Part I. Cation-Binding Properties of Nonactin.
In an effort to elucidate the cation specificity of
certain macrocyclic antibiotics in metabolic behavior, the
nature of the binding of K+, Na+ and Cs+ ions to the
macrotetrolide nonactin and the molecular structure of the nonactin
complexes have been investigated by 220 MHz proton
magnetic resonance spectroscopy. Studies were made in dry
acetone and in an acetone-water mixture as a function of
varying perchlorate salt concentrations. Salt-induced
chemical shifts were observed for all the nonactin protons,
except H2 and H21, with accompanying changes in the vicinal
coupling constants between the H2 and H3 protons and between
the H5 and H6-H6, protons. Analysis of the salt-induced
shifts yielded apparent formation constants for each of the
ions in both solvent systems. The results indicate that all
three ions bind to nonactin with equally high affinity in
dry acetone, but that in wet acetone the binding constants
are preferentially reduced making the potassium complex
highly favored. It is shown that in wet acetone the alkali
ion must be stripped of its hydration shell prior to its
accomodation in the nonactin cavity j and hence we surmise
that differences in hydration energies for the various ions
must contribute significantly to the ion selective behavior
of nonactin. Analysis of limiting chemical shifts and
coupling constants also indicates that the nonactin molecule
undergoes sizable conformation changes on complex formation,
but that the complexes formed, all spherical with a nonsolvated ion
at their center, differ little in exterior geometry. These results
are interpreted in terms of their implications on a simple
model for ion transport in a biological membrane.
Part II: Salt Effects on Nucleotide Conformation.
Aqueous solutions of uracil, uridine, deoxyuridine,
uridine 3'-monophosphate (3'-UMP), and uridine 5'-monophosphate
(5'-UMP) have been investigated by proton magnetic resonance
spectroscopy as a function of electrolyte concentration. The
addition of a "solvent-structure breaking"
salt such as Mg (ClO4)2 or NaClO4 to solutions of the
nucleosides and nucleotides was found to result in significant
upfield shifts of the uracil H6 resonance. In the cases of uridine and 5'-UMP,
these salt-induced shifts were accompanied by a decrease in the
ribose H1,-H2, coupling constant,
and in the case of 5'-UMP, significant changes in certain
intramolecular nuclear Overhauser ef...