Air-dried Crystals Research Articles

Exploring the impact of water on the morphology and crystallinity of xylan hydrate nanotiles

There has been a resurgence of studies on xylan particles describing various properties and exploring new applications. The aim of this study was to analyze xylan hydrate crystals in the wet state and after air-drying using state-of-art imaging techniques in order to assess the impact of water on both crystallinity and particle morphology. Xylan from esparto grass (Stipa tenacissima) was crystallized and formed convex platelets, termed ‘nanotiles’. Fully hydrated xylan crystals were examined in a layer of vitreous ice by cryogenic electron microscopy. Selected area electron diffraction of the xylan hydrate crystals revealed an oriented crystalline core, unlike the dried crystals that showed no orientation. The surface topographies and thickness of wet and air-dried xylan nanotiles were observed using atomic force microscopy imaging in both liquid and in air. X-ray diffraction was used to assess the crystallinity of xylan nanotiles after drying to varying levels. Air-dried crystals gave diffraction maxima corresponding to xylan hydrate, while wet crystals gave diffraction maxima corresponding to xylan dihydrate. This study offers new insight into xylan hydrate particles, focusing on the role of water on their crystallinity, ultrastructure, and orientation of the crystalline layers.

Extension of Azine-Triazole Synthesis to Azole-Triazoles Reduces Ligand Field, Leading to Spin Crossover in Tris-L Fe(II).

The first examples of azole-triazole Rat ligands, bidentate L4NMeIm(3-(1-methyl-1H-imidazol-4-yl)-5-phenyl-4-(p-tolyl)-4H-1,2,4-triazole) and L4SIm (4-(5-phenyl-4-(p-tolyl)-4H-1,2,4-triazol-3-yl)thiazole), have been prepared, by extension of the general synthesis used to access many examples of azine-triazoles. The tris-L FeII complexes of the azine-triazoles are consistently low spin (LS). As intended, these new azole-triazole ligands provide lower field strengths, resulting in high-spin (HS) [FeII(L4NMeIm)3](BF4)2 (1·4H2O) and spin crossover (SCO) active [FeII(L4SIm)3](BF4)2 (2·0.5H2O). Single-crystal structure determinations revealed that at 100 K 1·solvents is HS whereas 2·solvents is LS. Solid-state variable temperature magnetic studies of air-dried crystals showed that the methylimidazole-triazole complex 1·4H2O remains HS while the thiazole-triazole complex 2·0.5H2O undergoes a two-step gradual SCO (T1/2 approximately 275 and 350 K). Variable-temperature Evans method NMR studies of 2, in five different solvents (CD3NO2, CD3CN, CD3COCD3, CD2Cl2, and CDCl3) gave T1/2 values in a relatively narrow range, 214-259 K. These T1/2 values did not correlate with the solvent polarity index P' (R2 = 0.25) but did correlate with the solvent basicity parameter SB (R2 = 0.90). Variable-temperature UV-vis studies on a golden yellow CH3CN solution of 2, with monitoring of the d-d transition at 530 nm (ε = 39 L mol-1 cm-1 at 293 K) while the solution was heated from 253 to 303 K, showed that the high-spin fraction increased from 0.51 to 0.77. Cyclic voltammetry studies in CH3CN revealed a Fe(III)/Fe(II) redox process that was reversible for 1 and irreversible for 2, with significant tuning of the Epa value: the methylimidazole-triazole complex 1 is significantly easier to oxidize (0.46 V) than the thiazole-triazole complex 2 (0.68 V; both vs 0.01 M Ag/AgNO3).

Pseudopolymorphs of LB30870, a Direct Thrombin Inhibitor: One-Dimensional Solvent Channel Structures Explain Reversible Hydration/Dehydration

Three different hydrates of LB30870, a new direct thrombin inhibitor, were identified during the screening of solid form, and their interconversion relationship and relative thermodynamic stabilities were investigated. Form I (hexahydrate) changes to Form II (dihydrate) or Form III (tetrahydrate) by dehydration, while Form II becomes Form I by hydration, and both Form II and Form III change to Form I in the aqueous slurry. Single crystals obtained from two different crystallization conditions, wet or air-dried, were found to be isostructural with a difference in the solvent channels, and based on the simulated and experimental powder X-ray diffraction patterns, the air-dried crystals are assigned as Form I and Form III, respectively. In all crystal structures, LB30870 is in a folded conformation forced by the presence of strong hydrogen bonds by two structural water molecules. The solvent channel formed can hold up to six additional hydration water sites per each LB30870, and the one-dimensional solvent c...

