Conformational changes of cholesteryl palmitoleate in the crystal structure at low temperature

At 123 K, crystals of cholesteryl cis-9-hexadecenoate (cholesteryl palmitoleate, C45H74O2) are monoclinic, space group P2(1) with cell dimensions a = 12.917(7), b = 8.910(5), c = 34.04(1) A, beta = 94.95(7) degrees [lambda(CuK alpha) = 1.5424 A] having two independent molecules (A and B) per unit cell. The crystal structure has been determined from 6178 reflections with sin theta/lambda less than or equal to 0.56 A-1, of which 3406 gave [F] greater than 3 sigma. Structure refinement by alternating cycles of Fourier syntheses and block diagonal least squares gave R = 0.24 for all reflections, R = 0.13 for reflections [F] greater than 3 sigma. At 123 K, the crystal structure consists of closely packed layers very similar to those at 295 K. However, there are major conformational differences in the layer interface region, which affect the ester chain of molecule B and the C(17) tail of molecule A. Although the electron density is diffuse in this region, the B-chain, which is bent, appears to be ordered at 123 K and has a different conformation from the disordered B-chains at 295 K. The change in the A-tail, which is twisted at 123 K and extended at 295 K, is very similar to that which occurs in two of the molecules when anhydrous cholesterol undergoes phase transition. Measurements of the unit cell dimensions at twelve temperatures (295 K to 123 K) indicate that the major changes in the crystal structure of cholesteryl palmitoleate occur in a 10 K range near 173 K.     

Commensurate molecules in isostructural crystals of cholesteryl cis- and trans-9-hexadecenoate

At 295 K, crystals of form I of cholesteryl cis-9-hexadecenoate (palmitoleate) and cholesteryl trans-9-hexadecenoate (palmitelaidate) are difficult to distinguish by X-ray diffraction. Both form monoclinic thin plates, space group P21 with two molecules (C43H74O2) A and B in the asymmetric unit. Unit cell dimensions for cholesteryl palmitelaidate (I) are a = 12.827(4), b = 9.075(4), c = 35.67(1) A, beta = 93.42(3) degrees, very similar to those of the palmitoleate crystals. Other crystals (form II) of the palmitelaidate ester are described. The crystal structure of form I of cholesteryl palmitelaidate has been determined from 3657 reflections (sin theta/lambda less than 0.46 A-1) measured at 295 K using CuK alpha X-radiation and refined to give Rw(F) = 0.095. The molecular packing arrangement is isostructural to that of the previously determined crystal structure of cholesteryl palmitoleate. In both crystals, the fatty acid chains of the A molecules are kinked at the double bond but are nearly straight. The chains of B molecules have more complicated dislocations and are bent. It is remarkable that, neglecting their detailed conformations, corresponding fatty acid chains in the two crystal structures have similar overall shapes, although palmitoleate chains have cis-ethylenic groups and palmitelaidate chains have trans groups.     

Conformation and packing of unsaturated chains in cholesteryl linolelaidate at 123 K

At 123 K, crystals of cholesteryl trans-9-trans-12-octadecadienoate (cholesteryl linolelaidate, C47H76O2) are monoclinic, space group P2(1) with cell dimensions a = 13.03(3), b = 8.76(2), c = 17.90(4) A, beta = 89.7(2) degrees, having two molecules per unit cell. The crystal structure has been determined from 2041 X-ray intensities with sin theta/lambda less than 0.48 A-1, of which 922 gave I greater than 2 sigma(I). The hydrogen atoms were found in a difference Fourier synthesis. Block diagonal least squares refinement assuming isotropic thermal parameters has converged with Rw = 0.13. The molecule is fully extended (length 43.3 A), except for a symmetric bowing in the linolelaidate chain segment which contains the two unconjugated trans ethylenic bonds. The torsion angles at the four C--C bonds adjacent to the C=C bonds are all in the preferred (+/-)-skew range. Chain packing is efficient, without having a regular subcell structure. There is a similarity with the overall conformation of the oleate chains in crystals of cholesteryl oleate. Although chemically disparate, the oleate and linolelaidate chains have similar crystal environments.     

