Environment-Induced Silk Fibroin
Conformation Based on the Magnetic Resonance Spectroscopy
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4.5 Influence of Mn2+
Figure 12 shows the EPR spectra of Mn(II)/SF samples which were prepared from samples
with an initial pH’s of 7.5, 6.0 and 5.2 (Deng et al., 2011). All three spectra shown in Figure
12-c’s with the added Mn(II) contents of 40.0 μg/g were almost identical, indicating that the
environments of the Mn(II) ions were not pH dependent. Fig. 12-d’s show typical sextet
splitting along with double peaks between the adjacent peaks, very similar to that of MnCl 2
in 12 M HCl aqueous solution and in methyl alcohol solution at frozen state (T = 90 K)
(Allen & Nebert, 1964), where Mn2+ ion is in the Mn(H 2 O) 6 2+ complex. However, Figure 12-
a’s and 12-b’s with Mn(II) contents of 4.0 and 10.0 μg/g, respectively, show somewhat pH
dependency; the sextet splitting becomes evident as pH decrease from 7.5 to 5.2.
Fig. 12. Dependence of EPR spectra of [Mn(II)]/SF samples with different added contents of
Mn(II) upon pH. (a), (b), (c) represent the added [Mn(II)] of 4.0, 10.0, 40.0 μg/g under pH of
7.5 (A), 6.0 (B), 5.2 (C), respectively. The peaks marked with arrows are free radical signals,
respectively. The six stick-lines on the top of figure indicate the sextet splitting of Mn(II)
with A = 96 G. All the spectra were recorded at 100 K, υ = 9.45 GHz, sweep width = 1500 G
(From Deng et al., 2011 with permission).
Fig. 13. Dependence of total Silk II (summary of Silk II and Silk II-like) conformations on the
added [Mn(II)] at pH of 7.5 (a), 6.0 (b), and 5.2 (c) (From Deng et al., 2011 with permission).