INCREASING CONTACT ION-PAIRING IN THE SUPERCOOLED AND GLASSY STATES OF DILUTE AQUEOUS MAGNESIUM, CALCIUM, AND STRONTIUM NITRATE SOLUTION - IMPLICATIONS FOR BIOMOLECULES

FTIR spectra of aqueous D2O solutions of magnesium (0.5 and 0.05 M), calcium (0.5, 0.2, 0.05, and 0.02 M) and strontium (0.5 M) nitrate in their glassy states at 78 K and of 0.5 M calcium nitrate in supercooled solution from 300 to 235 K are reported and compared with those recorded at 300 K. All four fundamental vibrational regions of the nitrate ion and a combination band could be evaluated by careful subtraction of the D2O solvent. Evidence for increasing contact-ion pairing in going from ambient temperature to the glassy state is based on (i) development of a new band in the nu4 band region at 727 cm-1 (Mg) and increasing intensity of the band at almost-equal-to 740 cm-1 (Ca and Sr), (ii) doubling of bands in the nu1 and nu2 band regions, and (iii) fourth-derivative curves of the nu3 band region and its curve fitting (Ca). The latter also indicated, in addition to ''free' nitrate and the contact-ion pair, the presence of a third species which is tentatively assigned to ion triplets. Relative intensities of bands due to ''free'' nitrate and contact-ion pair were determined by curve fitting the nu4, nu2, and nu1 band region. For the glassy 0.5 M nitrate solutions, the relative area of the band assigned to the contact-ion pair is Sr > Ca almost-equal-to Mg for the nu4 band region and Sr > Ca > Mg for the nu1 band region. Evidence for increasing ion pairing is seen already in supercooled solution at less-than-or-equal-to 260 K. We suggest that water's structural changes toward a more open, fully hydrogen-bonded tetrahedral network in its supercooled and glassy state are at the bottom of increasing contact-ion pairing with decreasing temperature. Implications for biomolecules stabilized by contact-ion pairs are discussed with respect to cold denaturation of proteins, cryoenzymology, and cryofixation.