(217ca) All-in-One Peptide Biomaterial: Biomolecular Recognition, Ultra-Low Fouling, and Surface Anchoring
Proteins in the human body are able to avoid nonspecific adsorption and are stable in complex media. These are two desirable properties for many biomedical applications including biosensors, drug delivery, and tissue engineering. By examining the surfaces of proteins we can seek inspiration for new stealth materials that are biocompatible. The analysis of over a thousand protein surfaces indicates that glutamic acid (E) and lysine (K) are the two most prevalent amino acids on the surfaces of proteins. Based on this knowledge, a nonfouling peptide was rationally designed by alternating negatively charged E and positively charged K amino acid residues. The EK sequence forms a strong hydration layer similar to zwitterionic materials allowing it to resist nonspecific protein adsorption.
A surface anchoring linker was designed in order to form a robust coating. Cysteine is commonly used to attach peptides onto gold surfaces. Here we show that the inclusion of an additional linker with a length of four residues (-PPPPC) and a rigid, hydrophobic nature is a better choice for forming peptide self-assembled monolayers (SAMs) with a well-ordered structure and high surface density. We compared the structure and function of the nonfouling peptide EKEKEKE-PPPPC-Am with EKEKEKE-C-Am. Circular dichroism, attenuated total internal reflection Fourier transform IR spectroscopy, and molecular dynamics results showed that EKEKEKE-PPPPC-Am forms a secondary structure while EKEKEKE-C-Am has a random structure. Surface plasmon resonance sensor results showed that protein adsorption on EKEKEKE-PPPPC-Am/gold is very low with small variation while protein adsorption on EKEKEKE-C-Am/gold is high with large variation. X-ray photoelectron spectroscopy results showed that both peptides have strong gold−thiol binding with the gold surface, indicating that their difference in protein adsorption is due to their assembled structures. Further experimental and simulation studies were performed to show that -PPPPC is a better linker than -PC, -PPC, and -PPPC.
Finally, we extended the peptide sequence EKEKEKE-PPPPC-Am with the cell-binding sequence RGD to demonstrate control over specific vs. nonspecific cell adhesion without using synthetic polymers such as poly (ethylene) glycol (PEG). Adding a functional peptide to the nonfouling EK sequence avoids complex chemistries that are used for its connection to synthetic materials. The combination of peptide sequences with different properties allows for a versatile platform combining several functions in one material.