Structure and conformation of linear peptides. XII. Structure of tryptophanyl-glycyl-glycine dihydrate

The crystal structure of a tripeptide, tryptophanyl-glycyl-glycine dihydrate (C15H18N4O4.2H2O, molecular weight = 354) has been determined. The crystals are orthorhombic, space group P2(1)2(1)2(1), with a = 7.875 (1) A, b = 9.009(1), c = 24.307(1) and Z = 4. The final R-index is 0.058 for 1488 reflections [sin theta)/lambda less than or equal to 0.6 A-1) with I greater than 2 sigma (I). The molecule exists as a zwitterion, with terminal NH3+ and COO- groups. The peptide units are trans and nearly perpendicular to the plane of the carboxyl group. The backbone torsion angles are: psi 1 = 132.7 degrees, omega 1 = 174.2 degrees, phi 2 = 88.2 degrees, psi 2 = 8.6 degrees, omega 2 = -179.8 degrees, phi 3 = -85.2 degrees, psi 31 = -178.1 degrees, psi 32 = 5.0 degrees. For the sidechain of tryptophan, chi 1 = -171.6 degrees, chi 2 = 101.0 degrees.     

Conformation of the oleate chains in crystals of cholesteryl oleate at 123 K

At 123 K, crystals of cholesteryl cis-9-octadecenoate (cholesteryl oleate, C45H78O2) are monoclinic, space group P2(1) with unit cell dimensions a = 12.356(2), b = 8.980(3), c = 18.382(2) A, beta = 85.49(2) degrees, and have two molecules in the unit cell. The crystal structure including all H atoms has been determined from 3812 independent X-ray reflections with sin theta/lambda less than 0.61 A-1 and refined to give Rw = 0.08. At 123 K, the crystal structure consists of an antiparallel efficient packing of cholesteryl ring systems to form layers that are very similar to those observed in the room temperature structure. The oleate chains that protrude from these layers have a somewhat different packing arrangement from the room temperature structure because they have undergone a conformational change. At 123 K, the oleate chains are well ordered and are almost fully extended except for a kink at the cis double bond. The oleate chains at 123 K are 1.7 A longer than at 295 K due in part to an uncoiling whereby their helical character is lost. On cooling, there is a substantial change in the unit cell beta-angle from obtuse (93.3 degrees) to acute (85.5 degrees) which involves a shearing motion of 2.5 A between adjacent molecular layers. Cell dimension measurements at 10 temperatures in the range 295 K to 123 K show that much of the change occurs in two narrow ranges centered at 262 K and 215 K.     

Structure and conformation of linear peptides. XIII. Structure of L-phenylalanyl-glycyl-glycine

The crystal structure of a tripeptide, L-phenylalanyl-glycyl-glycine (C13H17N3O4), molecular weight = 279.3, has been determined. The crystals are orthorhombic, space group P2(1)2(1)2(1), with a = 5.462(1) A, b = 15.285(5), c = 16.056(4), Z = 4, and P (calc) = 1.384 g.cm-3. The final R-index is 0.052 for 866 reflections with sin theta/lambda less than or equal to 0.55 A-1 and I greater than 1 sigma. The molecule exists as a zwitterion, with the N-terminus protonated and the C-terminus in an ionized form. Both the peptide units are in the trans configuration and planar, though one of them shows significant deviations from planarity ([delta w[ = 5.1 degrees). The peptide backbone is folded, with the torsion angles of: psi 1 = 116.2(5) degrees, omega 1 = 178.8(4), phi 2 = -89.7(5). psi 2 = -28.9(6), omega 2 = -174.9(4), phi 3 = 134.9(5), psi 31 = 7.8(6), psi 32 = -172.6(4). The terminal glycine adopts a "D-residue" conformation. For the sidechain of phenylalanine, chi 1 = 175.5(4), chi 2 = -127.0(6).     

