The integral divalent cation within the intermolecular purine*puvine center dot pyrimidine structure: a variable determinant of the potential for andcharacteristics of the triple helical association

In vitro assembly of an intermolecular purine*purine pyrimidine triple heli x requires the presence of a divalent cation, The relationships between cat ion coordination and tripler assembly were investigated, and we have obtain ed new evidence for at least three functionally distinct potential modes of divalent cation coordination. (i) The positive influence of the divalent c ation on the affinity of the third strand for its specific target correlate s with affinity of the cation for coordination to phosphate, (ii) Once asse mbled, the integrity of the triple helical structure remains dependent upon its divalent cation component. A mode of heterocyclic coordination/chelati on is favorable to tripler formation by decreasing the relative tendency fo r efflux of integral cations from within the triple helical structure. (iii ) There is also a detrimental mode of base coordination through which a div alent cation may actively antagonize tripler assembly, even in the presence of other supportive divalent cations, These results demonstrate the consid erable impact of the cationic component, and suggest ways in which the trip le helical association might be positively or negatively modulated.