C O M M U N I C A T I O N S
rigidity. Sedimentation velocity measurements, on the other hand,
indicate that the predominant species at low protein/Ni concentra-
tions (<1 mM) is dimeric (Figure S10). We suggest that the trimeric
forms (including Ni3:MBP-Phen13), which should be entropically
disfavored relative to any dimeric species, are only significantly
populated at the high protein concentrations required for crystal-
lization.5 The high internal symmetry of Ni3:MBP-Phen13 and its
rigidity likely promote its selective crystallization from among other
species that coexist in solution (Figure 2d). Studies are currently
underway to stabilize Ni3:MBP-Phen13 and destabilize other
possible conformations through surface engineering in order to
isolate it in solution and assess the reactivity of the interfacial Ni
centers.6
Synthetic metal-coordinating functionalities have previously been
employed to stabilize coiled-coil assemblies,7 construct reactive
metal binding sites in protein interiors,4a and tune the potentials of
redox centers,8 among other applications.3 We have demonstrated
here that incorporation of such non-natural ligands onto protein
surfaces can lead to novel biological architectures as well as
coordinatively unsaturated metal sites within these scaffolds. The
wide array of functionalities available in the synthetic inorganic
chemist’s toolbox thus could provide a powerful means of generat-
ing structural and functional diversity in protein self-assembly.
Figure 2. (a) Four Ni3:MBP-Phen13 trimers in the asymmetric unit of the
P21 crystals and their triangular representation (Ni atoms as vertices) viewed
from the side and the top. (b, c) Lattice packing arrangements of Ni3:MBP-
Phen13 in the P21 and P6322 crystal forms. (d) Suggested Ni-induced
oligomerization behavior of MBP-Phen1 in solution.
Acknowledgment. This work was supported by UCSD, a
Beckman Young Investigator Award (F.A.T.), and an NIH Heme
and Blood Program Training Grant (R.J.R). Portions of this research
were carried out at SSRL, operated by Stanford University on behalf
of DOE.
square-pyramidal Ni coordination geometry. While His77 (dNi-N
) 2.1 ( 0.1 Å) and PhenC59 (dNi-N1/N2 ) 2.1 ( 0.1 Å) are clearly
defined in the electron density maps (Figure S11), the two other
coordination sites cannot be unambiguously assigned because of
the resolution limits. We tentatively ascribe the diffuse electron
density near Ni to two aquo/chloride ligands coordinated trans to
PhenC59 at distances of 2.8 and 2.6 ((0.3) Å from Ni and 2.7
((0.2) Å from each other, averaged over the 12 independent metal
sites in the asymmetric unit of P21 crystals (see the SI for a detailed
discussion).
Lattice packing interactions in both Ni3:MBP-Phen13 crystals are
particularly noteworthy. In the P21 form, there are four crystallo-
graphically distinct but identical (overall rmsd-CR ) 0.3 Å) copies
of the trimer in the asymmetric unit, which stack up along their
threefold symmetry axes to form a tubular architecture (Figure 2a).
Each of the four Ni3:MBP-Phen13 trimers adopts a different
orientation around the long axis of the tube, giving rise to three
distinct trimer-trimer interactions. In the lattice, the tubular units
are further stacked end-on-end infinitely and, because of the
superposition of the four different trimer orientations, adopt an
apparent hexagonal geometry. The resulting hexagonal tubes form
a tightly packed two-dimensional array (50% solvent content)
(Figure 2b). In the P6322 crystals, the Ni3:MBP-Phen13 trimers are
similarly arranged to form hexagonal tubular structures (Figure 2c).
In contrast to the P21 lattice, the trimers of all adjacent tubes are
coplanar, which is required for generation of the two-, three- and
sixfold symmetry components of the P6322 space group. Moreover,
a central hexagonal tube is not accommodated in this lattice, leading
to a large cavity (6 nm diameter) and an increased crystal solvent
content of 64%. Though observed only in crystals, such arrange-
ments suggest that open, symmetrical protein superstructures such
as Ni3:MBP-Phen13 could be in principle be utilized as building
blocks for porous protein frameworks.
Supporting Information Available: Additional experimental details
and discussion. This material is available free of charge via the Internet
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