Intrinsic and Extrinsic Control of the p Ka of Thiol Guests inside Yoctoliter Containers

Article abstract of DOI:10.1021/jacs.0c00907

Despite decades of research, there are still many open questions surrounding the mechanisms by which enzymes catalyze reactions. Understanding all the noncovalent forces involved has the potential to allow de novo catalysis design, and as a step toward this, understanding how to control the charge state of ionizable groups represents a powerful yet straightforward approach to probing complex systems. Here we utilize supramolecular capsules assembled via the hydrophobic effect to encapsulate guests and control their acidity. We find that the greatest influence on the acidity of bound guests is the location of the acidic group within the yoctoliter space. However, the nature of the electrostatic field generated by the (remote) charged solubilizing groups also plays a significant role in acidity, as does counterion complexation to the outer surfaces of the capsules. Taken together, these results suggest new ways by which to affect reactions in confined spaces.

Full text of DOI:10.1021/jacs.0c00907

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Article
Intrinsic and extrinsic control of the pKa of
thiol guests inside yocto-liter containers
Xiaoyang Cai, Rhea Kataria, and Bruce C Gibb
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c00907 • Publication Date (Web): 09 Apr 2020
Downloaded from pubs.acs.org on April 10, 2020
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Intrinsic and extrinsic control of the pKa of thiol guests inside
yocto-liter containers
Xiaoyang Cai , Rhea Kataria and Bruce C. Gibb *
Department of Chemistry, Tulane University, New Orleans, LA, 70118
*Corresponding author: bgibb@tulane.edu
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Abstract
Despite decades of research there are still many open questions surrounding the
mechanisms by which enzymes catalyze reactions. Understanding all the non-covalent forces
involved has the potential to allow de novo catalysis design, and as a step towards this,
understanding how to control the charge-state of ionizable groups represents a powerful yet
straightforward approach to probing complex systems. Here we utilize supramolecular capsules
assembled via the hydrophobic effect to encapsulate guests and control their acidity. We find
that the greatest influence on the acidity of bound guests is the location of the acidic group within
the yocto-liter space. However, the nature of the electrostatic field generated by the (remote)
charged solubilizing groups also plays a significant role on acidity, as does counter-ion
complexation to the outer surfaces of the capsules. Taken together, these results suggest new
ways in which to affect reactions in confined spaces.
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Introduction
Many different non-covalent interactions come together in an enzyme to contribute to its
catalytic properties, but at a general level it is currently believed that of all of these,1 Coulombic
interactions reign supreme.2 This is perhaps not surprising; in vacuum, two mono-valent ions will
experience an attraction (or repulsion) greater than the thermal energy kT at a distance of ~56 Å.
Even in a high dielectric medium such as water (εr = 78.4 versus 1 for vacuum), this Bjerrum
length is relatively long; ions must be within approximately 7 Å to feel each other above the energy
fluctuations of the solution. Hence an electrostatic potential field (EPF) emanating from a charged
group in a protein can extend some distance along the surface, and more deeply through the
lower dielectric of the inner structure.3
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One way to try and understand enzymes is to mimic them, and over the last decade or so
the field of supramolecular chemistry has made considerable advances in this regard.4 Thus,
supramolecular containers assembled via metal-ligand coordination have proven to be successful
catalysts and stoichiometric reagents,5 as have hydrogen-bonded containers,6 covalent hosts,7
and containers assembled via the hydrophobic effect.8 These many advances noted, because of
the overall complexity of chemical transformations, this body of work tends to identify new and
exciting chemical conversions rather than reveal the inner workings of enzymes.
To better parse out the non-covalent contributions to enzyme catalysis, more
straightforward metrics are preferable. One such metric is the acidity of an ionizable group. Thus,
in studying how context affects acidity, the pKa values of protonated lysine residues in engineered
variants of staphylococcal nuclease have been found to vary from the typical value of 10, to a
value as low as 5.3 depending on their position in the protein.9 Similarly, the pKa values of the
carboxylate groups of aspartic and glutamic acid have been found to be shifted up to 4-5 units
depending on their environment,10 whilst similar variations in the pKa of cysteine resides are
intimately linked to their redox and signaling properties.11 Complementary to these types of
studies, considerable effort has also gone into the modelling the effects of electrostatic charges
in proteins,12 and taken together with the empirical studies, this work reveals subtle contextual
effects on the ionization-state of functional groups.
