Biomedical Engineering Reference
In-Depth Information
2.1.2 Soft Corona
The molecules which are loosely bonded to the nanoparticle surface or have weak
interaction with the hard corona form the soft corona. In the case of some
nanoparticles, especially those with a preformed functional group such as pegylated
nanoparticles, there is only a weak corona covering the surface and no hard corona
is observed [
11
].
The theoretical challenge of understanding why certain proteins are adsorbed in
a competitive manner is unclear. Certainly there are many hints that this is a
collective process, and therefore, it will be difficult to rationalize on the basis of
individual protein-binding studies. Thus, while there is growing certainty that the
corona is what is “seen” by the cell, there is as yet relatively little progress on why
any NP chooses those particular proteins.
2.2 Protein Conformation
During adsorption on the nanoparticles, proteins may undergo structural
rearrangements called “conformational changes.” These changes are thermody-
namically favorable if they allow a hydrophobic or charged sequence within a
protein to interact with a hydrophobic or charged nanomaterial surface, respec-
tively. Changes in protein conformation are typically irreversible after desorp-
tion. For example, conformational changes in the iron-transport protein
transferrin are not recovered after desorption from iron oxide nanoparticles.
Conformation of adsorbed proteins is altered more in the presence of charged
or hydrophobic nanomaterials. For example, quantum dots grafted with
mercaptoundecanoic acid denature and inactivate the enzyme chymotrypsin,
while the same particles grafted with a structurally similar but hydrophilic
poly(ethylene glycol) (PEG) derivative adsorb the enzyme but do not denature
it to the same extent [
2
].
Binding of proteins to planar surfaces often induces significant changes in
secondary structure, but the high curvature of NPs can help proteins to retain
their original structure. However, study of a variety of NP surfaces and proteins
indicates that the perturbation of protein structure can appear. Lysozyme adsorbed
onto silica NPs or bovine serum albumin adsorbed on Au NPs surfaces showed a
rapid conformational change at both secondary and tertiary structure levels. Most of
the studies have reported that loss of
α
-helical content occurs as detected by circular
dichroism spectroscopy when proteins are adsorbed onto NPs and a significant
increase in sheet and turn structures.
