Biomedical subjects

Anwar Usman

Publications and source records attributed to Anwar Usman.

At least 19 recordsLinked to original sources

PubMed2006

trans-cis Photoisomerization of a photoactive yellow protein model chromophore in crystalline phase.

We have studied the photoinduced trans/cis isomerization of the protonated form of p-hydroxycinnamic thiophenyl ester, a model chromophore of the photoactive yellow protein (PYP), in crystalline phase, by both fluorescence and infrared spectroscopies. The conversion from trans to cis configuration is revealed by a shift of the fluorescence peak and by inspection of the infrared maker bands. The crystal packing apparently stabilizes the cis photoproduct, suggesting different environmental effects from the solvent molecules for this model chromophore in liquid solutions or from the amino acid residues for the PYP chromophore.

Bacterial Proteins↗

PubMed2005

Structural evolution of the chromophore in the primary stages of trans/cis isomerization in photoactive yellow protein.

We have studied the structural changes induced by optical excitation of the chromophore in wild-type photoactive yellow protein (PYP) in liquid solution with a combined approach of polarization-sensitive ultrafast infrared spectroscopy and density functional theory calculations. We identify the nuC8-C9 marker modes for solution phase PYP in the P and I0 states, from which we derive that the first intermediate state I0 that appears with a 3 ps time constant can be characterized to have a cis geometry. This is the first unequivocal demonstration that the formation of I0 correlates with the conversion from the trans to the cis state. For the P and I0 states we compare the experimentally measured vibrational band patterns and anisotropies with calculations and find that for both trans and cis configurations the planarity of the chromophore has a strong influence. The C7=C8-(C9=O)-S moiety of the chromophore in the dark P state has a trans geometry with the C=O group slightly tilted out-of-plane, in accordance with the earlier reported structure obtained in an X-ray diffraction study of PYP crystals. In the case of I0, experiment and theory are only in agreement when the C7=C8-(C9=O)-S moiety has a planar configuration. We find that the carboxylic side group of Glu46 that is hydrogen-bonded to the chromophore phenolate oxygen does not alter its orientation on going from the electronic ground P state, via the electronic excited P state to the intermediate I0 state, providing conclusive experimental evidence that the primary stages of PYP photoisomerization involve flipping of the enone thioester linkage without significant relocation of the phenolate moiety.

Bacterial Proteins↗

PubMed2005

Excited-state structure determination of the green fluorescent protein chromophore.

Ultrafast polarization-sensitive infrared (IR) spectroscopy of the C=O stretching mode of the chromophore of the green fluorescent protein reveals a near complete twisting around the ethylenic bridge between the phenolate and imidazolidinone groups upon electronic excitation, hinting at a decisive role of this motion in the efficient internal conversion process.

Green Fluorescent Proteins↗

PubMed2003

Redetermination of hydronium perchlorate at 193 and 293 K.

A sample of hydronium perchlorate, H(3)O(+) x ClO(4)(-), crystallized from ethanol at ambient temperature, was found to be orthorhombic (space group Pnma) at both 193 and 293 K, with no phase transition observed in this temperature range. This contrasts with the earlier observation [Nordman (1962). Acta Cryst. 15, 18-23] of a monoclinic phase (space group P2(1)/n) at 193 K for crystals grown at that temperature from perchloric acid. The hydronium and perchlorate ions lie across a mirror plane but it is not possible to define at either temperature a simple description of the H-atom positions due to the three-dimensional tumbling of the hydronium cation.

Journal Article↗

PubMed2003

Bis(2-aminopyridine-kappaN(1))(benzoato-kappaO)silver(I).

In the title compound, [Ag(C(7)H(5)O(2))(C(5)H(6)N(2))(2)], the Ag(I) atom is tricoordinated by two independent pyridine N atoms and one benzoate O atom in a nearly planar geometry. An intramolecular N-H.O hydrogen bond forms an S(8) graph ring. The packing is built from molecular layers stabilized by two types of N-H.O hydrogen bond. Intermolecular Ag.N and intramolecular Ag.O contacts were also observed, together with three weak intermolecular C-H.pi interactions.

