The liquidlike ordering of lipid A-diphosphate colloidal crystals: the influence of Ca2+, Mg2+, Na+, and K+ on the ordering of colloidal suspensions of lipid A-diphosphate in aqueous solutions.

A comprehensive study was performed on electrostatically stabilized aqueous dispersion of lipid A-diphosphate in the presence of bound Ca2+, Mg2+, K+, and Na+ ions at low ionic strength (0.10-10.0-mM NaCl, 25 degrees C) over a range of volume fraction of 1.0 x 10(-4)< or =phi< or =4.95 x 10(-4). These suspensions were characterized by light scattering (LS), quasielastic light scattering, small-angle x-ray scattering, transmission electron microscopy, scanning electron microscopy, conductivity measurements, and acid-base titrations. LS and electron microscopy yielded similar values for particle sizes, particle size distributions, and polydispersity. The measured static structure factor, S(Q), of lipid A-diphosphate was seen to be heavily dependent on the nature and concentration of the counterions, e.g., Ca2+ at 5.0 nM, Mg2+ at 15.0 microM, and K+ at 100.0 microM (25 degrees C). The magnitude and position of the S(Q) peaks depend not only on the divalent ion concentration (Ca2+ and Mg2+) but also on the order of addition of the counterions to the lipid A-diphosphate suspension in the presence of 0.1-microM NaCl. Significant changes in the rms radii of gyration (R2G) 1/2 of the lipid A-diphosphate particles were observed in the presence of Ca2+ (24.8+/-0.8 nm), Mg2+ (28.5+/-0.7 nm), and K+ (25.2+/-0.6 nm), whereas the Na+ salt (29.1+/-0.8 nm) has a value similar to the one found for the de-ionized lipid A-diphosphate suspensions (29.2+/-0.8 nm). Effective particle charges were determined by fits of the integral equation calculations of the polydisperse static structure factor, S(Q), to the light-scattering data and they were found to be in the range of Z*=700-750 for the lipid A-diphosphate salts under investigation. The light-scattering data indicated that only a small fraction of the ionizable surface sites (phosphate) of the lipid A-diphosphate was partly dissociated (approximately 30%). It was also discovered that a given amount of Ca2+ (1.0-5.0 nM) or K+ (100 microM) influenced the structure much more than Na+ (0.1-10.0-mM NaCl) or Mg2+ (50 microM). By comparing the heights and positions of the structure factor peaks S(Q) for lipid A-diphosphate-Na+ and lipid A-diphosphate-Ca2+, it was concluded that the structure factor does not depend simply on ionic strength but more importantly on the internal structural arrangements of the lipid A-diphosphate assembly in the presence of the bound cations. The liquidlike interactions revealed a considerable degree of ordering in solution accounting for the primary S(Q) peak and also the secondary minimum at large particle separation. The ordering of lipid A-diphosphate-Ca2+ colloidal crystals in suspension showed six to seven discrete diffraction peaks and revealed a face-centered-cubic (fcc) lattice type (a=56.3 nm) at a volume fraction of 3.2 x 10(-4)< or =phi< or =3.9 x 10(-4). The K+ salt also exhibited a fcc lattice (a=55.92 nm) at the same volume fractions, but reveals a different peak intensity distribution, as seen for the lipid A-diphosphate-Ca2+ salt. However, the Mg2+ and the Na+ salts of lipid A-diphosphate showed body-centered-cubic (bcc) lattices with a=45.50 nm and a=41.50 nm, respectively (3.2 x 10(-4)< or =phi< or =3.9 x 10(-4)), displaying the same intensity distribution with the exception of the (220) diffraction peaks, which differ in intensity for both salts of lipid A-diphosphate.

Physicochemical and structural studies of triamterene.