Synthesis and crystal structures of Boc-L-Asn-L-Pro-OBzl.CH3OH and dehydration side product, Boc-beta-cyano-L-alanine-L-Pro-OBzl

Boc-L-Asn-L-Pro-OBzl: C21H29O6N3.CH3OH, Mr = 419.48 + CH3 OH, monoclinic, P2(1), a = 10.049(1), b = 10.399(2), c = 11.702(1) A, beta = 92.50(1)degrees, V = 1221.7(3) A3, dx = 1.14 g.cm-3, Z = 2, CuK alpha (lambda = 1.54178 A), F(000) = 484 (with solvent), 23 degrees, unique reflections (I greater than 3 sigma(I)) = 1745, R = 0.043, Rw = 0.062, S = 1.66. Boc-beta-cyano-L-alanine-L-Pro-OBzl: C21H27O5N3, Mr = 401.46, orthorhombic, P2(1)2(1)2(1), a = 15.741(3), b = 21.060(3), c = 6.496(3) A, V = 2153(1) A3, dx = 1.24 g.cm-3, Z = 4, CuK alpha (lambda = 1.54178 A), F(000) = 856, 23 degrees, unique reflections (I greater than 3 sigma(I)) = 1573, R = 0.055, Rw = 0.078, S = 1.86. The tert.-butyloxycarbonyl (Boc) protected dipeptide benzyl ester (OBzl), Boc-L-Asn-L-Pro-OBzl, prepared from a mixed anhydride reaction using isobutylchloroformate, Boc-L-asparagine, and HCl.L-proline-OBzl, crystallized with one methanol per asymmetric unit in an extended conformation with the Asn-Pro peptide bond trans. Intermolecular hydrogen bonding occurs between the methanol and the Asn side chain and between the peptide backbone and the Asn side chain. A minor impurity due to the dehydration of the Asn side chain to a beta-CNala crystallized with a similar extended conformation and a single intermolecular hydrogen bond.     

A vibrating flexible chain in a molecular cage: crystal structure of the complex cyclomaltoheptaose (beta-cyclodextrin)-1,4-butanediol.6.25H2O.

A single crystal X-ray diffraction study of the title complex carried out at room temperature revealed space group P2(1), a = 21.199(12), b = 9.973(3), c = 15.271(8) A, beta = 110.87(3) degrees, V = 3017(3) A3, 4681 unique reflections with Fo greater than 1 sigma (Fo). The structure was refined to R = 0.069, resolution lambda/2sin theta max = 0.89 A. The crystal packing is of the cage type and is isomorphous to that of beta-cyclodextrin (beta CD) dodecahydrate. One 1,4-butanediol and approximately 1.25 water molecules are enclosed in each beta CD cavity. The hydroxyl groups of the 1,4-butanediol molecule are located at each end of the cavity and form hydrogen bonds with neighboring water and beta CD molecules. The flexible (CH2)4 moiety vibrates extensively in the central part of the cavity. Water molecules and hydroxyl groups are chelated between O-6 and O-5 of at least five glucose residues.     

The electrostatic potential for the phosphodiester group determined from X-ray diffraction.

