Intrinsic Persistence Research Articles

Monte Carlo test of electrostatic persistence length for short polymers

A Metropolis Monte Carlo program with reptation is used to generate sample configurations of short electrically charged polymers with thermally distributed Debye–Hückel electrostatic energies. The polymer is a three fold rotational isomeric state model with bond angle θ between 5° and 90° and number of units N between 10 and 225. To compare the resulting root-mean-square (rms) values for radius of gyration S, and end-to-end length R, to theory, we use a wormlike chain with contour length L equal to the stretched out length of the polymer, the same total charge, and an intrinsic persistence length set so that, for large N, for specified θ and L, S agrees with the rotational isomeric state model. The results are compared with the predictions for S, with correction for finite L, of Odijk [J. Polymer Sci., Polymer Phys. Ed. 15, 477 (1977)]. They are then compared with three attempted corrections for excluded volume: (1) Odijk and Houwaart [J. Polymer Sci., Polymer Phys. Ed. 16, 627 (1978)]; (2) correction (1) modified by using the electrostatic excluded volume of Fixman and Skolnick [Macromolecules 11, 863 (1978)]; (3) correction (2) modified by replacing the Yamakawa–Tanaka formula by an approximation due to Gupta and Forsman [Macromolecules 5, 779 (1972)]. Odijk’s prediction with correction for finite length works fairly well under conditions of small excluded volume. The excluded volume corrections are often but not always of about the right size; the latter two work better.

Double‐stranded helix of xanthan: Rigidity in 0.01M aqueous sodium chloride containing 0.01 N hydrochloric acid

AbstractSix samples of Na xanthan in 0.01M aqueous NaCl containing 0.01 N HCl (pH = 2) were studied by light scattering and viscosity. This study was motivated by the finding that the intrinsic viscosity [η] fairly sharply decreased when the pH of the solvent was lowered from about 6 to 2 by adding HCl to 0.01M aqueous NaCl in which Na xanthan dissolves as rigid dimers having a double‐helical structure. The data for weight‐average molecular weight, radius of gyration, and [η] showed that Na xanthan at pH = 2 remains a dimer behaving as a semiflexible chain. Data analysis in terms of known theories for unperturbed wormlike chains yielded 0.47 ± 0.02, 2.0 ± 0.6, and 68 ± 7 nm for the contour length h per main‐chain residue, diameter d, and persistence length q of the dimer, respectively. these h and d values agreed with the pitch per main‐chain residue and the diameter of the double helix of Na xanthan in 0.01 or 0.1M aqueous NaCl. However, the q value, which was close to the intrinsic persistence length q0 ( = q in the absence of electrostatic interaction) of Na xanthan at pH = 2, was much smaller than the q0 (106 nm) of this helix. We concluded that the xanthan dimer at pH = 2 assumes a double‐helical structure, which is geometrically the same as, but is more flexible than, that at neutral pH.

Viscosity behaviour and persistence length of sodium xanthan in aqueous sodium chloride

Zero-shear-rate intrinsic viscosities [eta] of sodium xanthan in aqueous NACl at 25 degrees C were determined for five samples ranging in weight- average molecular weight from 2 x 10(5) to 4 x 10(6) at salt concentrations Cs between 0.005 and 1 M, at which the polysaccharide maintains its double-helical structure. The measured [eta] for every sample was almost independent of Cs, in contrast to usual observations on flexible polyelectrolytes. The persistence length q of sodium xanthan was determined as a function of Cs by use of the theory of Yamakawa et al. for [eta] of an unperturbed worm-like cylinder, and from its Cs dependence the intrinsic persistence length q(o) ( = q at infinite ionic strength) was estimated to be 106 nm. This q(o) value was roughly twice as large as that of double-stranded DNA, indicating a high intrinsic rigidity of the xanthan double helix. The electrostatic contribution ( = q - q(o)) to q was only about 10% even at the lowest Cs of 0.005 M. Thus, it was concluded that above Cs = 0.005 M, the double- helical structure of sodium xanthan is hardly stiffened by electrostatic interactions between charged groups.

