Report No xxxx - Instytut Fizyki JÄ drowej PAN

CHEMICAL SHIFT TITRATION AT THE INTERMEDIATE
EXCHANGE RATES OF HOST – GUEST SYSTEM
Andrzej Ejchart
Institute of Biochemistry and Biophysics, Polish Academy of Sciences,
Pawińskiego 5A, 02-106 Warsaw, Poland; aejchart@ibb.waw.pl
NMR spectroscopy has been widely used for the determination of association
constants, K, observing NMR parameters, most often chemical shifts, in the titration
experiments. Owing to the specific time scale of NMR method the appearance of the NMR
spectrum of the host – guest mixture depends not only on association constant but also on the
rate of the exchange of guest molecules between free and bound states. Most of the studies
have been concerned with the case of fast exchange when only a time averaged mole fraction
weighted spectrum of guest (and/or host) in free and bound states can be observed.
δ obs = (1 – [C]/[G 0 ])δ G + ([C]/[G 0 ])δ C
where δ obs is observed chemical shift of guest resonance, δ G and δ C are chemical shifts in free
and bound states, respectively. [C] is the concentration of complex and [G 0 ] is initial
concentration of guest. On the other hand, the equation which couples concentrations and
association constant depends on the stoichiometry of the system. For the simplest 1 : 1
stoichiometry one obtains:
K = [C]/{([H 0 ]–[C])([G 0 ]–[C])}
where [H 0 ] is initial host concentration.
Intermediate exchange rates on the NMR time scale cause signal broadening which
combined with the titration shifts of multiple (often superposed) resonances can preclude
determination of their positions making such titration experiment useless.
An approach allowing to overcome this problem is proposed. It relies on the concerted
fit of the theoretical lineshapes determined by a number of parameters including association
constant to the set of experimental lineshapes obtained during titration experiment.
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