Organic & Biomolecular Chemistry
Paper
the scope of the current work, however it is further evidence followed by 1.85 mL of 9.25 pH NaHCO3 buffer. Immediately
against the prevailing notion that there is a sole factor following the addition of buffer, absorbance at the chalcone’s
(i.e. pKa of HA or, similarly, stability of chalconate 5) mediating λmax was measured every 5 seconds for 9000 seconds. A plot of
interconversion.
ln[A/A0] vs. time was fitted linearly; the slope of this line was
used to determine the relative rate.
Conclusions
Procedure for UV/Vis assay to determine flavanone to chalcone
rates
This work has elucidated several interesting substituent-
dependent features of the reversible interconversion of ortho-
hydroxychalcones and flavanones. It has been shown that the
putative equivalence point – the pH at which a solution con-
tains a 1 : 1 mixture of isomers – can be altered electronically
by substitution of either of the aromatic moieties. Analogues
featuring mono-fluorination of the ‘B’ ring (1d/2d, 1e/2e) show
similar activity to the previously described mono-methoxylated
structures (1b/2b, 1c/2c), while derivatives bearing a fluorine
on the ‘A’ ring (1p/2p, 1u/2u) exhibit significantly altered
switching properties when compared to their methyl ether
counterparts (1f/2f, 1k/2k). Of particular interest is the 1p/2p
equilibrium, which displayed the greatest midpoint pH shift of
all of the investigated mono-substituted derivatives.
A linear free energy relationship between substituents and
midpoint pH can be described by and understood through use
of the Swain–Lupton equation. This relationship cannot be
extended to the kinetics of the forward and reverse reactions
independently; the mechanistic complexity of this equilibrat-
ing scaffold is evident in this finding. The Swain–Lupton data
coupled with the lack of correlation between the midpoint pH
and either (a) HA 1H NMR chemical shift (despite the linear
substituent-dependence of this data) and (b) the hydrogen
bond strength indicates that the shift in midpoint pH is not
due solely to the pKa of HA. These findings contribute to our
understanding of this scaffold and will aid in the development
of structural derivatives for use as dynamic covalent molecular
switches.
A 4 × 10−4 M stock solution of flavanone (2) in ethanol was pre-
pared. Into a cuvette was added 150 µL of this solution fol-
lowed by 1.85 mL of 12.75 pH KCl buffer. Immediately
following the addition of buffer, absorbance at the corres-
ponding chalcone’s λmax was measured every 0.2 seconds for
240 seconds. A plot of ln[A/A0] vs. time was fitted linearly; the
slope of this line was used to determine the relative rate.
Acknowledgements
Funding graciously provided by Northern Illinois University.
The authors thank Dr Heike Hofstetter (NIU) for spectroscopic
assistance and Prof. Louise Charkoudian (Haverford College)
for helpful discussions.
Notes and references
1 B. L. Feringa and W. R. Browne, Molecular Switches, Wiley-
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Selected experimental procedures
Procedure for UV/Vis equilibration assay to determine
midpoint pH
A 4 × 10−4 M stock solution of chalcone (1) in ethanol was pre-
pared. Into a cuvette was added 150 µL of chalcone solution
followed by 1.85 mL of aqueous buffer by way of autopipette.
Each sample was prepared in triplicate and all samples were
equilibrated in a fridge at 2.6 °C for 2 hours. The samples were
9 A. A. Beharry and G. A. Woolley, Chem. Soc. Rev., 2011, 40,
4422–4437.
removed and stored at ambient temperature for 1 hour. The 10 T. Fehrentz, M. Schönberger and D. Trauner, Angew. Chem.,
UV-Vis spectrum was taken from 290–500 nm for each sample. Int. Ed., 2011, 50, 12156–12182.
The absorbance at λmax for each pH was then plotted in Origin 11 W. Szymański, J. M. Beierle, H. A. V. Kistemaker,
and fitted using a logistic sigmoidal function iteratively until
W. A. Velema and B. L. Feringa, Chem. Rev., 2013, 113,
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chi2 was minimized, with x0 representing the midpoint.
12 M. A. Kienzler, A. Reiner, E. Trautman, S. Yoo, D. Trauner
and E. Y. Isacoff, J. Am. Chem. Soc., 2013, 135, 17683–
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Procedure for UV/Vis assay to determine chalcone to flavanone
rates
A 4 × 10−4 M stock solution of chalcone (1) in ethanol was 13 X. Su, S. Voskian, R. P. Hughes and I. Aprahamian, Angew.
prepared. Into a cuvette was added 150 µL of this solution
Chem., Int. Ed., 2013, 52, 10734–10739.
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Org. Biomol. Chem., 2014, 12, 5108–5114 | 5113