Spin Crossover in Dinuclear N4S2 Iron(II) Thioether-Triazole Complexes: Access to [HS-HS], [HS-LS], and [LS-LS] States.

Access to a new family of thioether-linked PSRT ligands, 4-substituted-3,5-bis{[(2-pyridylmethyl)sulfanyl]methyl}-4H-1,2,4-triazoles (analogues of the previously studied amino-linked PMRT ligands), has been established. Four such ligands have been prepared, PSPhT, PS(i)BuT, PS(t-Bu)PhT, and PS(Me)PhT, with R = Ph, (i)Bu, (t-Bu)Ph, and (Me)Ph, respectively. Three dinuclear colorless to pale green iron(II) complexes, [Fe(II)2(PSRT)2](BF4)4·solvent, featuring N4S2 donor sets, were prepared. Single-crystal structure determinations on [Fe(II)2(PSPhT)2](BF4)4·2MeCN·H2O, [Fe(II)2(PSPhT)2](BF4)4·2(1)/2MeCN·(1)/2H2O·THF, [Fe(II)2(PS(Me)PhT)2](BF4)4·2MeCN, and [Fe(II)2(PS(i)BuT)2](BF4)4·4MeCN reveal that all four are stabilized in the [HS-HS] state to 100 K and that both possible binding modes of the bis-terdentate ligands, cis- and trans-axial, are observed. Variable-temperature magnetic susceptibility studies of air-dried crystals (solvatomorphs of the single crystal samples) reveal the first examples of spin crossover (SCO) for a dinuclear iron(II) complex with N4S2 coordination. Specifically, [Fe(II)2(PSPhT)2](BF4)4·2(1)/2H2O undergoes a multistep but complete SCO from [HS-HS] to [LS-LS], whereas [Fe(II)2(PS(Me)PhT)2](BF4)4·1(1)/2MeCN·2H2O exhibits a half-SCO from [HS-HS] to [HS-LS]. In contrast, [Fe(II)2(PS(i)BuT)2](BF4)4·MeCN·H2O remains [HS-HS] down to 50 K. The reflectance spectrum of pale green [Fe(II)2(PSPhT)2](BF4)4·(1)/2CHCl3·2(1)/2H2O (solvatomorph A) reveals a trace of LS character (572 nm band (1)A1g → (1)T1g). Evans' (1)H NMR method and UV-vis spectroscopy studies revealed that on cooling dark green acetonitrile solutions of these complexes from 313 to 233 K, all three undergo SCO centered at or near room temperature. The tendency of the complexes to go LS in solution reflects the electronic impact of R on the σ-donor strength of the PSRT ligand, whereas the opposite trend in stabilization of the LS state is seen in the solid state, where crystal packing effects, of the R group and solvent content, dominate the SCO behavior.

Environmental Influence on the Single-Molecule Magnet Behavior of [MnIII6CrIII]3+: Molecular Symmetry versus Solid-State Effects

