Behavior of ligand binding assays with crowded surfaces: Molecular model of antigen capture by antibody-conjugated nanoparticles
September
Behavior of ligand binding assays with crowded surfaces: Molecular model of antigen capture by antibody-conjugated nanoparticles
David C. Malaspina 1 2
Gabriel Longo 0 2
Igal Szleifer 1 2
0 Instituto de Investigaciones FisicoquÂõmicas , Teo ricas y Aplicadas (INIFTA), UNLP, CONICET, La Plata , Argentina , 3 Chemistry Department and Chemistry of Life Processes Institute , Evanston, Illinois , United States of America
1 Biomedical Engineering Department, Northwestern University , Evanston, Illinois , United States of America
2 Editor: Bing Xu, Brandeis University , UNITED STATES
Ligand-receptor binding is of utmost importance in several biologically related disciplines. Ligand binding assays (LBA) use the high specificity and high affinity of ligands to detect, target or measure a specific receptors. One particular example of ligand binding assays are Antibody conjugated Nanoparticles (AcNPs), edge-cutting technologies that are present in several novel biomedical approaches for imaging, detection and treatment of diseases. However, the nano-confinement in AcNPs and LBA nanostructures introduces extra complexity in the analysis of ligand-receptor equilibriums. Because antibodies are large voluminous ligands, the effective affinity in AcNPs is often determined by antibody orientation and surface coverage. Moreover, antibodies have two binding sites introducing an extra ligandreceptor binding equilibrium. As consequence of all this, experimental or theoretical studies providing a guidelines for the prediction of the binding behavior in AcNPs are scarce. In this work, we present a set of theoretical calculations to shed light into the complex binding behavior of AcNPs and its implications in biomedical applications. To investigate the ligandreceptor binding on AcNPs, we have used a molecular theory that predicts the probability of different molecular conformations of the system depending on the local environment. We have considered two different pathways for designing these devices: covalently conjugated antibodies and streptavidin-biotin conjugated antibodies. We also explore the effects of surface coverage, bulk concentrations, nanoparticle size and antibody-antigen affinity. Overall, this work offers a series of theoretical predictions that can be used as a guide in the design of antibody conjugated nanoparticles for different applications.
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Data Availability Statement: All relevant data are
within the paper and its Supporting Information
files.
Funding: This research was supported by Grant
No. EB005772 from the National Institute of
Biomedical Imaging and Bio- engineering (NIBIB)
at the National Institutes of Health (NIH) to IS. This
research was supported by National Science
Foundation CBET-1403058 (to IS). G.L. thanks the
support from the ANPCyT, Argentina
(PICT-20143377). The funders had no role in study design,
Introduction
The binding between a ligand and its receptor is the main area of research in several
biological related disciplines. Ligand-receptor binding is ubiquitous in many biological processes,
data collection and analysis, decision to publish, or
preparation of the manuscript.
including immune reactions, signaling, opening of ion channels and gene activity [1±4]. In
the pharmaceutical industry, around 70% of the total sales of drugs to treat cancer [
5
] and
autoimmune diseases [
6
] are therapies based on the binding of antibodies (the ligands) to
specific receptors. The main feature of ligand-receptor binding that makes this interaction
so attractive for a variety of applications is that it displays high specificity and high affinity.
For example, antibodies only bind strongly to their respective complementary epitopes
(high selectivity), with typical antibody-antigen dissociation constants (Kd) in the range of
10−8 to 10−11 M (high affinity) [
7
]. Due to these characteristics the biomedical research field
has introduce several techniques that exploit ligand-receptor interactions. In particular, we
can mention ligand-binding assays (LBA) that use ligands to detect, to target or to measure a
specific receptor [1±3].
During the last decade, the production of LBA combined with nanoparticles (NPs) has
increased due to the potential for in-vivo and in-vitro imaging and detection of different
analytes, as well as for specific therapies such as thermal-ablation, gene therapy or localized drug
delivery with nano-carriers [8±13]. However, nanoparticle mediated ligand-receptor binding
displays a particularly complex behavior that arises from the confinement of the molecular
species on a small surface. The chemical equilibrium between ligands and receptors can be
locally displaced according to the inhomogeneous concentration of the species, which results
in an effective affinity that highly depends on the local environment and the nature of the
confinement. For that, predicting of the outcome behavior of LBA in nano-structures represents a
challenging task that requires (...truncated)