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of the oligocholate and its guest binding were intimately related and seemed to benefit
each other, unlike what the principle of preorganization assumes.
The authors then investigated a series of diamines, H 2 N(CH 2 ) n NH 2 , which could only
form weak carboxylate-ammonium ion pairs with 14 in the polar MeOH/EA mixtures.
For diamines with intermediate chain length ( n
6 and 8), similar binding strengths were
observed, once again peaking at 15% methanol. The results were completely unexpected
for the hydrogen-bonded ion pairs, which are typically weakened by protic solvents such
as methanol. The same trend was observed in the binding of the stronger ion-pairing
diguanidine 19 and also in another oligocholate ( 18 ) that folded in a different solvent
mixture [69].
¼
The authors attributed the unusual solvent effect to the positive cooperativity between
the host's conformation and the host-guest binding. Essentially, strong interactions already
exist between different segments of the host and with the solvents when 14 is folded; the
guest-binding, under such a circumstance, brings little benefit to the host . When the host is
far into the unfolded region, the guest-binding has to overcome an unfavorable unfolding-
folding conformational change, which undermines the binding affinity [57]. When the host
is unfolded but near the unfolding-folding transition, however, the conformational change
that occurs during guest-binding could engage additional interactions within the host and
with the solvents. Because these additional interactions also contribute to the overall
binding equilibrium, the binding affinity is “magnified” by the synergism or positive
cooperativity between the conformation of the host and host-guest binding interactions.
 
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