It has been shown by several physicochemical techniques that the existence of different crystal forms of triamterene, as well as the crystalline products from different solvents with distinct differences in melting points, does not necessarily imply polymorphic crystal forms. The crystalline structure for triamterene is reported herein, revealing a N,N-dimethylformamide molecule and a water molecule within the crystal lattice. Triamterene crystallizes in a face-centered orthorhombic space group, Fdd2 (#43), when grown from aqueous solutions of dimethylformamide (N,N-DMF). In addition, the asymmetric unit contains a partially occupied molecule which is situated on a crystallographic twofold axis. Weak hydrogen bonding occurs between atoms O(1) and N(7), with a distance of 2.66 A, O(1) and N(5) with a distance of 2.96 A, O(2) and N(1), at two different symmetry sites of 2.99 A, and N(2) and N(7), with a distance of 2.91 A. The different solvated crystalline triamterene specimens have different melting points depending on organic solvation, water, or both.

Structure of tyrocidine micelles in isotropic aqueous solution.

Aqueous solutions of tyrocidine B, iodoacetyltyrocidine B, and diiodotyrocidine B in 40% H2O:60% ethanol (w/w) were investigated by analytical ultracentrifugation, light scattering, and small-angle X-ray scattering techniques. A reasonable hydrodynamic description of the aggregates of molecular weight 28,600 is a rod with a length of 170 A and a diameter of 30 A; this description is consistent with the X-ray scattering data. Over a broad range of concentrations, inelastic light scattering measurements and small-angle X-ray scattering experiments provide the same hydrodynamic values (e.g., Stokes radii and frictional ratios). The positions of the iodines were resolved and found at a radius (R) of 16 A. So, the iodine-iodine distance across the cross section is 34 A, indicating that labeled tyrocidine B is not affected during the aggregational process.

Structure of phenylbutazone and mofebutazone in the crystalline state and in solution.

The crystal structure of the keto form of phenylbutazone has been determined and compared with the molecular conformation of mofebutazone. The pronounced differences between these two structures are the conformation of the n-butyl side chain (which is extended and in trans conformation for mofebutazone, however, bent at the C gamma-position with hydrogen bonding of the gamma-hydrogen to the carbonyl C(3)--O(1) for phenylbutazone), and the in-plane conformation of the phenyl group with respect to the heterocyclic ring system in mofebutazone compared with two benzene rings at N(1) and N(2) which are almost perpendicular to the pyrazolidin-(3,5)-dione ring. Preliminary X-ray data reveal that the structures of the alkali enolates of both compounds are difficult to solve because the crystal quality is insufficient and highly disordered. However, the large unit cells are consistent with chelated structures of the alkali enolates with solvent. Sodium and potassium enolates of phenylbutazone in nonpolar solvents (e.g., benzene and cyclohexane) exist as inverse micelles comprised of 30-40 monomer units. In contrast, the salt appears to be essentially monomeric in 1,2-dimethoxyethane, and, in water for phenylbutazone, the critical micelle concentration (CMC) is greater than 0.30 M. Mofebutazone salts, however, reveal a CMC of 1.5 X 10(-3) M in water, where the basic unit of micellar aggregation seems to be the dimer. The micellar properties of phenylbutazone and mofebutazone enolates are strongly dependent on the nature of solvents, solvation of the enolate, and size of the cation. However, sodium and potassium enolates of mofebutazone in nonpolar solvents exist as small inverse micelles comprised of 8-10 monomer units only. However, in aqueous solution, weight average molecular weights of 13,100 +/- 500 were determined, with a radius of gyration of 12.85 +/- 0.55 A.

Size and shape of the alpha subunit of the (Ca,Mg)-dependent adenosinetriphosphate from Escherichia coli in solution in the presence and absence of ATP.

