From left, undergraduate Jason Lang, graduate students Yugang Bai and Hua Lu, and materials science and engineering professor Jianjun Cheng have developed a simple method of making short protein chains with spiral structures that can also dissolve in water, two desirable traits not often found together. Credit: L. Brian Stauffer
Researchers have developed a simple method of making short protein chains
with spiral structures that can also dissolve in water, two desirable traits
not often found together. Such structures could have applications as building
blocks for self-assembling nanostructures and as agents for drug and gene delivery.
Led by Jianjun Cheng, a professor of materials science and
engineering at the Univ.
of Illinois, the research
team will publish its findings in Nature
Materials scientists have been interested in designing large
polymer molecules that could be used as building blocks for self-assembling
structures. The challenge has been that the molecules generally adopt a
globular, spherical shape, limiting their ability to form orderly aggregates. However,
polypeptides can form helical structures. Short polypeptide chains that adopt a
spiral shape act like cylindrical rods.
“If you have two rigid rods, one positive and one negative,
right next to each other, they’re going to stick to each other. If you have a
way to put the charge on the surface then they can pack together in a close,
compact way, so they form a three-dimensional structure,” Cheng said.
However, it is difficult to make helical polypeptides that
are water-soluble so they can be used in solution. Polypeptides gain their
solubility from side chains—molecular structures that stem from each amino acid
link in the polypeptide chain. Amino acids with positive or negative charges in
their side chains are needed to make a polypeptide disperse in water.
The problem arises when chains with charged side chains form
helical structures. The charges cause a strong repulsion between the side
chains, which destabilizes the helical conformation. This causes water-soluble
polypeptides to form random coil structures instead of the desired helices.
In exploring solutions to the riddle of helical,
water-soluble polypeptides, researchers have tried several complicated methods.
For example, scientists have attempted grafting highly water-soluble chemicals
to the side chains to increase the polypeptides’ overall solubility, or
creating helices with charges only on one side.
“You can achieve the helical structure and the solubility
but you have to design the helical structure in a very special way. The peptide
design needs a very specific sequence. Then you’re very limited in the type of
polypeptide you can build, and it’s not easy to design or handle these
polypeptides,” Cheng said.
In contrast, Cheng’s group developed a very straightforward
solution. Since the close proximity of the charges causes the repulsion that
disrupts the helix, the researchers simply elongated the side chains, moving
the charges farther from the backbone and giving them more freedom to keep their
distance from one another.
Researchers found that elongating side chains with charged ends enabled short proteins to coil into a stable helix. Credit: Jianjun Cheng
The researchers observed that as they increased the length
of the side chains with charges on the end, the polypeptides’ propensity for forming
helices also increased.
“It’s such a simple idea—move the charge away from the
backbone,” Cheng said. “It’s not difficult at all to make the longer side
chains, and it has amazing properties for winding up helical structures simply
by pushing the distance between the charge and the backbone.”
The group found that not only do polypeptides with long side
chains form helices, they display remarkable stability even when compared to
non-charged helices. The helices seem immune to temperature, pH, and other
denaturing agents that would unwind most polypeptides.
This may explain why amino acids with large hydrophobic side
chains are not found in nature. Such immutability would preclude dynamic
winding and unwinding of protein structures, which is essential to many
biological processes. However, rigid stability is a desirable trait for the
types of applications Cheng’s group explores: nanostructures for drug and gene
delivery, particularly targeting cancerous tumors and stem cells.
“We want to test the correlation of the lengths of the
helices and the circulation in the body to see what’s the impact of the shape
and the charge and the side chains for clearance in the body,” Cheng said.
“Recent studies show that the aspect ratio of the nanostructures—spherical
structures versus tubes—has a huge impact on their penetration of tumor tissues
and circulation half-lives in the body.”
Cheng plans to create a library of short helical
polypeptides of varying backbone lengths, side chain lengths and types of
charge. He hopes to simplify the chemistry even further and make the materials
widely accessible. His lab already has demonstrated that helical structures can
be effective gene delivery and membrane transduction agents, and building the
library of soluble helical molecules will allow further investigation of
tailoring such nanostructures for specific biomedical applications.