The charge density distribution in the crystal structure of ammonium dimethylphosphate at 123 K has been determined from x-ray diffraction data (MoK alpha) using 8437 reflections with sin theta/lambda less than 1.33 A-1 [NH4+.(CH3)2PO4-, M(r) = 143.08, monoclinic, P2(1)/c, a = 10.007(1), b = 6.926(1), c = 9.599(2) A, beta = 105.40(1) degrees, V = 641.4(3) A3, Z = 4, F000 = 304, Dx = 1.4815 g.cm-3, mu = 3.726 cm-1]. Least-squares structure refinement assuming Stewart's rigid pseudoatom model (variables including Slater-type radial exponents and electron populations for multipole terms extending to octapoles for C, N, O, and P, and dipoles for H) gave R(F2) = 0.039 for all reflections. The dimethylphosphate anion is in the gauche-gauche conformation and has approximate twofold symmetry. One phosphoryl O atom forms three hydrogen bonds and the other forms one. Neither of the ester O atoms is hydrogen bonded. For the dimethylphosphate anion isolated from the crystal structure, a map of the electrostatic potential obtained using the pseudoatom charge parameters shows that the phosphoryl O atoms are considerably more electronegative than the ester O atoms. The electrostatic potential distribution obtained in this way has been fitted by least squares to a system of atom-centered point charges. The potential calculated from these point charges agrees with the experimental result. It also agrees reasonably well with potentials obtained from three other systems of point charges that are widely used as part of the semiempirical force field for molecular mechanics and molecular dynamics calculations involving nucleic acids.     

Crystal structure of cholesteryl decanoate.

Cholesteryl decanoate (C37H64O2) is monoclinic, space group P2I, with cell dimensions a = 12.931 (6), b = 9.066 (2), c = 30.22 (1) A, beta = 91.14 (4) degrees, and Z = 4. The atomic coordinates from cholesteryl laurate were used in an initial trial structure which was refined by block diagonal least-squares methods with 1846 observed X-ray reflections (R = 0.129). Molecules A and B have almost fully extended conformations, except at the ester bonds and towards the end of the decanoate B chain. The molecules are arranged in antiparallel array forming monolayers of thickness d001 = 30.22 A, with the molecular long axis making an angle of about 67 degrees with the layer interface. The crystal structure is very similar to that of cholesteryl nonanoate and cholesteryl laurate.     

Crystal structure of cholestanyl caprylate and binary phase behavior with cholesteryl caprylate

The crystal structure of cholestanyl n-octanoate (caprylate) (C35H62O2) is monoclinic with space group A2 and cell dimensions a = 10.103(7), b = 7.646(7), c = 87.63(7) A, beta = 90.51(6) degrees; Z = 8 [two molecules (A, B) in asymmetric unit], V = 6769 A3, Dc = 1.010 g cm-3. Integrated X-ray intensities for 3798 reflections with I greater than 2 sigma (I) were measured with a rotating anode diffractometer at room temperature. The structure was determined using direct methods. Block diagonal least squares refinement gave R = 0.111. Molecules A and B have almost fully extended conformations, but differ significantly in the rotation about the ester bond and in the C17 chains. The molecular packing in the crystal structure of cholestanyl caprylate consists of stacked bilayers each having d002 = 43.8 A in thickness and within each bilayer, cholestanols pack with cholestanols and caprylate chains pack with caprylate chains. The crystal structure is very similar to that of cholesteryl myristate but is quite different from that of cholesteryl caprylate. The phase equilibria of the cholestanyl caprylate/cholesteryl caprylate binary system have been shown to involve limited mutual solubility of the two components and to have a eutectic point at 73% cholestanyl caprylate. The cholesteric mesophase is monotropic at all compositions except for a narrow range near the eutectic point where it is enantiotropic.     

Rapid calculation of the solution scattering profile from a macromolecule of known structure

If one expands the structure factor equation in spherical coordinates, rotational averaging of the molecular Fourier transform, which leads directly to the solution scattering profile, is greatly simplified. It becomes a projection in the polar and azimuthal angular variables. The profile is given by I(R) = 1/2 infinity sigma n = 0 n sigma m = 0 epsilon mNm,n magnitude of Gm,n(R) 2 where Gm,n(R) = sigma jfjYm,n(theta j, phi j)jn(2 pi rjR) The index j runs over all atoms; r, theta, phi are atomic coordinates and epsilon and N are constants; the Ym,n are complex spherical harmonics, and jn are spherical Bessel functions; R = 2 sin theta/lambda. The effects of solvent have been modeled by subtracting from each protein atom a properly weighted water. Hydrogens have been included by using scattering curves fj derived from the spherical averaging of protein atoms with their attached hydrogens. This approach may also be satisfactory for neutron scattering. Published scattering profiles for lysozyme and BPTI have been accurately matched in less than one-tenth the time required by other methods. Separate, adjustable temperature factors for the protein, solvent waters, and bound waters are used, and appear to be needed. In the case of BPTI, as suggested by NMR observations, the observed diffraction pattern was much better accounted for by including only 4 tightly bound waters rather than the roughly 60 seen by crystallography.     