The structural, spectroscopic, and magnetic properties of a series of [Mn(III)(6)Cr(III)](3+) (= [{(talen(t-Bu(2)))Mn(III)(3)}(2){Cr(III)(CN)(6)}](3+)) compounds have been investigated by single-crystal X-ray diffraction (XRD), Fourier transform infrared (FT-IR) and electronic absorption spectroscopy, elemental analysis, electro spray ionization-mass spectrometry (ESI-MS) and matrix-assisted laser desorption ionization-mass spectrometry (MALDI-MS), cyclic voltammetry, AC and DC magnetic measurements, as well as theoretical analysis. The crystal structures obtained with [Cr(III)(CN)(6)](3-) as a counterion exhibit (quasi-)one-dimensional (1D) chains formed by hydrogen-bonded (1) or covalently linked (2) trications and trianions. The rod-shaped anion lactate enforces a rod packing of the [Mn(III)(6)Cr(III)](3+) complexes in the highly symmetric space group R3[overline] (3) with a collinear arrangement of the molecular S(6) axes. Incorporation of the spherical anion BPh(4)(-) leads to less-symmetric crystal structures (4-6) with noncollinear orientations of the [Mn(III)(6)Cr(III)](3+) complexes, as evidenced by the angle between the approximate molecular C(3) axes taking no specific values in the range of 2°-69°. AC magnetic measurements on freshly isolated crystals (1a and 3a-6a), air-dried crystals (3b-6b), and vacuum-dried powder samples (3c-6c) indicate single-molecule magnet (SMM) behavior for all samples with U(eff) values up to 28 K. The DC magnetic data are analyzed by a full-matrix diagonalization of the appropriate spin-Hamiltonian including isotropic exchange, zero-field splitting, and Zeeman interaction, taking into account the relative orientation of the D-tensors. Simulations for 3a-6a and 3c-6c indicate a weak antiferromagnetic exchange between the Mn(III) ions in the trinuclear subunits (J(Mn-Mn) = -0.70 to -0.85 cm(-1), Ĥ(ex) = -2∑(i<j )J(ij)Ŝ(i)·Ŝ(j)) that is overcome by the stronger antiferromagnetic interaction via the Cr-C≡N-Mn pathway (J(Cr-Mn) = -3.00 to -5.00 cm(-1)), leading to an overall ferrimagnetic coupling scheme with an S(t) = (21)/(2) spin ground state. The differences in U(eff), J(Mn-Mn), and J(Cr-Mn) for the investigated samples are rationalized in terms of subtle variations in the molecular and crystal structures. In particular, a magnetostructural correlation between the Mn-N(C≡N) bond length and the J(Cr-Mn) exchange coupling is inferred from the magnetic measurements and corroborated by DFT calculations. The results of this detailed study on [Mn(III)(6)Cr(III)](3+) allow the formulation of some key recipes for a rational improvement of the SMM behavior.

Atomic force microscopy of the three-dimensional crystal of membrane protein, OmpC porin

Three-dimensional microcrystals of OmpC osmoporin were air-dried slowly and imaged in air with an atomic force microscope (AFM). The overall structural features in AFM images are in good agreement with the X-ray diffraction data of these OmpC osmoporin crystals: monoclinic P2 1 with the unit cell constants a=117.6 Å, b=110 Å, c=298.4 Å, β=97°. Such a good correspondence between X-ray diffraction and AFM data suggests that the slow and mild air-drying of these crystals did not induce any significant alterations in the crystal lattices as expected upon crystal dehydration. At the (100) crystal face, individual trimeric protein–detergent complexes were resolved. These results show the potential for studying the molecular structure of microcrystals of integral membrane proteins. This study also suggests that the crystal grew in a fashion of rapid two-dimensional expansion along the bc plane followed by a slow deposition along the a axis, perhaps as a rate-limiting nucleation process. Thus, AFM imaging of air-dried crystals would also be of considerable use in the early stages of a project to grow large three-dimensional crystals of membrane proteins suitable for high-resolution X-ray diffraction studies.

Molecular and Crystal Structures of Inulin from Electron Diffraction Data

The crystal and molecular structures of inulin, the poly(2→1)-β-d-fructofuranan, hemihydrate and monohydrate, were determined by a constrained linked-atom least-squares (LALS) refinement, utilizing intensities measured from electron diffraction data and stereochemical restraints. Single crystals were grown from dilute aqueous ethanol solutions and their three-dimensional electron diffraction diagrams analyzed. In the hemihydrate form, there is half a molecule of water per fructosyl residue. In this allomorph, which corresponds to air-dried crystals, inulin adopts an orthorhombic P212121 space group with cell parameters a = 16.70 Å, b = 9.65 Å, and c (chain axis) = 14.4 Å. By using the base plane electron diffraction data combined with the upper layer line data, the best molecular model was refined to an R factor of 22.7% and R‘‘ factor of 22.3%. This model has two antiparallel sixfold helices centered on the twofold screw axes intersecting the base plane of the orthorhombic system. The six water molecules are distributed over eight sites in the unit cell. The helical conformation is characterized by φ = 66°, ψ = 154°, and ω = −82°. In the monohydrate form obtained when inulin crystals were kept in a moist environment, there is one molecule of water per fructosyl residue. The unit cell dimensions remained nearly the same (a = 16.70 Å, b = 9.80 Å, and c (chain axis) = 14.7 Å) but substantial differences in the electron diffraction intensities were observed. The best molecular model was refined to an R factor of 18.1% and an R‘‘ factor of 16.5% by using only hk0 reflections of the base plane electron diffraction data. Simulated diffraction patterns of this structure showed that its differences with the hemihydrate could be accounted for only by introducing extra water molecules into the gaps of the crystalline structure without major modification of the inulin conformation.