The relatively simple structures of macrocyclic hosts also offer considerable opportunity
to investigate how local environment affects the pKa of an ionizable group (and hence any property
that is itself controlled by ionization state).13 For example, in the presence of the weak dipole of
cyclodextrins, it is typically observed that the pKa shifts for bound guest are < 1 unit.14 15 In contrast,
the larger quadrupole of cucurbiturils can shift the pKa of bound ammonium dyes16 by 2-3 units.17
Moreover, shifts as large as ~5 pKa units have been observed in selected examples of 2:1
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complexes between CB7 and coumarin dyes. In such cases, it appears that bringing two
quadrupoles to bear on the central protonated N-atom of the bound dye induces the relatively
large observed shift in acidity.17b, 18
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To build on our understanding of how local charges and electrostatic potential fields (EPFs)
can affect acidity constants, we report here on the acidity of guests encapsulated within the two
supramolecular capsules formed by octa-carboxylate 1a and positand 1b (Figure 1).19 These
hosts dimerize around guests, internalizing them within identically shaped non-polar nano-spaces.
The only significant difference between the inner space of each capsule is that one is located
within a negative EPF created by sixteen remote negative charges (1a2), whilst the other resides
within a positive EPF emanating from trimethyl ammonium solubilizing groups (1b2). Figure S1b
shows the EPF for each host. We report here on the acidity of a range of thiol guests (2-10, n =
1-9, Figure 1) within these supramolecular capsules. Our acid-base titrations of bound guests
reveal how the EPF generated by the arrays of charge groups affects acidity, and how acidity is
also intimately tied to the precise position of the thiol group in the inner space. Finally, we
demonstrate how extrinsic factors – salts dissolved in the outer aqueous medium – can bind to
the outer surface of the host, modulate the EPF of the capsule, and hence change the acidity of
the bound guest. Taken together, these studies reveal new details of the extent to which different
factors can influence the acidity of internalized guests.
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Results and Discussion
Hosts 1a and 1b were synthesized as previously described.19 Guests 3, 4, 6, 8 and 10
were commercially available, whilst guests 2, 5, 7 and 9 were synthesized from the corresponding
1-bromoalkane, via formation of the S-alkyl ethanethioate (KSAc in THF) and acid hydrolysis (HCl
in MeOH). Details of the guest syntheses are given in the Supporting Information.
Figure 1: Hosts 1 and 2, and guests 2-10 (n = 1-9 respectively) used in this study.
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A combination of signal integration in the 1H NMR spectrum of each complex, and DOSY
NMR spectroscopy of the free host and the different complexes, confirmed 2:1 stoichiometries for
each combination of host and guest. Each complex was typified by significant shifts in the host
aromatic signals (Figure S3 and S4, Supporting Information), and by bound guest signals between
–1 and –4 ppm (Figures S5-S10). By recording the NMR spectra in buffered (8 mM NaOH) 95:5
H2O:D2O it was possible to observe the SH signal from the bound thiol group for the majority of
complexes, and in most cases COSY NMR spectroscopy allowed full peak assignment (Figures
S11 to S36). Figure 2 shows the guest binding region for three guests within the negative 1a2
capsule: n-decyl thiol 2, n-tetradecyl thiol 6, and n-octadecyl thiol 10. Peak assignments allowed
the calculation of the signal shift data of the guest between the bound and the free state (Δδ =
δbound – δfree), which revealed the approximate location of each group within a host and hence the
overall preferred binding motif of each guest.20 In general, we found (Figure 3 and Figure S12)
that the smaller guests such as 2 and 3 adopted an extended motif in which the two termini of the
guest bound to the two ‘poles’ of the capsule, whilst guests such as 8 or 10 that were too long for
the nano-space were forced to bind in a J-shaped motif.
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Figure 2: Guest binding region of the 1H NMR spectra of the capsular complexes of 1a with thiols 2, 6 and 10 (all 1.0
mM concentration, in 8 mM NaOH 95%H2O/5%D2O). Numbering is from the thiol (0) to the methyl terminus (10, 14
or 18 respectively). The indicated (*), broad signals in the complex with 10 are attributed to a minor guest motif (see
main text).
