Conspectus
This Account summarizes the progress in protein–calixarene
complexation, tracing the developments from binary recognition to
the
glue
activity of calixarenes and beyond to macrocycle-mediated
frameworks. During the past 10 years, we have been tackling the question
of protein–calixarene complexation in several ways, mainly
by cocrystallization and X-ray structure determination as well as
by solution state methods, NMR spectroscopy, isothermal titration
calorimetry (ITC), and light scattering. Much of this work benefitted
from collaboration, highlighted here. Our first breakthrough was the
cocrystallization of cationic cytochrome
c
with sulfonato-calix[4]arene
leading to a crystal structure defining three binding sites. Together
with NMR studies, a dynamic complexation was deduced in which the
calixarene explores the protein surface. Other cationic proteins were
similarly amenable to cocrystallization with sulfonato-calix[4]arene,
confirming calixarene–arginine/lysine encapsulation and consequent
protein assembly. Calixarenes bearing anionic substituents such as
sulfonate or phosphonate, but not carboxylate, have proven useful.
Studies with larger calix[
n
]arenes (
n
= 6, 8) demonstrated the
bigger better binder
phenomenon
with increased affinities and more interesting assemblies, including
solution-state oligomerization and porous frameworks. While the calix[4]arene
cavity accommodates a single cationic side chain, the larger macrocycles
adopt different conformations, molding to the protein surface and
accommodating several residues (hydrophobic, polar, and/or charged)
in small cavities. In addition to accommodating protein features,
the calixarene can bind exogenous components such as polyethylene
glycol (PEG), metal ions, buffer, and additives. Ternary cocrystallization
of cytochrome
c
, sulfonato-calix[8]arene, and spermine
resulted in altered framework fabrication due to calixarene encapsulation
of the tetraamine. Besides host–guest chemistry with exogenous
components, the calixarene can also self-assemble, with numerous instances
of macrocycle dimers.
Calixarene complexation enables protein
encapsulation, not merely
side chain encapsulation. Cocrystal structures of sulfonato-calix[8]arene
with cytochrome
c
or
Ralstonia solanacearum
lectin (RSL) provide evidence of encapsulation, with multiple calixarenes
masking the same protein. NMR studies of cytochrome
c
and sulfonato-calix[8]arene are also consistent with multisite binding.
In the case of RSL, a
C
3
symmetric trimer,
up to six calixarenes bind the protein yielding a cubic framework
mediated by calixarene dimers. Biomolecular calixarene complexation
has evolved from molecular recognition to framework construction.
This latter development contributes to the challeng...