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.