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Returning to Figure 2, wide signal anisotropy is typically observed with guests such as 2
that adopt an extended motif.21 This arises because the Δδ values for the two termini are large,
whereas the Δδ value for the mid-section of the bound guest is small. Specifically, in the case of
2 inside capsule 1a2 the Δδ values of the thiol S–H, the mid-section methylenes (C4/C5), and the
C10 terminal methyl signals were –3.98, –1.32/–1.29, and –3.92 ppm respectively. With
increasing size of guest, so there is a transition from an extended motif, to an extended motif with
a kink, to motifs possessing a full reverse turn.20a This is exemplified by the smaller upfield shifts
of the termini regions of guest 6 inside negative 1a2 (Figure 2); the Δδ values of the thiol S–H,
C7/C8 methylenes and C14 terminal methyl signals were –3.65, –1.17/–1.17, and –3.37 ppm
respectively. Interestingly, guest 8 (C16H33SH) lay at the transition between the extended and J-
motifs. This guest had a mostly unresolvable COSY NMR (Figure S26). To probe this further we
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examined the H and COSY NMR of the complex of the corresponding alcohol (C16H33OH);
reasoning that the equatorial region of the capsule is more polar/hydrophilic (vide infra) and that
therefore the alcohol guest would be more predisposed to form the J-motif(Me) with its polar OH
group at the equator (c.f Figure 3, right). This was indeed the case; the bound 1H NMR signals
were exceptionally well resolved and indicated a well-defined J-motif(Me) with the OH at the polar
region of the host (Figure S27 and S28). For thiol 8 however, there is evidently no such driving
force helping define its motif, and as a result the extended motif and both J-motifs must be
approximately isoenergetic. Finally, the 1H NMR signals of bound 10 showed a wider anisotropy
but were relatively broad (Figure 2). Additionally, the signal from the thiol group was not evident.
Nevertheless, the relatively large upfield shifts of the C8/9 signals (approximately –2 ppm, Figure
S32) suggest a J-motif in which one of the cavitand hemispheres is occupied by a turn in the
guest mainchain. However, this data alone is not sufficient to determine if the J motif has the Me
at the base of one cavitand and the thiol at the equatorial region of the capsule (J-motif(Me),
Figure 3 right), or the ‘opposite’ motif with thiol group at the base of the pocket and the Me at the
equator (J-motif(SH)). However, a key clue to the preferred binding motif of this guest is that the
1H NMR spectrum for the complex with 10 also revealed the presence of small, broad signals
suggestive of a minor secondary motif (Figure 2). Considering the positional similarity of the most
upfield signal of this minor motif to the Me signal of complexed 3 and 4, we tentatively attribute
this minor motif of 10 to the J-motif(Me) with the thiol at the equatorial region of the capsule. To
support this hypothesis, we formed the complex between negative 1a2 and the corresponding
C18 alcohol. The guest binding region of the 1H NMR spectrum of the complex revealed sharp,
well resolved signals with the terminal methyl group at –3.25 ppm. Moreover, the corresponding
COSY spectrum allowed full signal identification (Figures S35 and S36) and conformation of a J-
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motif(Me). Thus bound 10 primarily adopts a J-motif(SH) with the thiol group bound into the base
of a cavitand and the Me nearer the equator. But considering that both motifs can be observed
for bound 10, there can be little energy difference between them.
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The well-defined H NMR signal from the thiol S–H allowed acid-base titrations with
sodium hydroxide to determine the pKa of each bound guest (Figures S42-S64) for all host guest
pairs, except those involving guests 9 and 10.22 During these titrations, other than the
disappearance of the S–H signal, we saw only small changes to the guest binding region of the
1H NMR spectrum.23 Specifically, there were slight shifts in all the bound guest signals, and
changes to the (poorly defined) signal splitting of the bound C1 methylene (e.g., compair Figures
S15 and S45). We conclude that, at least as measured by 500 MHz 1H NMR, deprotonation did
not significantly change the guest binding motif.23 This tells us that alkyl group solvation (for a
free guest) is more energetically costly than binding the thiolate in a non-polar pocket. But what
is the nature of the bound thiolate? There are four possibilities: not solvated and not ion-paired;
not solvated and ion-paired; solvated and not ion-paired; and solvated and ion-paired. We have
previously demonstrated that partially solvated, polarizable anions have an affinity for the pockets
of hosts 1a and 1b,24 and therefore discount the first two of these options. In other words, we
believe the bound thiolate is stabilized by one or two waters of solvation. However, the location
of the counter cation (outside or co-encapsulated) is unclear. However, the fact that the guest
motif does not change much upon deprotonation, and the fact that the concentration of the guest
in the capsule is ~3.2 M, suggests a bound (but solvated) RSNa group with little in the way of
charge separation. Further efforts may ultimately discern between all of these possibilities.
