No abstract

Baicalein (5,6,7-trihydroxyflavone, 1) is of interest because of its broad spectrum of biological activity. It is a constituent of the east Asian herbal remedy, "Sho-saiko-to". The 3D structure of 1 was determined using X-ray diffraction. The compound exists in an almost planar conformation with a C-2-C-1' bond distance of 1.476(5) A. Hydrogen-bonding interactions predominate in the crystal structure. The position of the three hydroxyl groups maximizes intramolecular hydrogen bonding, and each of the hydroxyl hydrogen atoms is a donor in a three-center hydrogen bond. The carbonyl oxygen, O-4, is an acceptor in an intramolecular hydrogen bond (with OH-5). Two molecules of 1 exist as hydrogen-bonded dimers related by inversion center (-x + 1, -y, -z + 1). O-4 is also an acceptor in an intermolecular hydrogen bond with OH-6. The planarity of the flavone framework is dependent on structural and/or electronic forces that stabilize the negative charge on the exocyclic oxygen atom, O-4. Compound 1, therefore, is planar in any situation where forces can stabilize the negative charge on O-4. Consistent with this, UV absorbance studies performed on 1-DNA complexes with varying concentrations of 1 strongly suggest intercalation of 1 within the double helix, followed by possible interstrand cross-links.

Dynamical calculations of the intensities of normally incident low-energy electrons diffracted from GaAs(110}, performed using-a matrix-inversion method, are compared both with earlier kinematical calculations and with measured intensities. The insensitivity of the calculated intensities to the choice of exchange potential and vacuum-solid boundary conditions is displayed. Surface lattice vibrations are found to be adequately described by the bulk Debye temperature. %'e consider second-and third-layer structural distortions as well as top-layer reconstructions. This analysis leads to the selection of the most probable surface structure for GaAs(110) as one in which the top layer undergoes both a rigid rotation of 27. 4' and a O 0.05-A contraction with the As atoms moving outward and the Ga atoms inward, giving a relative vertical shear of 0.65 A. In the second layer the Ga moves outward 0.06 A and the second-layer As moves inward 0.06 A. The dynamical analysis reported herein shows no evidence for third-layer distortions.

An ordered (l x l) structure is formed by evaporating approximately one-half monolayer of Al on GaAs(llO) and subsequently annealing at 450 °C for 30 min. Both a dynamical analysis of the low-energy electron-diffraction intensities from the resulting half-monolayer of GaAs(110)~£(lx 1)-A1 structure and soft-x-ray photoemission spectroscopy indicate that the Al replaces the Ga in the second atomic layer beneath the surface. The uppermost layer retains the structure of clean GaAs(llO) but is relaxed 0.1 A toward the second AlAs layer.