Nm For Poly Research Articles

Isodichroic point and the β–random coil transition of poly(S‐carboxymethyl‐L‐cysteine) and poly(S‐carboxyethyl‐L‐cysteine) in the absence of added salt

AbstractThe β‐coil transition of poly(S‐carboxymethyl‐L‐cysteine) (poly[Cys(CH2CO2H)]) and poly(S‐carboxyethyl‐L‐cysteine) (poly[Cys((CH2)2CO2H)]) was followed by CD, potentiometric titration, and viscosity in the absence of added salt. These different properties give consistent results for poly[Cys((CH2)2CO2H)]. The CD spectra of poly[Cys(CH2CO2H)] change considerably with the degree of neutralization α even for a low‐molecular‐weight sample incapable of forming the β‐structure. Because of the superposition of this additional effect, the dependence of CD on α is inconsistent with titration data for the case of poly[Cys(CH2CO2H)], particularly when the nπ transition is used to follow the β‐coil transition. The change of CD inherent to the β‐coil transition is characterized by an isodichroic point: 215 nm for poly[Cys((CH2)2CO2H)] and 218 nm for poly[Cys(CH2CO2H)]. A criterion supporting the stacking of the pleated sheet is suggested based on the isodichroic point.

Electric birefringence of ionized polypeptides in solution and the effect of high electric fields on the helix-coil transition

Electric birefringence and circular dichroism measurements have been made on solutions of two po!y (L-lysine) homologs. The specific Kerr constant and the molar ellipticity at 222 nm of poly (L-alpha, gamma-diaminobutyric acid hydrochloride) in methanol/water mixtures underwent an abrupt change between 75 and 80 vol% methanol at 25 degrees C, corresponding to a solvent-induced helix-coil transition. On the helix side of the transition region, i.e., between 78 and 80 vol% methanol, anomalous birefringence transients indicative of field-induced helix-to-coil transition were observed at high fields. In the case of poly (L-ornithine hydrobromide) in methanol/water mixtures, a helix-coil transition was induced between 93 and 98 vol% methanol and anomalous birefringence transients were observed between 96 and 98 vol% methanol. The double logarithmic plots of the steady-state specific birefringence versus the square of field strength for various solvent compositions and polymer concentrations could be superimposed on one another by horizontal and vertical shifts, except for the range where anomalous birefringence transients were observed. This enabled us to estimate the threshold field strength.

Spectroscopic analysis of the interaction of Escherichia coli DNA-dependent RNA polymerase with T7 DNA and synthetic polynucleotides.

We have studied the circular dichroism and ultraviolet difference spectra of T7 bacteriophage DNA and various synthetic polynucleotides upon addition of Escherichia coli RNA polymerase. When RNA polymerase binds nonspecifically to T7 DNA, the CD spectrum shows a decrease in the maximum at 272 but no detectable changes in other regions of the spectrum. This CD change can be compared with those associated with known conformational changes in DNA. Nonspecific binding to RNA polymerase leads to an increase in the winding angle, theta, in T7 DNA. The CD and UV difference spectra for poly[d(A-T)] at 4 degrees C show similar effects. At 25 degrees C, binding of RNA polymerase to poly[d(A-T)] leads to hyperchromicity at 263 nm and to significant changes in CD. These effects are consistent with an opening of the double helix, i.e. melting of a short region of the DNA. The hyperchromicity observed at 263 nm for poly[d(A-T)] is used to determine the number of base pairs disrupted in the binding of RNA polymerase holoenzyme. The melting effect involves about 10 base pairs/RNA polymerase molecule. Changes in the CD of poly(dT) and poly(dA) on binding to RNA polymerase suggest an unstacking of the bases with a change in the backbone conformation. This is further confirmed by the UV difference spectra. We also show direct evidence for differences in the template binding site between holo- and core enzyme, presumably induced by the sigma subunit. By titration of the enzyme with poly(dT) the physical site size of RNA polymerase on single-stranded DNA is approximately equal to 30 bases for both holo- and core enzyme. Titration of poly[d(A-T)] with polymerase places the figure at approximately equal to 28 base pairs for double-stranded DNA.