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Figure 3: Representative examples of the guest binding-motifs observed for the complexes formed between the
hosts and guests in this study. Shown are molecular models of the complexes formed by positively charged 1b and
the C11 thiol guest 3 in the extended conformation (extreme left), and the J-motif for the C16 thiol guest (8, extreme
right), as well as their corresponding schematic representations (center).
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By way of example, Figure 4a shows the titration data for bound guest 6 inside the
positively and negatively charged capsules. Figure 4b shows the obtained pKa values for each
thiol guest in the different host-guest combinations (estimated errors: < ± 0.1 pKa units for guests
2-7, and < ± 0.2 pKa units for guest 8; see Table S4). These values compare to the typical pKa of
a (free) thiol in aqueous solution of approximately 10-11. The data suggests that for both capsules
there is a maximal pKa value for guest 3. With increasing guest length, the pKa of the thiol group
decreases: in the case of negative capsule 1a2 the pKa range varies from 13.0 to 8.4 (ΔpKa =
4.6), whilst in the case of positive capsule 1b2 the range is 12 to 6.3 (ΔpKa = 5.7). We attribute
this general trend to the position of the thiol group in the complex. Thus, for the shorter guests
the thiol group is located deep in the end of the capsule in a relatively non-polar environment
(Figure 3, left). As a result, its acidity is lower than that of a free thiol in solution. However, with
longer guests that energetically prefer to adopt a J-motif, the thiol group is at least in part located
near to the equatorial region of the capsule (Figure 3, right). Considering the propensity of these
capsules to “breath”, i.e., partially open to allow the entry and egression of small molecules,25 this
equatorial region would be expected to be more hydrophilic, and the trends in acidity constants
shown in Figure 4b support this notion. Thus, in the case of the capsule formed by positive 1b,
the position of the thiol group can alter its acidity by almost 6 pKa units.
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Figure 4: a) pH titrations for guest 6 encapsulated within negative and positive capsules 1a2 and 1b2. Individual data
points are shown as circles with attendant error bars, whilst data fitting (Supporting Information) is shown as the
corresponding lines. Each titration was recorded in at least duplicate. The degree of deprotonation was determined
by integrating the thiol proton signal relative to the terminal methyl signal of the guest as a function of added NaOH.
b) Bar graph of pKa values for complexes of guests 2-8 inside the capsules formed by 1a and 1b. Estimated errors
are less than ± 0.1 pKa units for guests 2-7, and ± 0.2 pKa units for guests 8 (see Table S4, Supporting Information).
The difference in the pKa of individual guests (ΔpKa) in the two capsules ranges from 1.0
–1
ΔΔ
to 2.6 units, which corresponds to 6.3 to 14.8 kJ mol of stabilization in the positive capsule (
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퐺 = 2.3푅푇Δ푝퐾
푎). Previous electrostatic potential and charge stabilization calculations suggest
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that a negative charge should be stabilized in the positive EPF of 1b2 by ~13 kJ mol–1 relative to
the negative EPF of 1a2.26 Moreover, these calculations also suggest that the EPFs in the inner-
spaces of the capsules are rather homogeneous (Figure S1b). Hence the relatively small ΔpKa
between the two capsules for the shorter guests is quite surprising. Why is this? Coulombic
interactions are non-directional, whereas ion-dipole interactions very much are. Thus, our
tentative hypothesis is that the latter is key to explain the small difference in pKa of small guests.
For example, the positive and negative charges of the two hosts may establish or alter
(sub)dipoles within the two hosts such that their Hb protons (Figure 1) form hydrogen bonds of
different strength to the bound thiol/thiolate. Alternatively, the opposite charges of the hosts may
alter the position and orientation of any co-encapsulated water molecules within the two inner-
spaces of the capsules. Thus, if the inner space of negatively charged host 1a2 was more strongly
solvated, then any bound water molecule co-encapsulated with a thiol would enhance the acidity
relative to a dryer pocket. Our data cannot of course differentiate between these or other
scenarios, but it seems quite likely that the inherent asymmetry of positive ion-dipole versus
negative ion-dipole interactions (e.g., M+⋅⋅⋅OH2 versus X–⋅⋅⋅HOH) is somehow a pKa equalizer with
thiol groups at the base of the pocket. It will be interesting to see in future work if the same applies
to other ionizable groups.
