Adsorption and protonation of peptides and proteins in pH responsive gels

By Dean Williams on August 18, 2016

Authors Gabriel Longo (CONICET, Argentina) and Igal Szleifer (Northwestern University, USA) introduce their latest review in Journal of Physics D: Applied Physics on how proteins protonate and deprotonate their amino acids to regulate electric charge under different conditions, and how understanding these processes could lead to more efficient drug delivery:

There is currently great interest in the pharmaceutical use of peptide and proteins, which are increasingly used in vaccination, disease diagnosis and treatment, and tissue engineering. Proteins are key players in most biological games, having highly specific functions and low toxicity, which are qualities difficult to achieve with the traditional small-molecule drugs. Efficient delivery of fully functional proteins, however, is still an important challenge in biomedical research.

Oral uptake is the desirable drug delivery route. This path is less invasive having several advantages that can improve patient quality of life, including high compliance, ease of administration, avoidance of irritation and pain, and relatively low production cost. Most small-molecule drugs in the market are delivered orally. On the contrary, due to the digestive system’s natural mechanisms for breaking down ingested proteins/peptides, oral delivery of these biomolecules without some means of protection is extremely inefficient. Thus, therapeutic peptides and proteins are generally administered through injections.

To bypass the barriers that the gastrointestinal environment imposes to the oral delivery, some researchers have considered pH-responsive hydrogels as functional vehicles for the oral administration of therapeutic peptides and proteins to the upper small intestine. Polyacid networks are relatively collapsed at the acidic stomach environment (pH ~1.2-2). This high polymer density prevents the encapsulated agent from escaping the hydrogel. On the contrary, at the alkaline conditions of the intestines (pH ~7-8), the hydrogel swells and release can occur due to drug diffusion through the swollen network.

Applications requiring peptide/protein adsorption inside pH-responsive hydrogels face several challenges, and despite all the research conducted in recent years the physical chemistry involved in protein adsorption and protonation is not completely understood. In this review, we summarize our recent theoretical work in order to describe some of the non-trivial features of the adsorption and protonation of peptides/proteins in pH-responsive hydrogels.

Schematization of the molecular model used to study lysozyme adsorption in pH-responsive hydrogel thin-films. A network composed of crosslinked poly(acrylic acid) chains is in contact with a protein solution whose chemical composition is experimentally controlled. The table gives the frequency (composition number) and pKa of each amino acid in lysozyme. Image taken from Gabriel S Longo and Igal Szleifer 2016 J. Phys. D: Appl. Phys. 49 323001, © IOP Publishing. All Rights Reserved.

In these systems, molecular confinement in nanometer-sized environments modifies the balance between chemical state, physical interactions and molecular organization, which results in novel behavior that is qualitatively different from the behavior that proteins display in solution. To enhance adsorption, the protonation state of all amino acids of adsorbed proteins is significantly different than that of solution proteins. This behavior depends on the specific amino acid, which gives the adsorbed protein degrees of freedom to regulate electric charge and enhance the electrostatic attractions with the hydrogel’s polymer network under a wide range of experimental conditions. These electrostatic attractions are the driving force for adsorption. In addition, protein adsorption modifies the microenvironment inside the hydrogel, particularly the pH. As a result, the state of protonation of the polymer network is different before and after adsorption.

You can read the full review now in Journal of Physics D: Applied Physics.

About the authors

Gabriel S. Longo is a researcher at the National Scientific and Technical Research Council (CONICET), Argentina. He obtained his Licentiate degree in Physics from the National University of Cordoba, Argentina. He received his PhD in Chemistry from Purdue University under supervision of Professor I. Szleifer. Currently, Dr. Longo’s main research interests focus on the molecular modeling of soft materials, particularly stimuli-responsive hydrogels.

Igal Szleifer is the Christina Enroth-Cugell Professor of Biomedical Engineering at Northwestern University. He received his BSc in Chemistry and PhD, summa cum laude, from the Hebrew University of Jerusalem, Israel. Dr. Szleifer’s research is aimed at the fundamental understanding of the properties of complex molecular systems that encompass problems at the interface between medicine, biology, chemistry, physics and materials science. His research group concentrates on the development and application of molecular theoretical approaches to study biomaterials, drug delivery systems, models for cell membranes and biological processes.

This work is licensed under a Creative Commons Attribution 3.0 Unported License. Image taken from Gabriel S Longo and Igal Szleifer 2016 J. Phys. D: Appl. Phys. 49 323001 © IOP Publishing, All Rights Reserved.