showed stability maxima, between 0 and 20 percent DMSO by
volume. Peptide A- 2 showed maximal stability at 20 percent,
implying if this set of assays were to be extended to a greater
proportion of DMSO in solution, this peptide’s stability could
further increase. Regardless of hydrophilicity, the addition of
DMSO was beneficial for stability.
The stability gains achieved by the addition of DMSO were
not observed in the case of the MeCN addition. MeCN caused
rapid aggregation of the A group peptides, but it caused stabilization of the B group peptides. This can be accounted for
by changes in solution polarity. Decreased solution polarity
can decrease enthalpic penalties of solvation for hydrophobic
pH-dependent stability was highly variable between samples. More extreme pHs tended to represent increased stability
against aggregation, or conversion to an insoluble species.
Extreme pHs can also be problematic because of acid- or
base-catalyzed degradation pathways. In most cases, there were
moderate pHs in which samples demonstrated relatively stable
behavior and were not prone to hydrolysis.
Group A peptides showed decreasing stability with increasing concentration of peptide. Since these peptides easily dis-
Figure 1: Hydrophilic peptides exhibited increased stability as
ionic strength of guanidinium chloride increased. Their optimal
stability was achieved between 2 and 3 M.
Figure 2: (peptide: A-1) As the final concentration of acetonitrile increases, the CCD reaches saturation faster. There are
two explanations for this: 1) A-1 is quickly aggregating due
to a change in the solution polarity, or 2) the addition of the
MeCN:H2O cosolvent solution drastically changes the optical
properties of the solution when the peptide is present.
Figure 3: B-1 rapidly precipitated in basic conditions. At pH 2,
gradual aggregation occurs at the end of the experiment, but
there was almost no aggregation observed at pH 4. pH 6 and
pH 8 showed identical behavior as pH 10, but at a slower rate.
Both are consistent with observation of pH-dependent precipitation from change in protonation state rather than irreversible
Figure 4: Peptide B-1 exhibited colloidal stability. The stability
granted by higher concentration is only a slight increase and
is best observed by the gradual aggregation at the end of the
experiment when the dilute sample dimerizes.
solve into their solution, they can effortlessly explore conformations with favorable intermolecular interactions. Different from
Group A peptides, Peptide B-1 showed colloidal stability. As the
concentration of peptide increased, the solution became more
stable. This is normally sustained by intermolecular repulsion
preventing peptides from associating.
ARGEN was used to formulate four pharmaceutical peptides
for long-term storage in aqueous form based on continuously collected data and a kinetic approach to analysis and data
interpretation. Using ARGEN’s kinetic data, it was also possible
to quickly rule out certain excipients such as guanidinium
chloride for the hydrophobic B-group peptides and acetonitrile
for the hydrophilic A-group peptides. Future research could
utilize all 16 of ARGEN’s cells to better finely tune the formulation and probe for interactions between excipients. Lastly,
future research could also look into the effects of more exotic
additives that would be beneficial given a known primary