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How does the ionic strength of the external aqueous solution affect the acidity of internally
bound guests? We have previously shown that anions bind more strongly to the outer surface of
–
positive 1b than cations are attracted to the surface of negative 1a.27 For example, ClO4 binds
to the inner pocket and outer crown of four, trimethylammonium pendent groups of 1b with
affinities of 4,300 and 2,400 M–1 respectively. This means that perchlorate can bring about charge
neutralization and precipitation of the host at ~ 10 mM.24a In contrast, the alkali metal ion that
binds most strongly to 1a, Cs+,28 fails to induce the precipitation of this host even in the hundreds
of mM concentration range.27 Being the strongest binding alkali metal ion, we selected Cs+ (as
the chloride salt) to examine the effects of cation binding to the pKa of guest 5 bound to 1a2. As
anticipated, binding was weak; the addition of 200 equivalents of the salt reduced the negative
EPF of the capsule somewhat and shifted the pKa of bound 5 from to 12.0 to 11.8 (Figures S65-
67).29 In contrast, the addition of 200 equivalents of NaBr to the complex of guest 5 inside positive
capsule 1b2 led to a larger pKa shift: from 9.9 to 10.4; this despite Br– being a relatively weakly
binding anion to 1b (Ka for binding to the crown of four trimethyl ammonium pendent groups =
710 M–1).24a
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Larger and more polarizable anions bind even more strongly to positive 1b (see, for
–
example, the aforementioned ClO4 affinity data) and induce precipitation via charge
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neutralization at millimolar concentrations.24a Thus, we observed that more than 10 equivalents
of NaClO4 caused aggregation of the 5.1b2 complex. Correspondingly, to examine the effects
that this stronger binding anion has on the pKa of a bound guest, we carried out acid-base titrations
of the complex in the presence of 0, 1, 2, 3, 4, 6, and 8 equivalents of NaClO4 (Figures S73-S83).
During each 1H NMR spectroscopy titration we saw no evidence of either co-encapsulation or the
anion displacing the large thiol/thiolate guest (Figure S73),24a only signal shifts indicative of
binding to the crowns of four trimethylammonium pendent groups at the ‘poles’ of the capsule.
Figure 5 shows a plot of the measured pKa value of bound guest 5 as a function of different
equivalents of NaClO4. As this figure reveals, the acidity of the guest decreased from pKa = 9.9
to ~10.4 over the salt concentration range examined (ΔpKa = 0.5). Thus, perchlorate had the
same net effect as bromide, but at much lower concentrations.
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Figure 5: The obtained pKa values from pH titrations of bound 5 within the 1b2 capsule, plotted against the
equivalents of NaClO4 present in each titration. The red line is a fit to 1:1 binding model. Inset: the source pH
titration curves for guest 5 encapsulated within capsule 1b2. Each titration was recorded in at least duplicate.
Fitting the data in Figure 5 to a 1:1 model for ion complexation gave a Kapp = 1,553 M–1,
–
Figures S81 and S82). As discussed above, the binding constant of ClO4 to the crown of four
trimethyl ammonium pendent groups of 1b, corresponding to the ‘poles’ of the capsule ‘was found
to be 2,400 M–1.24a We suspect that the Kapp obtained here represents the average affinity for
anion binding to the capsule surface. For example, in addition to the ‘pole’ binding sites, in the
positive 1b2 capsule, it may be the case that there are weaker binding sites between pairs of
equatorially-located benzyltrimethylammonium groups on opposing hemispheres. Interestingly,
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that saturation of the change in pKa of bound guest 5 occurred close to the aggregation point of
–
the complex suggests that many of its sixteen positive charges are neutralized by binding ClO4 .
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To confirm this, we utilized zeta potential measurements (Figure S84). Thus, in the presence of
60 equivalents of NaCl (to ensure good conductivity) the zeta potential (ζ-potential) of the 1b2
capsular complex was measured to be approximately +30 mV, whereas in the presence of 50
equivalents of NaCl and 10 equivalents of NaClO4, the ζ-potential of the complex was only +10
mV. This compares to the capsular complex 5.1a2 in the presence of 60 equivalents of NaCl,
where the ζ-potential was –60 mV. This last point again illustrates the lower degree of counter-
ion binding to negatively charged 1a2 versus 1b2. Overall, these results demonstrate that ion
binding to the outer surface of the capsule is greater in the case of 1b2, but that the effect of this
on the acidity of the bound guest is relatively small compared to the effect of the location of the
thiol group within the inner space.
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In conclusion, despite the relative remoteness of the water-solubilizing groups of the
supramolecular capsules 1a2 and 1b2, the acidity of guests within their inner yocto-liter spaces is
considerably influenced by the electrostatic field generated by the solubilizing groups. In these
capsules however a bigger influence is the position that the acidic thiol group adopts within the
inner-space; pKa measurements suggest that acidity can be altered by up to almost 6 pKa units
depending on whether the acidic group is located in the pole or equatorial region of the capsule.