Magnetic birefringence study of the electrostatic and intrinsic persistence length of DNA.

AbstractMagnetic birefringence experiments were performed on solutions of DNA of intermediate molecular weight at several concentrations (cp) over a wide range of ionic strengths (of NaCl and MgCl2). The specific Cotton‐Mouton constant (CM/cp) is found to be independent of cp when contributions from cp to the ionic strength (μeff) are taken into account according to the concept of counterion condensation. For μeff ≳ 10−2M, CM/cp is also independent of the ionic strength; the plateau value results in an acceptable value of the intrinsic persistence length, when a revised theoretical expression for the magnetic birefringence of wormlike chains is used, combined with new experimental data for the monomeric optical and magnetic anisotropy. For μeff < 10−2M, CM/cp strongly, or wealky, increases with decreasing μeff, depending on the valency of the counterion used (Na+ or Mg2+, respectively). This increase agrees quantitatively with the variation of the electrostatic persistence length as predicted by Odijk [(1977) J. Polym. Sci. Polym. Phys. Ed. 15, 477–483], Odijk and Houwaart [(1978) J. Polym. Sci. Polym. Phys. Ed. 16, 627–639], and by Skolnick and Fixman [(1977) Macromolecules 10, 944–948]. A comparison with other experimental data seems to reveal the importance of excluded‐volume effects, which are particularly pronounced in the low‐salt regime.

Activity of juvenile hormone mimics in egg-laying ticks.

Treatment of freshly engorged Boophilus decoloratus, B. microplus and Amblyomma hebraeum adult females with juvenile hormone mimics causes desiccation of eggs laid. The number of eggs desiccating is dose-dependent, suggesting that activity is a function of both intrinsic activity and persistence. In B. decoloratus highest activity was shown by N-ethyl 3,7,11-trimethyldodeca-2,4-dienamide (ZR 615), while isopropyl 11-methoxy-3,7,11-trimethyldodeca-2,4-dienoate (ZR 515, Altosid®) was less active. Hexadecyl cyclopane-carboxylate (ZR 856), which is active against mites, showed low activity towards ticks. Structure-activity relationships indicate high activity associated with the presence of nitrogen in the molecule and the absence of functional groups that can serve as sites for rapid inactivation.

The toxicity of insecticides to larvae of the codling moth, Cydia pomonella (L.). I. Intrinsic toxicity and persistence.

The deposits found on apples at various times after application of 11 insecticides were determined by chemical analysis of samples from an orchard in south-eastern England sprayed in 1959 and 1960. The results, from samples taken simultaneously, of studies of the effectiveness of these and other chemicals for control of the codling moth, Cydia pomonella (L.), have already been reported.The period in which 50 per cent, of an initial deposit disappeared in the field (half-life) was calculated from the chemical data and used as a measure of persistence. For comparisons of intrinsic toxicity, the levels of fresh deposits causing 50 per cent, mortality of newly emerged codling-moth larvae (LD50) were determined in the laboratory by chemical and biological tests on apples dipped in a range of concentrations of commercial formulations. The LD50's, in μg./cm.2, were less than 0·025 for carbophenothion, parathion and diazinon, between 0·025 and 0·05 for azinphos-methyl and -ethyl, between 0·05 and 0·10 for malathion, DDT and carbaryl, between 0·1 and 0·5 for DDD and dimethoate, between 0·5 and 1·5 for lead arsenate and above 1·5 for phenkapton.Chemicals with an LD50 exceeding about 0·1 μg./cm.2 were considered not worth further investigation as codling-moth larvicides. The LD90's of the remainder were found and were taken as the minimum deposits that would have an adequate effect from the standpoint of control (minimum effective levels). From the half-life values, the periods for which the deposits found in the orchard in 1960 should have remained above the minimum effective levels were calculated; these were 24 days for DDT emulsion, 22 for DDT wettable powder, 15 for carbophenothion, 14 for carbaryl, 12 for azinphos-ethyl, 11 for parathion, 7 for malathion and 6 for diazinon. They were in good agreement with the results of the biological tests except in the case of DDT emulsion, the effectiveness of which was unexpectedly short-lived.