X-ray and neutron diffraction studies of the crystal and molecular structure of the predominant monocarboxylic acid obtained by mild acid hydrolysis of cyanocobalamin. Part I. X-ray diffraction studies of airdried crystals

The crystal structure of air-dried crystals of the O-monocarboxylic acid of cyanocobalamin, E2, has been solved by x-ray analysis. The lattice constants area =14-51 A,b = 1709 A,c = 16.35A, β = 103°; space group P21. Phase angles were derived from observations on anomalous dispersion effects for 2567 of the measured 2875 pairs of reflections,hkl andhkl, and used in the solution of the structure. The intensities of reflexions were eye estimated from Weissenberg photographs. The molecular arrangement is very different from that of other cyanocobalamins; the corrin rings of neighbouring molecules are atca 90° to one another. It was not possible to recognise the one acid group among the amides with certainty.

Tyrocidine A: Crystal growth and preliminary X-ray diffraction data

Tyrocidine A was crystallized from 2-methyl-2,4-pentanediol and water, or methanol, to yield crystals that are large enough for X-ray diffraction studies. Four crystals were examined; three were in equilibrium with mother liquor and the fourth was air-dried. They belong to the rhombohedral space group R32. Parameters of the hexagonal cell of the fully solvated crystals vary slightly and are approximately a = 34 A ̊ and c = 50 A ̊ . The asymmetric unit consists of one molecule of tyrocidine and several molecules of 2-methyl-2,4-pentanediol. Air-dried crystals appear to contain about half the number of solvent molecules. Three-dimensional X-ray diffraction data showing a maximum resolution of s = (1.7 A ̊ ) −1 have been recorded.

The structure of vitamin B12. VIII. The crystal structure of vitamin B12-5'-phosphate

The crystal structure of vitamin B12-5' -phosphate, a biosynthetic precursor of vitamin B12, has been determined by X-ray diffraction methods. The air-dried crystals are orthorhombic, space groupP212121, witha= 23.72 Å,b= 21.74 Å andc= 16.07 Å. The observed density of 1.362 g cm-3indicates four vitamin B12-5' -phosphate and about sixty water molecules in the unit cell. The X-ray diffraction data to 1.2 Å resolution were collected with a PAILRED linear diffractometer using Cu Kαradiation from a silicon monochromator. The structure was solved by using Patterson methods in conjunction with the tangent formula and was refined by Fourier and least-squares methods. The finalRvalue for the 2112 observed reflexions is 16.2 %. The structure is very similar to that found by Hodgkin and co-workers for air-dried vitamin B12crystals. The difference Fourier synthesis between this compound and vitamin B12shows that two water molecules move into phosphate oxygen positions when the phosphate in the precursor is removed, and one acetamide in contact with these water molecules in the vitamin is rotated out of the way in the phosphate. A second acetamide is also differently oriented in the two crystals. Theα-glycosidic bond between the ribose and the dimethylbenzimidazole is 16° out of the plane of the base. The ribose conformation is C2'&gt;-exo, and the phosphate ester conformations are similar to those found in other B12crystals.

The crystal structure of insulin: II. An investigation of rhombohedral zinc insulin crystals and a report of other crystalline forms

X-Ray diffraction photographs have been taken of zinc-free cubic insulin and of monoclinic zinc insulin crystals, prepared from buffers containing phenol. The wet cubic crystals are very small rhombic dodecahedra, a = 76 Å, ρ = 1·09 approx. and space group, I23 or I213; there is one insulin monomer (mol. wt about 6000) in the asymmetric unit. Wet monoclinic crystals have a = 62·3, b = 61·8, c = 47·8 Å, β = 110·7°, ρ = 1·19 g cm−3 and air-dried crystals have a = 61·4, b = 53·4, c = 43·8 Å, β = 134·5°, ρ = 1·29 g cm−3; the space group is P21 and there are six insulin monomers in the asymmetric unit.