Journal Article↗

PubMed2003

9,10-Diphenyl-9,10-epidioxyanthracene and 9,10-dihydro-10,10-dimethoxy-9-phenylanthracen-9-ol.

9,10-Diphenyl-9,10-epidioxyanthracene, C(26)H(18)O(2), (I), was accidentally used in a photooxygenation reaction that produced 9,10-dihydro-10,10-dimethoxy-9-phenylanthracen-9-ol, C(22)H(20)O(3), (II). In both compounds, the phenyl rings are approximately orthogonal to the anthracene moiety. The conformation of the anthracene moiety differs as a result of substitution. Intramolecular C-H.O interactions in (I) form two approximately planar S(5) rings in each of the two crystallographically independent molecules. The packing of (I) and (II) consists of molecular dimers stabilized by C-H.O interactions and of molecular chains stabilized by O-H.O interactions, respectively.

Journal Article↗

PubMed2003

1-Acetyl-4'-ethyl-2a'-phenyl-2',2a',5',5a'-tetrahydrospiro[1H-indole-3,2'-oxeto[5,4-b]oxazol]-2(3H)-one and 1-acetyl-4',5a'-diphenyl-2',2a',5',5a'-tetrahydrospiro[1H-indole-3,2'-oxeto[5,4-b]oxazol]-2(3H)-one.

In the title compounds, C(21)H(18)N(2)O(4) and C(25)H(18)N(2)O(4), respectively, the five-membered ring of the indole system is almost planar. The oxetane and oxazole rings are individually planar. The orientations of the indole, oxetane, oxazole and phenyl moieties are conditioned by the sp(3) nature of the spiro-C atoms. In both compounds, the relative orientation of the indole and oxazole rings is opposite.

Journal Article↗

PubMed2003

meso-Bis(phenylsulfinyl)methane.

The title compound, meso-C(13)H(12)O(2)S(2), is in an anti conformation, with R and S configurations around the S atoms. The two O atoms are trans to each other, and the same applies for the two benzene rings. The phenylsulfinyl groups are nearly orthogonal to the central dithiomethane group, and the orientation of the two phenyl rings are determined by the interactions in which they are involved. The packing is built from molecular columns stabilized by weak C-H...O interactions.

Journal Article↗

PubMed2003

trans-Diaquabis(thiosemicarbazido-kappa2N,S)nickel(II) dimaleate dihydrate.

In the title compound, [Ni(CH(5)N(3)S)(2)(H(2)O)(2)](C(4)H(3)O(4))(2).2H(2)O, the Ni atom lies on a center of symmetry and is coordinated by N and S atoms from two thiosemicarbazide ligands and the O atoms of two water molecules in a distorted octahedral geometry. In the asymmetric unit, the three components are linked together by one O-H...O and two N-H...O hydrogen bonds. The packing is built from molecular ribbons parallel to the b direction, stabilized by intramolecular hydrogen bonds, and by one N-H...S and two N-H...O intermolecular hydrogen bonds. The ribbons are further connected into columns by N-H...O interactions and then into a three-dimensional network by three O-H...O hydrogen bonds.

Journal Article↗

PubMed2003

Polymeric structure of (ethylenediamine)silver(I) 3-nitrobenzoate monohydrate.

In the ternary title compound, catena-poly[[silver(I)-mu-ethylenediamine-kappa(2)N:N'] 3-nitrobenzoate monohydrate], [[Ag(C(2)H(8)N(2))](C(7)H(4)NO(4)) x H(2)O](n), the Ag atom is bicoordinated in a linear configuration by two different N atoms from two symmetry-related ethylenediamine ligands, thus giving linear polymeric chains with an [-Ag-N-C-C-N-](n) backbone running parallel to the a axis. In the crystal packing, these linear chains are interconnected by N-H...O and O-H...O hydrogen bonds to form layers parallel to the ab plane.