(Counter) ion binding to the outside of the capsule also affects the pKa of internalized guests,
especially in the case of the positive capsule 1b2. However, the extent to which the acidity can
be modulated appears to be relatively small and is bounded by aggregation effects at low ζ-
potential values. Overall, these results help define the impact-potential of different factors in
reactions in confined spaces, and suggest new subtleties for controlling chemical conversions
within such inner-spaces.
Acknowledgements
The authors wish to express their gratitude to the National Science Foundation for financial
support (CHE-1807101).
Supporting Information. Synthesis of guests 2-10, 1H, COSY and DOSY NMR characterization
of host-guest complexes, pKa determinations in the absence and presence of added salts, and
zeta-potential measurements of selected complexes.
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22. There are multiple equilibria involved in the deprotonation of the bond guest. Specifically, 1:1
complex formation, capping of this to make the 2:1 capsular complex, and potentially the
equilibrium between the 2:1 complex and the empty capsule and the free guest. However, we
see no evidence of any of these equilibria and therefore, in defining pKa, assume that the 2:1
complex is an integral molecule.
23. An exception to this was guest 10. Although in this case there was no apparent SH to titrate
with base, the addition of excess base to the complex (to give a pH = 12.3 solution) led to
significant changes in the guest binding region of the 1H NMR spectrum (Figure S33 shows the
COSY NMR spectrum of the complex with host 1a). At this high pH, the signals attributed to the
J-motif(SH) disappeared, whilst those corresponding to a J-motif(Me), with the thiolate at the
equatorial region, increased (e.g., the terminal Me signal at approximately –3.25 ppm; for Δδ
vaules of all signals see Figure S34). This further supports the assignment of the motif of this
guest at close to neutral pH (Figure 2 and accompanying text). However, at pH 12.3 other broad
signals indicative of a fourth motif also appeared. Unfortunately, COSY NMR (Figure S33) could
not resolve most of the signals in this motif and its identity is unknown.
24. (a) Jordan, J. H.; Gibb, C. L. D.; Wishard, A.; Pham, T.; Gibb, B. C., Ion-Hydrocarbon and/or
Ion-Ion Interactions: Direct and Reverse Hofmeister Effects in a Synthetic Host, J. Am. Chem.
Soc., 2018, 140 (11), 4092-4099; (b) Carnagie, R.; Gibb, C. L. D.; Gibb, B. C., Anion Complexation
and The Hofmeister Effect, Angew. Chem. Int. Ed., 2014, 53 (43), 11498-11500; (c) Gibb, C. L.;
Gibb, B. C., Anion Binding to Hydrophobic Concavity Is Central to the Salting-in Effects of
Hofmeister Chaotropes, J, Am, Chem. Soc., 2011, 133, 7344-7347; (d) Gibb, C. L. D.; Gibb, B.
C., Anion binding to hydrophobic concavity is central to the salting-in effects of Hofmeister
chaotropes, J. Am. Chem. Soc., 2011, 133 (19), 7344-7347.
25. Tang, H.; de Oliveira, C. S.; Sonntag, G.; Gibb, C. L.; Gibb, B. C.; Bohne, C., Dynamics of a
supramolecular capsule assembly with pyrene, J, Am, Chem. Soc., 2012, 134 (12), 5544-7.
26. Wang, K.; Cai, X.; Yao, W.; Tang, D.; Kataria, R.; Ashbaugh, H. S.; Byers, L. D.; Gibb, B. C.,
Electrostatic Control of Macrocyclization Reactions within Nanospaces, J. Am. Chem. Soc., 2019,
141, 6740–6747.
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27. Wishard, A.; Gibb, B. C., Dynamic light scattering studies of the effects of salts on the
diffusivity of cationic and anionic cavitands, Beilstein J. Org. Chem., 2018, 14, 2212-2219.
28. Hillyer, M. B.; Gan, H.; Gibb, B. C., Precision Switching in a Discrete Supramolecular
Assembly: Alkali Metal Ion-Carboxylate Selectivities and the Cationic Hofmeister Effect,
Chemphyschem, 2018, 19 (18), 2285-2289.
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29. We also carried out a titration using CsOH instead of NaOH as base (Figure S68 and S69).
This revealed a shift in pKa for guest 5 in the capsule formed by 1a from 11.97 to 11.73 when
NaOH was replaced by CsOH.
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