapolite (21.8), each of which also has a smallest four-ring loop. The 1H --> 29Si CP MAS measurements suggest sample heterogeneity, that is, a portion of the sample that is strongly coupled to hydrogen and efficiently cross polarizes and another portion that does not.

Journal Article↗
PubMed2001

Hydrothermal synthesis and structure determination from powder data of new three-dimensional titanium(IV) diphosphonates Ti(O(3)P-(CH(2))(n)-PO(3)) or MIL-25(n) (n = 2, 3).

Ti(O(3)P-(CH(2))(n)-PO(3)) or MIL-25(n) (n = 2, 3) were prepared under hydrothermal conditions (4 days, 463 K, autogenous pressure). Their structures were determined ab initio from X-ray diffraction powder data. MIL-25(2) is triclinic (space group P-1 (no. 2)), with a = 5.033(1), b = 5.092(1), c = 6.859(1) A, alpha = 95.860(1) degrees, beta = 99.994(1) degrees, gamma = 118.217(1) degrees, and Z = 2. MIL-25(3) exhibits an orthorhombic symmetry (space group Cm2m (no. 38)), with a = 5.230(1), b = 8.451(1), c = 17.400(2) A, and Z = 4. Their three-dimensional structures are built up from TiO(6) titanium(IV) octahedra linked together via diphosphonate groups. This leads to pillared structures whose inorganic sheets are closely related to those of the alphaTiP titanium phosphate structure.

Journal Article↗
PubMed1986

Solid-state 13C NMR and X-ray diffraction of dermatan sulfate.

Dermatan sulfate in the solid state has been studied by 13C CP/MAS nmr and X-ray diffraction in order to establish the ring conformation of the L-iduronate moiety. The solid state nmr spectrum is similar to the solution spectrum obtained previously, indicating that a ring conformation at least approximating to 1C4 predominates in the solid state. X-ray powder diffraction data from the same sample indicate the presence of the 8-fold helix form previously observed by fiber diffraction, and interpreted in terms of a 4C1 ring form. A likely explanation of the results is that a distorted 1C4 L-iduronate ring conformation, not considered in the initial X-ray analysis, may emerge to provide a satisfactory interpretation of all available physical-chemical data.

Animals↗