A combination of ultrasound velocimetry, density, and UV spectroscopy has been employed to study the hydration effects of binding of Mn(2+) and alkaline-earth
cations to
poly(rA) and
poly(rU) single strands. The hydration effects, obtained from volume and compressibility measurements, are positive due to overlapping the hydration shells of interacting molecules and consequently releasing the water molecules to bulk state. The volume effects of the binding to
poly(rA), calculated per mole of
cations, range from 30.6 to 40.6 cm(3) mol(-1) and the compressibility effects range from 59.2 x 10(-4) to 73.6 x 10(-4) cm(3) mol(-1) bar(-1). The volume and compressibility effects for
poly(rU) are approximately 17 cm(3) mol(-1) and approximately 50 x 10(-4) cm(3) mol(-1) bar(-1), respectively. The comparative analysis of the
dehydration effects suggests that the
divalent cations bind to the
polynucleotides in inner-sphere manner. In the case of
poly(rU) the
dehydration effects correspond to two direct coordination, probably between adjacent
phosphate groups. The optical study did not reveal any effects of
cation on the secondary structure or aggregation of
poly(rU). In the case of single-helical
poly(rA) binding is more specific:
dehydration effects correspond to three to five direct contacts and must involve atomic groups of adenines, and the
divalent cations stabilize and aggregate the
polynucleotide.