Biomedical Engineering Reference
In-Depth Information
acids, which are major nitrogen-containing organic components of living organisms.
For delivery of exogenous proteins and peptides to the living system, it is essential to
understand the properties and the stability of this class of compounds.
9.3.4.1 Physical Properties of Peptide and Protein Drugs
9.3.4.1.1 Solubility, Aggregation, and Precipitation
Peptide solubility can be viewed as the summation of the solubility properties of indi-
vidual constituent amino acids, much as the solubility properties of organic chemicals
can be estimated from contributions made by molecular fragments. When a polypeptide
is in liquid medium, the less polarized groups tend to occupy the most preferred posi-
tion and to have minimal contact with the surrounding water molecules. In a large pep-
tide chain, the polar groups interact with water molecules and become hydrated. These
highly hydrated peptide chains have less of a chance of interacting with each other, so
aggregation is minimized. Peptides or proteins are least soluble in water at their isoelec-
tric point at which electrostatic forces responsible for stabilization of the bound water
layer are minimal and the potential for intermolecular interaction is maximal.
9.3.4.1.2 Unfolding and Refolding of Proteins and Peptides
The secondary structure of the proteins is extensively stabilized by intramolecular
hydrogen bonds and partially stabilized by the formation of disulfide crosslinkages
between cysteine moieties. Further arrangement of these secondary structures leads
to compact domains (tertiary) and beyond that, to aggregates (quaternary structures).
Partial unfolding of the peptide chain is accomplished by changing the solution pH,
by the addition of strong hydrogen bond disrupting (chaotropic) agents (e.g., urea or
guanidinium chloride) or surface-active agents, or by heating the solution. The meta-
stable state can often be converted back to the native state by returning the protein to
its original unfolded state. Thus, during the protein-peptide formulation these param-
eters must be evaluated for compatibility and stability.
9.3.4.1.3 Interfacial Properties
The presence of an air-water or solid-water interface disrupts the normal forces sta-
bilizing protein secondary, tertiary, and quaternary structure in solution [8] . Partial
unfolding occurs, which may then lead to adsorption to the surface through interaction
between hydrophobic amino acid residues and the surface, and electrostatic forces
between polar amino acid residues to charged surfaces. Thus, peptides and proteins
are adsorbed quite readily to both nonpolar solid surfaces (such as polyperfluoroeth-
ylene and polystyrene) and to surfaces with ion-exchange properties (such as glass).
These parameters must be considered during manufacturing, packaging, and storage
of proteins and peptide formulations. Some proteins contain hydrophobic amino acids
that form cylindrical sheet like structures that readily penetrate lipid membranes,
which contributes to a high affinity of the protein for oil-water interfaces.
9.3.4.1.4 Effect of Water on Peptide and Protein Structure
Even crystalline proteins contain a high proportion of bound water (up to 70% by
weight). Consequently, the removal of water from proteins profoundly affect their
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