An Evolutionary Approach to the Discovery of New Materials for the
Production and Utilization of Alternative Energy
An affordable energy supply that is less reliant on fossil fuels
is essential in curbing global warming and ocean acidification,
maintaining sufficient global food production, and improving air
quality. Lack of access to affordable energy is particularly serious
and widespread in the developing world, where more than 2 billion
people are without electricity. Without access to clean energy,
these people cannot perform basic tasks such as cooking food properly
or doing homework at night.
The challenge of providing cost-effective sources of non-fossil
fuel energy demands new materials for converting sunlight to electricity
or stored chemical energy such as hydrogen. Unfortunately, the
landscape of possible inorganic solid-state materials is infinitely
vast, and predicting the right combination of elements for a desired
application is nearly impossible. How can we possibly hope to
find the perfect combination of elements for the materials needed
advanced energy technology?
Our lab has been applying an approach to materials discovery
that seeks to exploit biological evolution and selection. Indeed,
nature has provided some of the best examples of materials synthesis.
Nearly every living organism on Earth synthesizes a solid-state
material, some for structural integrity or protection, others
for biosphere function such as light focusing or magnetotaxis.
We have asked the question: Can biomolecules evolve in vitro in
response to certain selection criteria until they are capable
of forming a solid-state material with a desired property? The
answer is yes, and the particles shown below are some examples.
These particles were all grown in the presence of specific RNA
sequences that were selected from large random RNA sequence libraries
(~1014 unique sequences) using a process known as in
RNA vitro selection. These results are important because they
suggest that biomolecules such as RNA can assemble metal precursors
into nanoparticles with sizes, shapes, and compositions that depend
upon RNA sequence. This in turn suggests that new materials with
advanced catalytic or photonic properties could emerge from a
large library of RNA sequences. We are currently attempting to
exploit RNA in vitro selection to discover new catalyst and photonic
materials for photovoltaics and fuel cells.
||Pt spheres (left) and porous decahedra synthesized with
Cobalt-doped iron oxide nanoparticles
(left), and Pd hexagons and cubes synthesized with different
RNA sequences isolated from large random sequence RNA libraries
using RNA in vitro selection.
Nanocrystal Approach to the Treatment of Multi-Drug Resistant Pathogens
Approximately 1/5 to 1/3 of people worldwide are living with
tuberculosis (TB) and 33 million people are living with HIV. Many
of these people have contracted both TB and HIV, a lethal combination
in part because drug interactions preclude the treatment of both
pathogens. In addition, both HIV and TB have become resistant
to current treatments and can hide in "sanctuary sites"
such as the brain, where small molecule drugs often have difficulty
TB or NoTB. Synthetic nanometer-scale systems
have the potential to overcome many limitations of conventional
small molecule therapeutic agents. For instance, small molecules
typically have short blod circulation times (hrs), rely on a single
high-affinity contact to a disease target, and are incapable of
disrupting protein-protein interactions that often drive disease
pathogenisis. In contrast, nanoscale systems can provide long
circulation times (days to weeks), have tunable valency, and are
adept at preventing protein-protein interactions. We have hypothesized
that gold nanocrystals may possess a number of attributes that
make them useful drug candidates. Gold nanocrystals are now accessible
in a range of well defined sizes from ca. 1.0 nm to 10 nm. Gold
nanocrystals also enable one to rapidly synthesize combinatorial
libraries of nanoscale compounds; using organothiol exchange reactions,
combinations of two or more chemically distinct ligands can be
attached to a single particle to create multi-ligand and multi-functional
systems. The ability to rapidly assemble mixed thiol monolayers
on a nanoscale platform provides a powerful tool that can be used
to tune particle binding affinity to a disease target, and control
cellular internalization and sub-cellular localization.
The potential benefits of gold nanocrystal therapeutics were demonstrated
in our lab recently with the synthesis of a multivalent gold conjugate
that effectively inhibited HIV entry in T-cells. We have also
shown that lactam antibiotics, rendered ineffective at bacterial
growth inhibition due to bacterial mutation, can be converted
back into potent therapeutics via appropriate design and conjugation
to gold nanocrystals. We are currently adapting these formulations
to cross the blood-brain barrier and enter other sanctuary sites.
||Model of a 2.0 nm diameter gold nanocrystal therapeutic that
effectively inhibits HIV entry into human T cells.
the Visual Interactome
In addition to developing nanocrystals
as new therapeutics for treating disease, our lab is developing
new cellular imaging tools for understanding disease at the molecular
level. Our long-term ambition is to generate a complete 3D map of
the spatial arrangement of RNA and proteins in eukaryotic cells:
the "visual interactome". This goal will be accomplished
through the implementation of a new concept in biomolecule tagging,
which will be used in conjunction with cryoelectron tomography (cryoET).
CryoET combines multiple images of a biological sample acquired
at different tilt angles to obtain a 3D volume representation of
the sample. With achievable 5 nm resolution and perfect preservation
of cellular structure, cryoET now represents the highest resolution
technique for examining cells in their native state. As advanced
as cryoET is, one problem still prevents the localization and identification
of all but the most dense of structures in a cell: it is exceedingly
difficult to know which biomolecule one is visualizing when looking
at a 3D whole cell reconstruction. The problem is analogous to the
one in fluorescence microscopy. Although many proteins autofluoresce,
one cannot tell them apart in the absence of a specific fluorescent
marker, such as the green fluorescent protein (GFP). Our lab is
thus developing a new tagging strategy for use in cryoET experiments.
The molecular tags we are developing consist of RNA or peptide sequences
that are selected in vitro to mediate the formation of inorganic
nanoparticles. Once genetically encoded in cells as RNA concatemers
and protein chimeras, these sequences will be able to mediate the
formation of inorganic nanoparticles inside living cells exclusively
at the site of interest. These particles will be electron dense
for visualization in a cryoET reconstruction; when a biomolecule
is observed in a tomogram, the presence of a nanoparticle will allow
the identity of the biomolecule to be known.
To date we have identified several peptides and
proteins that mediate the formation of inorganic nanoparticles such
as gold, silver, and iron oxide. Peptides have been genetically
cloned onto cellular proteins such as the Eg5 motor protein (important
in cell division and thus a target for disease treatment). When
incubated with gold and silver precursors, nanoparticles form on
the protein chimera.
||Gold nanoparticles synthesized on Eg5 motor proteins using a peptide/Eg5