Proteomics - Challenges in Delivery of Therapeutic Genomics and Proteomics - page 406

Though a lot of emphasis has been given to the native conformation of a globu-
lar protein, conservation of native conformation may not be observed with noncyclic
peptides. Endogeneous endorphins and enkephalins are a few such examples that dis-
play multiple conformations.
�.5.2.1.2 Solubility
Classically, proteins were classified on the basis of their solubility profile. Most
nonconjugated globular proteins are soluble in water, dilute acid, and dilute salt
solutions, but insoluble in concentrated salt solutions. Peptides with fewer ionic
groups are apparently less water soluble. Intermolecular attractions leading to aggre-
gation result in reduced water solubility of proteins. This may be the rationale for
higher dissolution rates and enhanced solubility of proteins in denaturing solvents.
It is customary to obtain protein as a solid from an aqueous solution. However,
thermolability of proteins puts a major restraint on the selection of evaporation or
drying procedures. It is preferable to choose degassing and freezing followed by
lyophilization to avoid protein degradation and loss. One of the drawbacks of lyophi-
lized powders of proteins is their high bulk volume and hygroscopicity, making
them moisture sensitive. Hence, such lyophilized powders should be stored over a
desiccant.
�.5.2.1.� Chiral Stability
Stereochemically active protein molecules mainly raise the issue of chiral stability.
All amino acids other than glycine exhibit stereoisomerism. These stereoisomers
are mainly enantiomers—nonsuperimposable on their own mirror images or
diastereomers—stereoisomers that are not enantiomers. An optical rotation or change
in the optical angle, like conversion of an L-amino acid to a D-amino acid form, leads
to altered properties.
�.5.2.1.4 Chemical Stability
Stability of conformation has been found to be crucial for overall chemical stability.
For instance, a chemical reaction could lead to lowering of conformational energy
of a protein in a highly strained conformation, or vice versa. Addition of chemical
reagents may also lead to stability issues. The effect of degradation products on the
stability of proteins must not be overlooked and is determined quantitatively by mod-
ern techniques like HPLC. The destabilization mechanisms are mainly composed
of diketopiperazine formation (seen in case of proline, glycine-containing proteins),
imide formation (in asparagines, glutamine resulting in formation of cyclic aspartim-
ide, or glutarimide), transpeptidation, oxidation, and so on.
A paradigm shift in biotechnology has led to exploration of proteins as bioactives
with high potency. These proteins are major challenges to chemists from a formula-
tion point of view, owing to their large macroscopic size and structural complexity,
which render some critical issues during formulation; as well, there is the challenge
of averting degradation. As seen earlier, three-dimensional conformational structures
of proteins are largely dependent on the presence of chemicals, processing steps, pH
conditions, and so forth. Hence, all the materials, processes, and accessories related