Catalytic Synthesis and Characterization of Biodegradable ...

Chapter 6
Figure 6.6 Cyclic voltammograms of (A) MPEG-b-PTAm and (B) MPEG-b-PLA-b-PTAm
micelles in 0.1 M HCl aqueous solution.
6.5 EPR study of the micelles in PBS containing ascorbic acid
It is known that a reducing environment is necessary for biological systems. The
antioxidants such as ascorbic acid (Vc) and glutathione (GSH) are important to keep the
redox status in living cells and exist in most of parts in vivo. Thus, we evaluated the reduction
resistance of the micelles in the presence of antioxidants. Since the reduction of nitroxyl
radical into hydroxylamine group by GSH is much slower than Vc and there is higher
concentration of Vc in the blood stream, 22, 23 we selected antioxidant Vc for our study. The
EPR measurements were conducted in the presence of Vc at concentration of 5.0 mM which
is 50 times more than the concentration of Vc in the blood.
As shown in Figure 6.7 A, the EPR signal decreased over time in PBS solution
containing Vc, ascribing to the reduction of nitroxyl radicals. And the EPR signal pattern of
both MPEG-b-PTAm and MPEG-b-PLA-b-PTAm micelles were broad single, which is
distinct from typical triplet signal of free TEMPOL radical in diluted solution, due to the
spin-spin interaction of TEMPO in the side chains. 19, 24, 25 Furthermore, we observed that the
EPR signal in aqueous media is broader than in chloroform, which is consisted with previous
report. 18 These results indicated that the polymer segment containing nitroxyl radicals had
been incorporated into micelles in aggregated state in the hydrophobic core. By virtue of the
micelle structure, the nitroxyl radicals in the core could resist reduction for a longer time than
free TEMPOL radical even in the presence of higher concentration of Vc (Figure 6.7 B). The
half-life time for both MPEG-b-PTAm and MPEG-b-PLA-b-PTAm micelles were about 8
‐ 120 ‐