Hydrocarbon chain packing modes in lipids: effect of altered sub-cell dimensions and chain rotation

The lateral hydrocarbon chain packing modes of lipids have been described in terms of specific hydrocarbon sub-cells as deduced from single crystal structural studies. To understand the changes in hydrocarbon chain packing in lipid bilayers induced by variations in temperature, hydration, ion-binding, etc., we have examined the effect on the calculated X-ray diffraction pattern of (a) systematic variations in the dimensions of the hydrocarbon sub-cell and (b) the effect of chain rotation at fixed lattice sites. For the O perpendicular (orthorhombic) sub-cell, the a and b sub-cell parameters were varied from as = 4.96 to 4.85 A and bs = 7.42 to 8.40 A in six steps and the positions (s = 2 sin theta/lambda) and intensities (Icalc = F2) of the strong sub-cell reflections calculated. In this way, the conversion of the O perpendicular sub-cell (with either fixed chain orientations or simulated chain rotation) to the hexagonal (H) sub-cell (with chain rotation) was followed. Notably, the two strong reflections characteristic of the O perpendicular sub-cell at 4.12 A (110) and 3.71 A (020) show progressive shifts in position and intensity, finally merging to give the strong (O1O) reflection at 4.2 A characteristic of the hexagonal sub-cell. Similar calculations were performed for the orthorhombic (O' perpendicular) and monoclinic (M parallel) sub-cells. This approach can be used to analyze changes in the X-ray diffraction data due to modifications of the hydrocarbon chain packing modes characteristic of simple and complex lipids.     

ESR studies on the orientation of cholesteryl ester in phosphatidylcholine multilayers.

The alignment of cholesteryl esters in multilayer phosphatidylcholine membranes was investigated using two spin-labelled cholesteryl esters: 10 : 3 ester (I) and 1 : 14 ester (II). The nitroxide label of I is aligned in the membrane with a very large angle of tilt (47 degrees +/- 1.5 degrees) with respect to the normal to the membrane surface; II does not show such a tilt. I gives spectra corresponding to immobilized label while II gives nearly isotropic spectra. Ascorbate treatment of the multilayers shows that the labels in I and II are not present at the phosphatidylcholine-water interphase. The data supports a 'horseshoe' configuration for the cholesteryl ester in the bilayer, with both the fatty acid chain and the cholesteryl moiety extending deep into the hydrophobic region of the membrane and with the ester linkage near the surface.     

Low-temperature crystal structures of tetrakis-mu-3,5-diisopropylsalicylatobis-dimethylformamidodico pper(II) and tetrakis-mu-3,5-diisopropylsalicylatobis-diethyletheratodicopp er(II) and their role in modulating polymorphonuclear leukocyte activity in overcoming seizures

