Photoresponsive peptide and polypeptide systems. VIII. Synthesis and reversible photochromism of azo aromatic poly(L-ornithine)

Poly(L -ornithine) having azo aromatic side chain was synthesized by the water-soluble carbodiimide procedure. The photochemical properties of the azo polypeptide poly[N^δ-p-(phenylazo)benzoyl-L -ornithine] (PPABLO) was investigated by absorption and circular dichroism spectroscopy in hexafluoro-2-propanol (HFIP). The photochromism of the dichroic bands in the visible and ultraviolet wavelength regions was found to be reversible as a function of irradiation time at different wavelengths due to the photostationary state (about 70% trans)–cis photoisomerization of the azo aromatic moieties. The reversible photoinduced solubility change was also studied. On irradiation PPABLO is soluble under ultraviolet light (cis) and precipitate under visible light (70% trans) in HFIP–water. A discussion was presented that includes results on azo aromatic poly(L -lysine).     

Synthesis and antitumor activity of lipid A analogs having a phosphonooxyethyl group with alpha- or beta-configuration at position 1

Three novel lipid A analogs, which have an alpha- or beta-glycosidically bound phosphonooxyethyl group instead of the alpha-glycosyl phosphate group of natural lipid A, were synthesized. The first analog (2) had an alpha-phosphonooxyethyl group on the identical acylated disaccharide 4'-phosphate structure found in natural lipid A (from Escherichia coli) and hence differed from the latter only in the nature of the acidic group at position 1. The second one (3) had tetradecanoyl groups in place of the two (R)-3-hydroxytetradecanoyl groups bound to the 2- and 3-hydroxyl function of 2, retaining the alpha-phosphonooxyethyl group. The structure of the third analog (4) was the same as that of 3 except that the phosphonooxyethyl group of the former was beta-oriented. Compounds 2 and 3 exhibited potent activity against Meth A at the same level as natural lipid A, whereas 4 showed less activity. This fact revealed that the glycosidic phosphate is not a prerequisite for the antitumor activity of lipopolysaccharide. It can be replaced with a phosphonooxyethyl group without any loss of activity provided that the alpha-anomeric configuration at C-1 is retained. The replacement of the hydroxytetradecanoyl groups with tetradecanoyl groups does not change the activity either.     

Structures of moracins E,F, G,and H,new phytoalexins from diseased mulberry

The structures and antifungal activity of four new mulberry phytoalexins, designated as moracins E, F, G, and H, are described.     

The role of metal cations in the homogeneous phase redox reaction of a NADH model compound

The catalytic activity of magnesium cations in the charge transfer between 1-benzyl-1,4-dihydronicotinamide (BNAH) and several aromatic compounds has been studied in acetonitrile by electrochemical techniques. Magnesium cations form complex compounds with both BNAH and the organic substrates studied. At the same time, magnesium forms ion-associates with negatively charged substrate molecules. Energetically, the effect of ion-pairing is much greater than the negative effect of complex formation. The rate of homogeneous phase reaction was also studied to show that the Michaelis-Menten type mechanism is operating.     

Epimerization of diastereomeric alpha-amino nitriles to single stereoisomers in the solid state

[reaction: see text] A diastereomeric mixture of the alpha-amino nitrile prepared by the Strecker reaction of benzaldehyde, (1S,2R)-1-aminoindan-2-ol, and cyanotrimethylsilane thermally epimerizes in the solid state to give a single diastereomer with an (S)-configuration at the alpha position to the nitrile moiety. This shows a sharp contrast to the reaction conducted in DMSO at room temperature, which gives a 1:1 mixture of (S)- and (R)-isomers. Several other alpha-amino nitriles also epimerize in the solid-state toward single diastereomers.     

Carbon-carbon bonding made easy

The palladium-catalyzed cross-coupling reaction between organoboranes and organic electrophiles in the presence of base was first developed 30 years ago. It offers a powerful and general methodology for forming carbon-carbon bonds. The scope of the reaction has continued to evolve and broaden to meet modern synthetic requirements.     

The concentration of surface VO species on oxidized vanadium oxide catalysts

The hollandite-type phase K_1.33Mg_3.11Sb_4.89O₁₆ has been studied by X-ray and electron diffraction as well as high resolution electron microscopy at 500 kV. This material was found to adopt the tetragonal hollandite structure, space group $\text{I4}\text{m}$, with a = 10.315 (4)Å; c = 3.080 (4) Å. The formation of a 3c body-centered supercell was observed and this was shown to be due to ordering of potassium cations within the tunnel sites. Computer image simulations established that ordering of the tunnel cations alone rather than the octahedrally coordinated framework cations was responsible for superlattice formation. In some crystals the supercell ordering appeared to occur in domains.     

Rotational diffusion of tungstic acid colloids in microgravity as studied by aircraft experiments

 Rotational relaxation times (τ) of anisotropic tungstic acid colloids (3.24 μm in major axis) in aqueous suspension are measured in microgravity (0G), normal gravity (1G) and at 2G. The measurements at 0G and 2G are achieved by parabolic and circular flights, respectively. The limiting slopes of the relaxation curves in the plots of the transmitted light intensity against time are close to zero at 0G irrespective of the flow directions in the flow cell, whereas those at 1G and especially at 2G depend on the flow direction by the convection of the suspension and particle sedimentation. Experimental errors at the τ values at 0G are small compared with those at 1G and 2G, which is ascribed to the lack of movement of impurities in the suspension such as quite small air bubbles, which cannot be recognized with the naked eye, and the convection of the suspension in microgravity. More reliable rotational relaxation times are obtained in microgravity; however, the relaxation times themselves are quite insensitive to gravity. Theτ values observed are larger than those calculated from the particle size, which indicates the important contribution of the electrical double layers formed around the colloidal particles. Received: 22 February 2001 Accepted: 13 June 2001     

Ionic photodissociation of some polymers quenched with electron donor or acceptor in solution

Ionic photodissociation processes of vinyl polymers with pendant carbazolyl groups in solution have been studied by nonsecond laser photolysis. The ionic dissociation yield decreases in the order of monomer, oligomers, and polymers.