The first derivative of a function of circular dichroism spectra: biophysical study of human telomeric G-quadruplex

Depending on conditions and base modifications, telomeric repeats can form many topological structures; parallel, antiparallel and hybrid forms. The influence of salts and some specific ligands on conformational changes has already been established. In this study, we analyze the human telomeric repeats 5'-GGG(TTAGGG)(3)-3' because this sequence forms topologically different structures under various conditions which have been well described by many authors. CD results are compared with electrophoretic and UV absorption spectroscopy results obtained under corresponding conditions in the presence of different ratios of sodium and potassium ions and polyethylene glycol (PEG). We confirmed that the most stable G-quadruplexes could only form under crowding conditions with PEG-200 and K(+) ion, but the molecularity is increased. Other monovalent ions without the presence of K(+) are unable to form the parallel quadruplex conformer and no change of stoichiometry is observed, even when PEG-200 is present. The first derivative of a function applied to CD spectra seems to be a powerful tool for spectra evaluation of any G-quadruplex, and could be more unambiguous than a direct analysis of original spectra.