The alpha subunit of the (Ca, Mg)-dependent ATPase from Escherichia coli was studied in solution by means of X-ray scattering experiments at variable contrast and in the presence of ATP. The experiments were carried out on an absolute scale in the range of (45.0 nm-1) less than h less than (1.5 nm-1) at pH 8.0. The experiments show that this system and the complex of the alpha subunit with ATP can be considered to be ideal and monodisperse so that the following parameters for the alpha subunit and its complex with ATP were determined: the radius of gyration, the volume, the degree of hydration, the maximum particle diameter, and the molecular weight. The molecular weight of the alpha subunit was determined as 58 500 +/- 3000 by means of light scattering measurements, and 57 700 +/- 25 000 by the small-angle X-ray scattering experiments. The radius of gyration of the alpha subunit at pH 8.0 was determined to 2.64 +/- 0.02 nm, the maximum chord in solution to 10.0 +/- 0.5 nm, the volume to 102.4 +/- 2.5 nm3, and the degree of hydration to 0.34 +/- 0.02 mg X g-1. Binding of ATP to the alpha subunit causes structural changes. These changes are reflected in the radius of gyration, Rg = 2.45 +/- 0.015 nm, in the volume, 117.6 +/- 1.5 nm3, and in the maximum chord length in solution, 7.8 +/- 0.5 nm. These changes imply a decrease of the axial ratio from 2.4 to 1.4, i.e. ATP-binding induced an increase in isometry of the alpha subunit.

Structure of cross-linked rabbit muscle phosphofructokinase in solution.

Cross-linked rabbit muscle phosphofructokinase in the active tetrameric and octameric state was studied in solution by hydrodynamic methods and small angle x-ray scattering techniques. The translational diffusion coefficients were determined by means of inelastic light scattering and were found to be 3.60 (+/- 0.02) x 10(-7) cm2 . s-1 for the tetramer and 2.54 (+/- 0.15) x 10(-7) cm2 . s-1 for the octamer. From small angle x-ray scattering measurements the radius of gyration, the specific inner surface area, and the volume were determined for both enzyme forms, revealing that the octameric cross-linked form is approximately spherical, with a diameter of 120.0 A, whereas the tetrameric form is asymmetric having an axial ratio of 2. By comparison of the scattering curves with triaxial geometric bodies which are equivalent in scattering, the tetrameric enzyme is described as a rectangular prism, with overall dimensions of A = 131.0 A, B = 131.0 A, and C = 65.0 A, and the octameric form as that of a cube with A = B = C = 120.0 A. The shape of the protomer, having a radius of gyration of 24.8 A, in the tetramer and octamer is similar to that for the native tetramer at pH 10 in the presence of 5 mM fructose 6-phosphate or 15 mM fructose 1,6-bis-phosphate. From the different shapes of the scattering curves of the native phosphofructokinase at pH 7.5 in the presence of 15 mM ATP and of the cross-linked tetramer or octamer, it can be inferred that the shapes of the protomers are different: in the presence of ATP the protomers are elongated, having an axial ratio of 1.8 to 2.0; the cross-linked state reveals a spherical protomer of radius 33.0 A, similar to that of the native enzyme at pH 7.5 in the presence of fructose 6-phosphate or fructose 1,6-bisphosphate.

Size and molecular parameters of adenosine triphosphatase from Escherichia coli.

The Mg2+- and Ca2+-stimulated ATPase (bacterial coupling factor) has been investigated in solution with different independent techniques. The molecular weight of the five-subunit enzyme was found to be 345,000 +/- 5,000 by means of light scattering, 350,000 by sedimentation equilibrium experiments, and 358,000 by means of small-angle x-ray scattering. The radius of gyration was found to be 41.9 A, the volume 7.39 x 10(5) A3, and the surface to volume ratio 5.5 x 10(-2) A-1 from small-angle x-ray scattering measurements of the enzyme in solution. The degree of hydration was found to be 0.62 ml of H2O/g of ATPase. The translational diffusion coefficient was determined to be 3.47 x 10(-7) cm2 s-1 by means of inelastic light scattering. The distribution of the scattered intensity near the origin appears to be bimodal, suggesting that the ATPase molecule is composed of spherical parts bound together by a flexible polypeptide chain. The largest dimension of the ATPase in solution is 120.0 A, determined from the pair distribution function.