- Conformational States of myo-Inositol Hexakis(phosphate) in Aqueous Solution. A 13C NMR, 31P NMR, and Raman Spectroscopic Investigation
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The longstanding question of the solution conformation of myo-inositol hexakis(phosphate) (IHP) has been resolved through combined use of NMR and Raman spectroscopic methods.IHP exists in aqueous solution in either of two conformations, depending on pH: the low-pH form possesses one axial and five equatorial phosphates (1-ax/5-eq), whereas the high-pH form has the inverted 5-ax/1-eq structure.Equal amounts of the two conformers coexist at pH 9.4 +/- 0.1 for 0.10 M Na12IHP at 27 deg C.The 13C NMR spectra of IHP (recorded over the pH range 0.5-12.5) are relatively insensitive to phosphate deprotonation, but do exhibit a single, major change in appearance centered around pH 9.4; this provides strong support for a conformational change.The 31P NMR spectra as a function of pH reveal, among other things, that the three least acidic phosphate protons have apparent pKa values of 9.2-9.6.The IHP ring inversion is most probably triggered by one or more of these acid-dissociation steps.The Raman spectra are sensitive both to acid dissociation and conformation, and also allow key spectral comparisons between aqueous solutions of IHP and solids of known structure (viz., solid Na12IHP having 5-ax/1-eq phosphates and solid myo-inositol having 1-ax/5-eq hydroxyls).Raman data indicate that alkali metal ions preferentially bind to, and thus stabilize, the aqueous high-pH IHP conformer in the order Li+ ca.Na+ > Cs+.The Raman spectrum of solid Ca6IHP is characteristic of the 1-ax/5-eq conformer, in contrast to the 5-ax/1-eq structure of the dodecasodium salt.
- Isbrandt, Lester R.,Oertel, Richard P.
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p. 3144 - 3148
(2007/10/02)
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