The structure of Vitamin B 12 VI. The structure of crystals of vitamin B 12 grown from and immersed in water

Vitamin B 12 crystals have been grown from water and photographed in their mother liquor by means of copper K α X-radiation. The crystal unit cell dimensions are a = 25·33 Å, b = 22·32 Å and c = 15·92 Å, space group P2 1 2 1 2 1 and the formula for the asymmetric unit probably C 63 H 88 O 14 N 14 PCo.22H 2 O. From the intensities of some 2900 X-ray reflexions visually measured in 1949, calculations have been made of the electron density in the crystal through a series of approximations. The calculations lead to positions of all the atoms in the B 12 molecule, not counting hydrogen atoms; these positions agree closely with those derived for the air-dried crystals except over certain of the amide side chains which differ in conformational details. In the wet crystals the molecules make contact with one another through hydrogen bonds between the amide groups. Between them is a region occupied by water molecules. Some eight or nine of these have definite positions in the crystal; the remainder appear considerably disordered probably as a result of movement in the crystal. The disordered positions lie in or near channels which run between the B 12 molecules continuously throughout the crystal and through which it seems likely that water passes out on drying.

The structure of vitamin B 12 III. The crystal structure analysis of the air-dried B 12 crystals

The X-ray structure analysis of vitamin B 12 in the air-dried crystals is described in detail up to the point at which the molecular structure was considered established apart from certain double bond and hydrogen atom positions. The heavy-atom technique was used, based on the cobalt atom which is an intrinsic part of the B 12 molecule. While this atom was much more effective in phase determination than a formal calculation of its scattering power as against that of the rest of the crystal asymmetric unit would indicate, it was not a sufficiently dominating influence, and consequently the electron density maps based on cobalt phases alone contained too many spurious peaks for a complete structure to be postulated at the initial stage. Some form of trial was therefore necessary; this took the form of comparing the densities generated by several rather different postulated scattering nuclei included in the phase calculations. The techniques by which the very strong bias towards atoms inserted in the calculations in the non-centrosymmetric case could be circumvented are described, as are other aspects of the work which have a bearing on future structure analyses of very complex organic molecules.

The structure of vitamin B 12 - IV. The X-ray analysis of air-dried crystals of B 12

Measurements were made during 1948-9 of all the intensities of hkl reflexions observable with chromium Koc X-radiation from air-dried crystals of vitamin B 12 . Calculations parallel with those of J. G. White were carried out on these data. The positions of the cobalt atoms in the crystal structure were found from three-dimensional Patterson series and the positions of 90 atoms of the B 12 molecule and 7 water molecules were derived through three successive approximations to the three-dimensional electron density distribution. The choice of atomic positions was checked against superposition maps derived from the original Patterson series, and assisted by comparisons with other B 12 derivatives. Minor differences appear between the positions derived here and in III; some of these may be real differences due to the state of dryness of the crystals.

Some observations on the structures of wet and air-dried crystals of vitamin B12

Some observations on the structures of wet and air-dried crystals of vitamin B₁₂

Some observations on the structures of wet and air-dried crystals of vitamin B12

Some observations on the structures of wet and air-dried crystals of vitamin B₁₂

The crystal structure of ribonuclease. I

A detailed investigation of the monoclinic form of crystalline ribonuclease has been undertaken. The unit cell dimensions of the crystals in the wet and air-dried states are as follows: a b c β a sin β (A) (A) (A) (°) (A) wet crystal 30*90 38-80 54-06 106-0 29-30 dry crystal 29*10 30-08 51-03 114-0 26-4 The densities of the wet and air-dried crystals were found to be 1.220 and 1.269 respectively, and the air-dried crystals contained 13.6 % by weight of water. From the latter observations and the dry-cell dimensions, the molecular weight of ribonuclease, assuming two molecules in the unit cell, was calculated to be 13400. This figure is in reasonable agreement with the determinations of molecular weight made by osmotic pressure and sedimentation-diffusion methods. The Patterson projection maps along the three axes, a, band c, show vectors of 9.5 and 4.5 to 5.0 A, which are characteristic of protein crystals. In the case of the c-axis projection map it has been possible to identify peaks separated by 9.5 A lying on an hexagonal net. Perpendicular to this plane there is a vector repeat of 4.9 A lying parallel to the c axis of the unit cell. These vector distances are similar to those obtained from measurements on other proteins, where 9*5 A is thought to be the distance between polypeptide chains lying parallel to one another in an hexagonal array and 4.5 to 5 A are repeat units along the chains. The attempt has been made to interpret the Patterson vector projection maps in term s of sets of points chosen from the vector diagrams in such a manner that there is agreement between calculated and observed structure factors. Phase angles were thus obtained, and these were used with observed | F | values to calculate electron-density maps on the c- and a-axis projections of the unit cell. In all this work the minimum of hypothesis about protein structure has been employed. The selection of points made it possible to construct a vector model of ribonuclease, which suggests that it contains five polypeptide chains, each with a minimum length of about 46 A, lying parallel to one another on an hexagonal network and separated from each other by distances of about 9.5 A. These preliminary investigations are insufficient to establish any connexions between the ends of the polypeptide chains, or of the nature of the intramolecular fold of the chains themselves. The dimensions of the molecule are approximately 18 x 30 x 48 A, and the evidence favours the existence of two amino-acids per 4.9 A repeat unit along the axes of the polypeptide chains, assuming these chains to contain the a-type fold. The molecule contains something between 90 and 110 amino-acid residues. Independent confirmation of the direction of the polypeptide chains in the crystal has been provided from polarized infra-red absorption studies of the —N—H — combination bands of the internal hydrogen bonds of the polypeptide chains.