Étude par spectroscopie ESR de la photodégradation de la poly(vinyl‐2 pyridine) et des copolymères vinyl‐2 pyridine/méthacrylate de méthyle

AbstractIrradiation of films of poly(2‐vinylpyridine), (poly[1‐(2‐pyridyl)ethylene]), and poly(4‐vinylpyridine), (poly[1‐(4‐pyridyl)ethylene]) at wavelengths λ > 300 nm gives rise to the corresponding vinylpyridinyl and azabenzyl radicals, the concentrations of which do not exceed 1015 spins/g. Introduction of naphthyl substituents into the polymer chain enhances appreciably the rate of radical formation from the copolymer. Irradiation at 254 nm of poly(methyl methacrylate), (PMMA) and its copolymers containing various amounts of 2‐vinylpyridine monomeric units shows that the presence of pyridyl groups along the chain of PMMA reduces the rate of accumulation of radicals. This result is ascribed to the screening effect of pyridine. The photoprotection by the pyridyl ring is likewise observed in the phenanthrene sensitized photolysis at λ > 300 nm of the various 2‐vinylpyridine/methyl methacrylate copolymers and its mechanism is discussed.

A small-angle- x-ray scattering study on poly (d(A-t)-d(A-T)), poly(d(A-s4T)-d(A-s4T)) and poly (d(A-s4U)-d(A-s4U)).

A small‐angle X‐ray scattering study was performed with solutions of the alternating polydeoxynucleotide poly[d(A‐T) · d(A‐T)] and its analogs poly[d(A‐s4T) · d(A‐s4T)] and poly[d(A‐s4U) · d(A‐s4U)]. At low concentration of NaCl and neutral pH almost identical results were obtained for these polydeoxynucleotides. At small scattering angles the same cross section radius of gyration Rc= 0.92 ± 0.03 nm for all three polydeoxynucleotides was derived from the scattering curves. At larger scattering angles a second cross‐section radius of gyration Rc2 could be determined which amounts to 0.855 ± 0.02 nm for poly[d(A‐T) · d(A‐T)], 0.86 ± 0.01 nm for poly‐[d(A‐s4T) · d(A‐s4T)] and 0.82 ± 0.01 nm for poly[d(A‐s4U) · d(A‐s4U)], respectively. The length per nucleotide pair, 0.33 ± 0.03 nm, was found to be the same for all three polydeoxynucleotides. These results are similar to those for natural DNA.The above results for poly[d(A‐s4T) · d(A‐s4T)] and poly[d(A‐s4U) · d(A‐s4U)] refer to the modification of these polydeoxynucleotides which prevails at low concentrations of 1:1 electrolytes and was designated as helix I. The length per nucleotide pair obtained in this study appears to be consistent with the presence of strong 0 0 8 and 0 0 16 reflections in the high‐humidity X‐ray fibre diffraction patterns for the sodium salt of poy[d(A‐s4T) · d(A‐s4T)]. The values for the cross‐section radii of gyration, on the other hand, seem to be inconsistent with the reversed Hoogsteen base‐pairing scheme proposed for poly[d(A‐s4T) · d(A‐s4T)] and poly[d(A‐s4U) · d(A‐s4U)], because the similarity of these values to those for poly[d(A‐T) · d(A‐T)] and natural DNA is contrary to the results of theoretical calculations on models built up from reversed Hoogsteen base‐pairs. According to these calculations, the cross‐section radius of gyration of poly‐[d(A‐s4T) · d(A‐s4T)] ought to be smaller than that of natural DNA by about 0.16–0.2 nm if this polydeoxynucleotide really were of the reversed Hoogsteen type.

A new polynucleotide complex poly(s 2 C)-poly(I).

Poly(s2C) · poly(I) complex formation (where s2C = 2‐thiocytidylate) was shown by spectrophotometric titration, absorption‐difference spectroscopy and hydroxyl‐ion titration. The absorption temperature profile of the complex does not display a transition between 10 and 100°C. Cooperative melting with a transition midpoint at 77°C was observed only after destabilisation by 30% ethyleneglycol. A stabilizing effect of s2CMP residues in hellces is further shown by the observation that 25% s2CMP in poly(C3,s2C) · poly(I) significantly increased the thermal stability compared to poly(C) · poly(I). The ultraviolet absorption spectrum, first derivative absorption spectrum and circular dichroism spectrum suggest a weak n—π* transition at 320 nm in poly(s2C) · poly(I). Unlike other nucleoside diphosphates the polymerisation of s2CDP by polynucleotide phosphorylase from Escherichia coli occurred with a pronounced lag phase which disappeared after the addition of oligonucleotide primers.