Conformational studies of polymers and copolymers of L-aspartate esters. I. Preparation and solution studies

An alcoholysis method is described for the modification of high molecular weight poly(β-benzyl L -asparatate); by this method the benzyl groups in the polypeptide have been replaced by methyl, ethyl, isopropyl, n-propyl, and phenethyl groups to give a series of copolymers of each of the corresponding aspartate esters with benzyl L -aspartate. By repeating the reactions, replacement of better than 99% has been achieved in some cases to give in effect the homopolymer. Optical rotatory dispersion studies show that of all the systems studied only poly(β-methyl L -aspartate) has the left-handed helix sense, the others are right-handed. It is shown further that the helix sense is not an intrinsic property of the nature of the aspartate side chain. Raising the temperature of chloroform solutions of the right-handed form of the copolymers of benzyl L -aspartate and ethyl L -aspartate results in a transition to the left-handed helix, the temperature of the transition being dependent on the composition of the copolymer. Also poly(β-n-propyl L -aspartate) undergoes a transition from the right- to the left-handed helix form at 59°C. These results suggest a general pattern of behavior of poly(aspartate esters) and that with suitable conditions of solvent and temperature they may be in either the right- or left-handed helical form.     

Uridine-cytidine kinase. Kinetic studies and reaction mechanism.

The initial velocity pattern has been determined for uridine-cytidine kinase purified from the murine mast cell neoplasm P815. With either uridine or cytidine as phosphate acceptor, and ATP as phosphate donor, the pattern observed was one of intersecting lines, ruling out a ping-pong reaction mechanism, and suggesting that the reaction probably proceeds by the sequential addition of both substrates to the enzyme to form a ternary complex, followed by the sequential release of the two products. This pattern was obtained whether the reaction was run in 0.01 m potassium phosphate buffer, pH 7.5, or in 0.1 m Tris-HCl, pH 7.2. When analyzed by the Sequen computer program, the data indicated an apparent K_m of the enzyme for uridine of 1.5 × 10^?4m, an apparent K_m for cytidine of 4.5 × 10^?5m, and a K_m for ATP, with uridine or cytidine as phosphate acceptor, of 3.6 × 10^?3m or 2.1 × 10^?3m, respectively. The V was 1.83 μmol phosphorylated/min/mg enzyme protein for the uridine kinase reaction and 0.91 μmol for the cytidine kinase reaction.     

Genetical studies onOenothera. V.

Molecular Genetics and Genomics -     

Cytogenetic studies of three triazine herbicides. I. In vitro studies

Atrazine, simazine, and cyanazine are widely used pre-emergence and post-emergence triazine herbicides that have made their way into the potable water supply of many agricultural communities. Because of this and the prevalence of contradictory cytogenetic studies in the literature on atrazine, simazine, and cyanazine, a series of in vitro experiments was performed to investigate the ability of these three triazines to induce sister chromatid exchanges (SCEs) and chromosome aberrations (CAs) in human lymphocyte cultures. Our results showed that all three triazines failed to produce any significant increases in SCEs or CAs up to the limits of solubility [using 0.5% dimethyl sulfoxide (DMSO)]. Our results are discussed in light of contradictory results in the literature.