Appendix. Crystallographic measurements on the anti-pernicious anaemia factor

Since May 1948, five different samples of crystalline anti-pernicious anaemia factor have been examined in Oxford by various crystallographic techniques. These all, from their general characteristics, particularly the intensities of a number of X-ray reflexions, contain the same molecular structure. But the different samples have shown small variations in unit-cell dimensions and in crystal habit, which are probably due mainly to differences in solvent content, combined with traces of different impurities. Small changes in, for example, some side chain in the molecule, might also conceivably contribute to these effects. The crystals, as grown both from water and from aqueous acetone, are dark red and show marked pleochroism . They vary in habit from long thin needles to short thick prisms on which different crystal faces appear (figures A 1 and A 2). They all contain solvent, probably water, of crystallization, part at least of which they lose slowly on exposure to the air. Crystals kept in their mother-liquor are transparent and show beautifully clear reflecting faces; they give sharp X-ray reflexions extending to spacings of 1·1Å. On removal from the liquid they tend to crack and to become opaque; the faces are distorted and the X-ray reflexions become first multiple, and then blurred, corresponding to the presence of disorder within the dried crystal structure. But the rate of loss of solvent appears to vary both with the size of crystals and with the different samples studied. Individual air-dried crystals for example, have been observed which have given good and sharp X-ray reflexions some weeks after exposure to the atmosphere.

A comparative X-ray study of foetal and adult sheep haemoglobins

Haemoglobins from adult sheep and from foetuses of different ages were crystallized, using a standard procedure. The optical properties, cell dimensions and space-groups of the wet and, where possible, of the air-dried crystals were determined, and surveys were made of the lower order diffraction patterns in each of the principal crystal zones. The results obtained provide additional evidence for the non-identity of the foetal and adult haemoglobins and lead to certain conclusions relevant to the general problem of haemoglobin structure. Methaemoglobin from foetuses aged less than 120 days was found to crystallize in either of two forms, each of which was different from methaemoglobin crystals of adult sheep pre­pared by the same method. Haemoglobin from later foetuses is difficult to crystallize, and its properties resemble those of artificial mixtures of adult and early foetal haemoglobins. No general picture of the structural differences between adult and foetal haemoglobin has emerged from this study, mainly because these were obscured by differences in molecular packing. Indications were found of a difference in molecular symmetry between these two proteins, and this could be correlated with different splitting properties in solution. There is also evidence of a pronounced intramolecular layering in foetal haemoglobin. A new method of controlled shrinkage of protein crystals at constant humidity was developed and applied to foetal haemoglobin crystals, which were shown to pass through a series of at least four well-defined lattices. The densities of wet adult and foetal haemoglobin crystals were measured and used for the estimation of ‘bound water’. The amounts of ‘bound water’ were of the same order as those obtained from haemoglobin crystals of other species.

Points from Foregoing Letters

X-RAY photographs of zinc insulin crystals in several immersion media have been obtained by D. Crowfoot and D. Riley. The wet crystal unit cell is found to be a moderately expanded version of that previously found for the air-dried crystals. Preliminary calculations show that the striking differences in the X-ray intensities given by the different crystal structures is to be correlated mainly with a reorientation of the molecules relative to the crystal axes which takes place on drying.