Two binuclear copper(II) complexes of 3,5-diisopropylsalicylic acid were characterized by single crystal X-ray diffraction methods and examined for anti-inflammatory activity using activated polymorphonuclear leukocytes and for anticonvulsant activities using electroshock and metrazol models of seizures. These complexes were crystallized from dimethylformamide (DMF) or diethylether. Tetrakis-mu-3,5-diisopropylsalicylatobis-dimethylformamidodicop per(II) [Cu(II)2(3,5-DIPS)4(DMF)2] I is in space group P 1; a = 10.393 (2), b = 11.258 (2), c = 12.734 (2) A, alpha = 96.64 (2), beta = 92.95 (2), gamma = 94.90 (2) degrees; V = 1471.7 (4) A3; Z = 1. Tetrakis-mu-3,5-diisopropylsalicylatobis-etheratodicopper(II ) [Cu(II)2(3,5-DIPS)4(ether)2] II is in space group P 1; a = 10.409 (3), b = 11.901 (4), c = 12.687 (6) A, alpha = 91.12 (5), beta = 90.84 (5), gamma = 100.90 (4) degrees; V = 1542 (1) A3; Z = 1. The structure of I was determined at 140 K from 4361 unique reflections (I > 2sigma(1)) and refined on F2 to R1 = 0.04 and wR2 = 0.09. The structure of II was determined at 180 K from 4605 unique reflections (I > 2sigma(I)) and refined on F2 to R1 = 0.05 and wR2 = 0.13. Each compound is a crystallographically centrosymmetric binuclear complex with Cu atoms bridged by four 3,5-diisopropylsalicylate ligands related by a symmetry center [Cu-Cu(i): 2.6139 (9) A in I and 2.613 (1) in II]. The four nearest O atoms around each Cu atom form a nearly rectangular planar arrangement with the square pyramidal coordination completed by the dimethylformamide (or diethylether) oxygen atom occupying an apical position, at a distance of 2.129 (2) A in I and 2.230 (3) A in II. Each Cu atom is displaced towards the DMF (or diethylether) ligand, by 0.189 A in I and 0.184 A in II, from the plane of the four O atoms. The crystal structures of I and II are essentially similar to each other, except for the DMF or diethylether accommodation. Many disorder phenomena were found in the crystal structure of I. Copper(II)2(3,5-DIPS)4(DMF)2 inhibited polymorphonuclear leukocyte (PMNL) oxidative metabolism in vitro. This effect was concentration related and significant for concentrations higher than 10 microg or 0.68 nmol/ml. Copper(II)2(3,5-DIPS)4(DMF)2 was more active than the parent ligand, 3,5-DIPS, as has been demonstrated with copper complexes of other non-steroidal anti-inflammatory drugs. The DMF and diethylether ternary complexes of Cu(II)2(3,5-DIPS)4 were found to have anticonvulsant activity in the maximal electroshock model of grand mal epilepsy in doses ranging from 26 to 258 micromol/kg of body mass following intraperitoneal, subcutaneous, or oral treatment. The DMF ternary complex was also found to be effective in the subcutaneous injection of metrazol model of petit mal epilepsy. We conclude that both ternary copper complexes are lipophilic and bioavailable, capable of facilitating the inflammatory response to brain injury and causing the subsidence of this response in bringing about remission of these disease states.     

Structure and conformation of linear peptides. V. Structure of L-prolyl-glycyl-glycine

The tripeptide, L-prolyl-glycyl-glycine, crystallizes in the trigonal space group P3(2), with a = b = 8.682(2) A, c = 12.008(2) and Z = 3. The structure was solved by direct methods and refined to an R-value of 0.07 for 727 reflections (I greater than 1.0 sigma). The molecule exists as a zwitterion in the crystal. The peptide units are trans and show significant deviations from planarity (omega 1 = 169.7 degrees, omega 2 = -170.1 degrees). The peptide backbone adopts a left-handed helical conformation similar to that of polyglycine II and polyproline II.     

