[Interactions between pigments and nucleic acids. 5. Stoichiometry of complex 1 formed between luteoskyrine, magnesium ions and nucleic acids].

Luteoskyrine can be bound to nucleic acids in presence of Mg 2+ ions. Two types of complexes are found. Complex I is made with single stranded nucleic acids containing purine bases such as denatured DNA, heated apyrimidinic acid, poly adenylic acid, poly inosinic acid or heated poly guanylic acid. Its visible spectrum is shifted towards shorter wavelengths compared to that of free luteoskyrine. Complex II is formed with double stranded nucleic acids or with single stranded ones deprived of purine bases such as a purinic acid, poly uridylic or poly cytidylic acid. The visible spectrum of complex II is shifted towards longer wavelengths with respect to that of free pigment. In complex I, one mole of luteoskyrine is associated to one atom of magnesium. Prolonged dialysis or heating does not dissociate the magnesium from the complex I but addition of EDTA does, with the simultaneous appearance of the free dye spectrum. Any purine residue in a polynucleotide is a potential site for complex I formation, but neither the nature of the purine (adenine or guanine) nor the nature of the sugar (ribose or deoxyribose) is relevant. However, the maximum amount of luteoskyrine bound as complex I by poly adenylic acid corresponds to a dye/purine ratio of 1/2. This ratio is somewhat larger with the denatured DNAs studied, being comprised between 1/1,2 and 1/1,7. It is suggested that this limit for complex I formation results from steric hindrance due to the size of the luteoskyrine molecule. Being bound to a given purine residue, a luteoskyrine molecule hinders the approach of another dye molecule to the adjacent purine residue but not to the one second next. According to this hypothesis, the maximum amount of complex I formed with given DNA can be calculated from the frequencies of purines sequences, and the predictions made correspond to the observed values. It is therefore likely that the stoechiometry of complex I corresponds to the association of one purine residue, one molecule of luteoskyrine and one atom of magnesium. To be able to bind luteoskyrine and magnesium as a complex I, a purine‐containing oligo‐nucleotide has to have a certain minimum size of 5 to 6 nucleotides (at 20 °). This suggests that a certain conformation of the relevant purine nucleotide residue is required for the binding of luteoskyrine as complex I.