Circular dichroism studies on the interaction of the Lac repressor with Poly d(A-T) and nucleic acid components

Nonspecific binding of lac repressor to DNA. I. An absorption and circular dichroism study

Nonspecific binding of lac repressor on DNA has been studied by absorption and circular dichroism (CD) spectroscopies. In a first step, the complex formation is accompanied by an absorption difference spectrum and a change of the CD signal of the DNA. The absorption difference spectrum is mainly due to a spectral change of the DNA. The variation of the CD signal has been analyzed according to a model calculation, which takes into account the fact that the excluded site is shorter than the perturbed site. We found that in this first step one repressor can bind every 14 +/- 2 base-pairs, whereas one repressor perturbs 22 +/- 2 base-pairs. In a second step, more repressor can bind on DNA, but without further change in the absorption and CD spectrum, indicating that another binding process occurs. The model calculation developed here is general for all binding processes inducing a perturbation over a length of DNA longer than that of the excluded site.

Pyridine interactions with phenolic groups in water: evidence for hydrogen bonding and hydrophobic association

Pyridine interactions with phenol, substituted phenol, tyrosine and poly(Glu50,Tyr50) in aqueous solutions have been studied by ultraviolet (UV) difference spectroscopy, spectrophotometric pH titration, circular dichroism (CD) and proton magnetic resonance (PMR) spectroscopy. A red shift and spectral sharpening of the near-UV spectrum of phenol in water was noted at pyridine concentrations greater than 0.25 M. In addition, the spectrophotometric equivalence point for the phenol- or substituted phenol-phenolate equilibrium was increased about 0.5 pH units upon the addition of 1.0 M pyridine. PMR studies were consistent with the formation of a 1 : 1 phenol-pyridine hydrogen bonded complex. The equilibrium constant derived for this interaction, 0.6-0.7 M-1, is greater than the corresponding value for phenol-acetate hydrogen bonding in water. Enhancement of thepyridine hydrogen bond interaction with Tyr within poly(Glu50,Tyr50) was observed at pH greater than 12 due to a hydrophobic microenvironment produced by pyridine molecules intercalating between neighboring tyrosyl residues.