residues of the native protein are only partially located on the surface and exposed to water. Significant parts are located inside the
globule. An excess of gelatin results in a decrease in the fluorescence intensity and in a small shift of max from 339 to 344 nm.
These facts indicate a partial unfolding of BSA with an additional
exposure of the hydrophobic tryptophan residues to the surface.
What is the reason for the great difference in structure formation
between native and BSA
TA
in systems containing gelatin? We have
considered three possible reasons for such a behavior of BSA: (1)
the structure and the charge of thermally aggregated BSA could be
different in the presence of gelatin as compared to those of native
BSA. (2) Change in solvent quality for BSA molecules after limited
thermoaggregation. (3) Steric reasons affecting complex formation.
Let us consider all these possibilities.
If partial destabilization of the secondary structure of BSA
TA
and
an additional exposition of the hydrophobic tryptophan residues to
the surface in the presence gelatin take place, they might increase
the charge of the interacting molecules and affect the structure
of the complexes formed. Actually, the z-average potential of BSA
(z
za
BSA) is −8.45 mV ± 0.8 mV), whereas z
za
BSA
TA
= −11.6 ± 0.8 mV.
At the compositions corresponding to the maximal binding of BSA
by gelatin, the average z potential of the gelatin–BSA complex particles is (z
za
BSA–gel)∼=0 [20], against +3.8 mV for BSA
TA
. It is obvious
that the difference between the surface charge of the complex particles when formed by native or by thermally aggregated BSA can
be one of the reasons their great difference in size.
Capacity of bovine serum albumin to interact with gelatin could
be also changed after its thermoaggregation due to a change in its
activity in its saturated solution.
Therefore Rosenberg’s method was applied to evaluate a
possible changes in the activity of BSA after its limited thermoaggregation. Fig. 8 shows the effect of PEG on the solubility of BSA in
water before and after thermally induced aggregation at pH 5.4. The
dependences obtained proved to be rectilinear. Extrapolation of
this dependence to CPEG= 0 gives the value for the effective activity
of the protein in its saturated solution (log Cbiopolymer) [34].
The results obtained show that thermally induced aggregation
of BSA does not affect appreciably the activity of saturated BSA