Matrix-assisted diffusion-ordered NMR spectroscopy with an invisible, tuneable matrix

Abstract

Perfluorooctanoate is a useful “invisible matrix” for mixture analysis by diffusion-ordered NMR (DOSY) with pH-controlled diffusion resolution.

Matrix-assisted DOSY

“…An alternative to deuteration, which is only practical (if costly) for a few common surfactants, or the salt-induced aggregation of micelles, is to use fluorinated surfactants, originally suggested by Zielinski and Morris [137], and elegantly demonstrated by Hernandez-Cid et al [146]. In this case sodium perfluorooctanoate (PFO) is used as the matrix, being inexpensive, readily available, and invisible in the 1 H NMR spectrum, just as protonated micelles are invisible when used in 19 F matrixassisted diffusion NMR experiments [130,132].…”

“…Perfluorinated surfactants are not "drop-in" replacements for common micelle forming systems, such as SDS, CTAB or AOT. The differences in polarity, electronegativity etc between a proton and fluorine atom means that the nature of the interactions [147], and the selectivity as a DOSY matrix, will be very different [146]. This is illustrated by the fact that PFO does not cause effective separation of common test mixtures such as dihydroxybenzenes, which are effectively separated by SDS micelles.…”

“…This is illustrated by the fact that PFO does not cause effective separation of common test mixtures such as dihydroxybenzenes, which are effectively separated by SDS micelles. Charge-charge interactions dominate the interaction between highly fluorinated species and PFO has a lower affinity for long alkyl groups [148], hence it should be a good matrix for the separation of charged species, and should have a reasonable range of tunability by virtue of pH adjustment of the bulk solution [146]. Separation of a quaternary mixture of amino acids at neutral pH has been demonstrated using 100 mM PFO micelles [146].…”

“…Charge-charge interactions dominate the interaction between highly fluorinated species and PFO has a lower affinity for long alkyl groups [148], hence it should be a good matrix for the separation of charged species, and should have a reasonable range of tunability by virtue of pH adjustment of the bulk solution [146]. Separation of a quaternary mixture of amino acids at neutral pH has been demonstrated using 100 mM PFO micelles [146]. The models used to described the diffusion coefficients of the various species in a micelle matrix diffusion experiment, as describe above [117], can be modified to account for the charge state of the analyte species as a function of pH.…”

“…The models used to described the diffusion coefficients of the various species in a micelle matrix diffusion experiment, as describe above [117], can be modified to account for the charge state of the analyte species as a function of pH. This model has been used to construct ionisation state curves for a range of amino acids and hence provides an alternative approach to the determination of the ionisable group pKa [146].…”