Optical rotatory dispersion and circular dichroism of sulfur-containing nucleosides and nucleotide and of the ribonuclease-thionucleotide complex

Circular dichroic spectra of 6-thioguanosine nucleotides and their complexes with myosin subfragment 1

The circular dichroic spectra of the thione form of 2-amino-6-mercapto-9 beta-ribofuranosylpurine 5'-triphosphate (thioGTP), thioGDP, thioGTP(gamma S), thioGMP-P(NH)P, and 6-mercapto-9 beta-ribofuranosylpurine 5'-diphosphate (thioIDP) and their complexes with myosin subfragment 1 and heavy meromyosin have been measured between 300 and 400 nm. The free nucleotides have a weak negative circular dichroic spectral peak at their absorption maxima, with a longer wavelength shoulder. On binding to the proteolytic fragments of myosin, the negative peak in enhanced approximately 10-fold and a longer wavelength positive peak appears. These effects are attributed to a change in the stereochemical structure of the nucleotide and specific interactions within the nucleotide binding site. All four thioguanosine nucleotides give these spectral changes although minor significant differences do occur. The CD spectra of the subfragment 1 steady-state complexes with thioGTP and thioGTP(gamma S) are similar to each other but different from the complexes of subfragment 1 with thioGDP and thioGMP-P(NH)P. Interactions between nucleotides in the complexes are excluded by a study of their spectra in 2-propanol when base stacking occurs. This is the first investigation where circular dichroism has been used to determine structural differences between different myosin--nucleotide states and provides evidence that the nucleotide in the gamma S bound state is similar to that of the triphosphate bound state.

CD studies on ribonuclease A - oligonucleotides interactions

The interaction of ApU, Aps4U, Aps4Up, ApAps4Up and Gps4U with RNase A was studied by CD difference spectroscopy. The use of 4-thiouridine (s4U) containing oligonucleotides enables to distinguish between the interaction of the different components of the ligand with the enzyme. The mode of binding of the oligonucleotides to the enzyme is described. From this mode of binding it is explained why Aps4U, for example, inhibits RNase A, while s4UpA serves as a substrate.

Some observations on the near ultraviolet circular dichroism of tRNA from E. coli

The circular dichroism arising from 4-thiouridine residues in E. coli transfer RNA has been studied under conditions in which the secondary and tertiary structure of the macromolecule is either intact or totally disrupted. Studies on both unfractionated tRNA and on highly purified species of methionine-, valine-, and lysine-specific tRNA suggest that the circular dichroism of the 4-thiouridine residue is highly sensitive to its local environment in the macro-molecule and reveal interesting differences between these purified species. A new chromophore, resembling 4-thiouridine in some respects but showing distinctive chemical and optical properties, has been detected. The implications of these results on model-building studies of tRNA are discussed.