Structure and conformation of peptides containing the sulphonamide junction. I. N-acetyl-tauryl-L-phenylalanine methyl ester

N-acetyl-tauryl-L-phenylalanine methyl ester 1 has been synthesized. The crystal structure and molecular conformation of 1 have been determined. Crystals are monoclinic, space group P2(1) with a = 5.088(2), b = 17.112(17), c = 9.581(6) A, beta = 92.34(4) degrees, Z = 2. The structure has been solved by direct methods and refined to R = 0.043 for 2279 reflections with I greater than 1.5 sigma(I). The sulphonamide junction maintains the peptide backbone folded with Tau and Phe C alpha atoms in a cisoidal arrangement, the torsion angle around the S-N bond being 65.4 degrees. In this conformation the p-orbital of the sulphonamide nitrogen lies in the region of the plane bisecting the O-S-O angle, thus favouring d pi-p pi interactions between nitrogen and sulphur atoms. The S-N bond with a length of 1.618 A has significant pi-bond character. The CO-NH is planar and adopts trans conformation. The Tau residue is extended with the Tau-C1 alpha-Ca beta bond anti-periplanar to the S-N bond. The Phe side chain conformation corresponds to the statistically most favoured g- rotamer and exhibits a chi 1 torsion angle of -67.5 degrees. The packing is characterized by intermolecular H-bonds which the Tau and Phe NH groups form with the acetyl carbonyl and one of the two sulphonamide oxygens, respectively.     

Peptide crystal growth via vapor diffusion. Crystal structure of glycyl-L-tyrosyl-L-alanine dihydrate.

The structure of a dihydrated form of glycyl-L-tyrosyl-L-alanine (GYA) has been determined as part of a series of peptide structural investigations and development of microscale vapor diffusion experiments for peptide crystal growth. Crystals were grown by the hanging-drop method against sodium acetate. The tripeptide is a zwitterion in the crystal, adopting an extended conformation through glycine, a nearly perpendicular bend at tyrosine and a reverse turn for the C-terminal carboxylate. Principal backbone torsion angles are psi 1 175(1) degrees, omega 2 173(1) degrees, phi 2 -119(1) degrees, psi 2 120(1) degrees, omega 3 172(1) degrees, phi 3 -73(1) degrees, psi 31 -9(1) degrees, psi 32 171(1) degrees. The tyrosyl side chain adopts an unusual orientation (chi 1/2 = -86(1) degrees). The relationship of the GYA.2H2O structure to GYA sequences in proteins is examined, particularly as regards its helix-forming potential. Crystal data: C14H19N3O4.2H2O, M(r) = 345.36, orthorhombic, P2(1)2(1)2(1), a = 4.810 (4), b = 11.400(7), c = 30.162(23)A, V = 1653.8(24)A-3, Z = 4, Dx = 1.387 Mgm-3, lambda(CuK- alpha) = 1.540 A, mu = 9.053 mm-1, F(000) = 736, T = 199 K, R = 0.041 for 1458 observations with I greater than or equal to 3 sigma(I).     

Dynamic light-scattering study of muscle F-actin. II

By dynamic light scattering, the intensity autocorrelation function, G2(tau) = B[1 + beta[g1(tau)[2], was obtained over the scattering angles (theta) from 30 to 130 degrees in steps of 10 degrees for semidilute solutions of muscle F-actin and of F-actin complexed with heavy meromyosin in the absence of ATP (acto-HMM), where B is the baseline and beta a constant. The main findings were: (1) A 0.5 mg/ml F-actin solution gave nonreproducible spectra at theta less than or equal to 40 degrees but quite reproducible spectra at theta greater than or equal to 50 degrees, with beta = 0.9-0.8 at all theta values. Nonreproducibility of spectra at low theta values was concluded to be due to restricted motions of very long filaments confined in cages or zig-zag tubing formed by a major fraction of filaments, where the very long filaments were those at a distant tail of an exponential length distribution and the major fraction of filaments were those with lengths around Ln-2Ln, Ln being the number-average length. Spectral widths were compared with theoretical ones for rigid rods averaged over the length distribution with Ln = 900 nm, and were suggested to be largely contributed at high theta values from bending motions of filaments. (2) Acto-HMM solutions at 0.5 mg/ml F-actin and at weight ratios of HMM to F-actin of 0.5-2 gave spectra which, with respect to theta, behaved very similarly to those of F-actin alone. The spectral widths, however, drastically decreased with the weight ratio up to unity and stayed virtually constant above unity. In contrast to a previous study (F.D. Carlson and A.B. Fraser, J. Mol. Biol. 89 (1974) 273), beta values of acto-HMM were as large as those of F-actin alone. Acto-HMM was concluded to travel a distance far greater than 1/K with a mobility smaller than that of F-actin, where K = (4 pi/lambda) sin(theta/2), lambda being the wavelength of light in the medium. These results suggest that acto-HMM gels are very soft even though they did not pour from an inverted cell. Based on several intuitive models which give a mutual relationship between the beta value and modes of motion of scatterers, we discuss the restricted motions responsible for nonreproducibility of spectra at low angles and large beta values of acto-HMM gels at all theta values and weight ratios so far studied.     