Journal Article↗

PubMed2003

N-[4-bromo-2-[(3-bromo-1-phenylsulfonyl-1H-indol-2-yl)methyl]-5-methoxyphenyl]acetamide.

In the title compound, C(24)H(20)Br(2)N(2)O(4)S, the indole ring system is planar and the S atom has a distorted tetrahedral configuration. The sulfonyl-bound phenyl ring is orthogonal to the indole ring system and the conformation of the phenylsulfonyl substituent with respect to the indole moiety is influenced by intramolecular C-H...O hydrogen bonds involving the two sulfonyl O atoms. The mean plane through the acetylamido group makes a dihedral angle of 57.0 (1) degrees with the phenyl ring of the benzyl moiety. In the crystal, glide-related molecules are linked together by N-H...O hydrogen bonds and C-H...pi interactions to form molecular chains, which extend through the crystal. Inversion-related chains are interlinked by C-H...pi interactions to form molecular layers parallel to the bc plane. These layers are interconnected through pi-pi interactions involving the five- and six-membered rings of the indole moiety.

Journal Article↗

PubMed2003

Bis[14beta-hydroxy-3beta-O-(L-thevetosyl)-5beta-card-20(22)-enolide] methanol solvate monohydrate and 3beta-O-(L-2'-o-acetylthevetosyl)-14beta-hydroxy-5beta-card-20(22)-enolide.

The title compounds, 2C(30)H(46)O(8).CH(3)OH.H(2)O, (I), and C(32)H(48)O(9), (II), respectively, are cardenolide glycosides which were isolated from the seeds of Cerbera odollam. There are two crystallographically independent cardenolide molecules in (I), together with one methanol and one water solvate molecule. In both (I) and (II), the steroid nuclei are in cis/trans/cis configurations, with the cyclopentane rings showing conformational flexibility, viz. an envelope conformation in (I) and a twisted conformation in (II). In both compounds, the lactone ring is nearly orthogonal to the cyclopentane ring. The packing of (I) is composed of molecular layers stabilized by five O-H...O hydrogen bonds. In the packing of (II), the molecules are packed into columns by one O-H...O hydrogen bond, and are further interconnected into a three-dimensional network by one O-H...O and three C-H...O interactions.

Apocynaceae↗

PubMed2003

Spectral studies and structure of a 2-hydroxyacetophenone 3-hexamethyleneiminyl thiosemicarbazonate(-2) copper(II) complex containing 1,10-phenanthroline.

The spectral studies and structure of a ternary complex of copper(II) with 2-hydroxyacetophenone 3-hexamethyliminylthiosemicarbazonate (L(2-)) and 1,10-phenanthroline (phen) are reported. The thiosemicarbazone binds to the metal as a dianionic ONS-donor (L(2-)) ligand, and forms a complex of the stoichiometry [CuLphen]. The copper(II) complex was characterized by IR and UV/Vis spectroscopies, as well as by solid state room-temperature magnetic susceptibility. Spin Hamiltonian and bonding parameters of the compound are calculated from the EPR spectra. Computer simulation of EPR spectrum in DMF at 77 K aided the calculation of magnetic and bonding parameters of the compound. The structure of the compound is solved by single crystal X-ray diffraction. The geometry around copper is distorted square pyramidal.

Acetophenones↗

PubMed2002

Barium(II)-2,4-dinitrophenolate-18-crown-6 at 183 K.

The title compound, bis(2,4-dinitrophenolato-kappa2O,O')(1,4,7,10,13,16-hexaoxadecane-kappa6O)barium(II), [Ba(C6H3N2O5)2(C12H24O6)], is a 1:1 complex of barium(II)-2,4-dinitrophenolate and 1,4,7,10,13,16-hexaoxacyclooctadecane (18-crown-6). Its structure is located on a crystallographic inversion centre. The temperature dependence of the crystal structure has been studied. The monoclinic beta angle of the P2(1)/n space group increases with increasing temperature. The packing structure of the complex is stabilized by intermolecular C-H...O interactions.

Journal Article↗

PubMed2002

2-Hydroxy-3-(4,5,6,7-tetrahydro-1H-imidazo[4,5-c]pyridin-5-ium-4-yl)benzoate monohydrate.

The title molecule, C(13)H(13)N(3)O(3).H(2)O, is in the form of a monohydrated zwitterion. The tetrahydropyridinium ring adopts an envelope conformation and is nearly coplanar with the plane of the imidazoline ring. The water solvate molecule plays an important role as a bridge between zwitterions, forming molecular chains running along the c direction, which are interconnected by N-H.O hydrogen bonds into molecular ribbons. The crystal packing is further stabilized by another N-H.O and one O-H.N hydrogen bond, which interconnect the molecular ribbons.

Journal Article↗

PubMed2002

Hexamethylenetetramine-4-nitrocatechol-water (1/2/1).

In the title adduct, 1,3,5,7-tetraazatricyclo[3.3.1.1(3,7)]decane-4-nitrobenzene-1,2-diol-water (1/2/1), C(6)H(12)N(4).2C(6)H(5)NO(4).H(2)O, the hexamethylenetetramine molecule acts as an acceptor of intermolecular O-H.N hydrogen-bonding interactions from the water molecule and the hydroxy groups of one of the two symmetry-independent 4-nitrocatechol molecules. The structure is built from molecular layers which are stabilized by three intermolecular O-H.O, two intermolecular O-H.N and four intermolecular C-H.O hydrogen bonds. The layers are further interconnected by one additional intermolecular O-H.N and two intermolecular C-H.O hydrogen bonds.

Journal Article↗

PubMed2002

Di-2-pyridyl ketone N4,N4-(butane-1,4-diyl)thiosemicarbazone.

The title compound, C(16)H(17)N(5)S, is in the thione form and crystallizes with two independent molecules in the asymmetric unit. In both molecules, the pentamethyleneimine five-membered ring adopts an envelope conformation, and in one of the molecules this ring shows positional disorder. The thione S and hydrazine N atoms are in the Z configuration with respect to the C-N bond.

Journal Article↗

PubMed2002

A ternary complex of aqua(18-crown-6)bis(o-nitrophenolato)barium(II), the triaqua(18-crown-6)(o-nitrophenolato)barium(II) cation and the o-nitrophenolate anion.

In the title compound [systematic name: triaqua(1,4,7,10,13,16-hexaoxacyclooctadecane-kappa(6)O)(2-nitrophenolato-kappaO)barium(II)-aqua(1,4,7,10,13,16-hexaoxacyclooctadecane-kappa(6)O)bis(2-nitrophenolato-kappa(2)O,O')barium(II)-2-nitrophenolate (1/1/1)], [Ba(C(12)H(24)O(6))(C(6)H(4)NO(3))(H(2)O)(3)][Ba(C(12)H(24)O(6))(C(6)H(4)NO(3))(2)(H(2)O)](C(6)H(4)NO(3)), the two Ba(II) atoms encapsulated by the 18-crown-6 rings have different coordinations. Although both Ba(II) atoms are coordinated to the six O atoms of the crowns, in the neutral moiety, the Ba(II) atom is coordinated to one terminal O atom from a water molecule, two phenolate O atoms and two nitro-group O atoms, while in the cationic moiety, the Ba(II) atom is coordinated to three terminal O atoms from water molecules and one phenolate O atom. Both the crowns are eclipsed and translated along the b direction. In the asymmetric unit, the three components are interconnected by four O-H.O interactions. The packing is stabilized by two intermolecular C-H.O interactions and by one O-H.O interaction.

Journal Article↗