Synthesis, structure and spectroscopic properties of two models for the active site of the oxygenated state of cytochrome P450 [corrected

Two dioxygen adducts of thiolato-iron(II) porphyrins, [K(222)][Fe(TPpivP)(SC6HF4)(O2)] 1a and [Na(18c.6)][Fe(TPpivP)(SC6HF4)(O2)] 2 were synthesized by reaction of O2 with five-coordinate, high-spin, cryptated alkali metal thiolato-iron(II) 'picket fence' porphyrinate. They were characterized by visible and infrared spectroscopy: lambda max (log epsilon) = 360 nm (4), 427 nm (4.69), 560 nm (3.69), 610 nm (3.40) for both compounds; v(16O-16O) = 1139 cm-1 in chlorobenzene and fluorobenzene for 1a and 2. Single crystals of composition [K(222)][Fe(TPpivP)(SC6HF4)(O2)].[K(222)](SC6HF4)(C 6H5Cl)(H2O) 1b were obtained by diffusion of pentane/xylene mixtures into chlorobenzene solutions of 1a at -5 degrees C. Single crystals of composition [Na(18c.6)][Fe(TPpivP)(SC6HF4)(O2)] were obtained by slow diffusion of pentane into benzene solutions of 2. Structures of 1b and 2 were studied at 20 degrees C (1b) and -100 degrees C (1b and 2). 1b: space group P2(1)/c (monoclinic), a = 16.806(5) A (1.6806 nm), b = 14.331(4) A (1.4331 nm), c = 52.000(15) A (5.2000 nm), beta = 92.95(2) degrees, V = 12.507 A3 (12.507 nm3), Z = 4, Dcal = 1.28 g.cm-3 (t = 20 degrees C). The final R1 factor was 0.085 for 5238 reflections having I greater than 3 sigma(I). 2: space group P2(1)/c (monoclinic), a = 13.107(3) A (1.3107 nm), b = 27.055(4) A (2.7055 nm), c = 25.029(4) A (2.5029 nm), beta = 96.84(2) degrees, V = 8812 A3 (8.812 nm3), Z = 4, Dcal = 1.18 g.cm-3 (t = -100 degrees C). The final R1 factor was 0.088 for 6587 reflections having I greater than 3 sigma(I). The iron atom is, in both compounds, bonded to the four porphyrinato nitrogens (Np), the sulfur atom of the axial thiolate and one oxygen atom of the axially end-on bonded dioxygen molecule. The average Fe-Np distance found in 1b [1.994(4) A, 0.1994 nm] is not significantly different from that found in 2 [1.993(3) A, 0.1993 nm]. The Fe-S bond length is 2.367(3) A (0.2367 nm) in 1b and 2.365(2) A (0.2365 nm) in 2. The Fe-O1 distances with the oxygen atom of O2 bonded to iron are respectively 1.837(9) A (0.1837 nm) and 1.850(4) A (0.1850 nm). The end-on bonded O2 molecule is disordered in both complexes 1b and 2.(ABSTRACT TRUNCATED AT 400 WORDS)