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Fe3 / Fe2 , and pH value on two of silk fibroin aggregation structure

Author: JiDan
Tutor: ZhouPing
School: Fudan University
Course: Polymer Chemistry and Physics
Keywords: Silk fibroin Iron / ferrous ion pH Solid-state NMR Electron paramagnetic resonance Raman spectroscopy
CLC: R318.08
Type: Master's thesis
Year: 2009
Downloads: 15
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Abstract


Silk fiber is attractive due to its outstanding mechanical properties as well as the potential in biomaterial and medical applications. Silk fibroin is thought to be the supplier for the excellent performance of silk fiber and the sequence and secondary structure directly determines the properties. There are three components of silk fibroin: heavy chain（391KD）, light chain（28KD） and P25（25KD） and heavy chain is the most important that accont for around 80wt% of silk fibroin. Motif GAGAGS and GA（V）GAGY appears repeatedly in the crystal（semi-crystal） region while motif GTGSSGFGPYVAN（H）GGYSGYEYAWSSESDFGT is located in ’link’ region as a loop structure. The crystal region contains mainly two structural components: Silk I （helix） and Silk II （β-sheet）. Under influence such as shearing force, pH values and metal ions etc. Silk I could transform to Silk II structure. We have already studied systematically the effects of pH values, Cu2+, Zn2+, Ca2+, K+, Na+ on the conformation of silk fibroin but there are few reports about the interaction between iron and silk fibroin. Therefore, we pay attention to iron ions which has not been studied deeply before and both ferric and ferrous ions are concerned. The content of iron in silk gland is 1.8±0.8μg/g while in cocoon is 6.1±0.3μg/g. It is evident cocoon contains more iron than silk gland which suggests that iron might play a role in the fibrosis process of silk fibroin.Furthermore, the fibrosis process is not peculiar since the symptom and pathogeny of many neurodegenerated disease such as Alzheimer and Parkinson diseases prove to be related. The overload of iron ions are observed in these diseases. However, the basic reasons are still unclear now. And the transform between ferric and ferrous ions through Fenton reaction makes the problem even more complicated. All of these neurodegenerated diseases share a common feature, that is, proteins involved originally with a soluble form turn to an insoluble form and the latter contain abundantβ-sheet structures. It is quite similar to the folding process of silk fibroin so that we took silk fibroin as model protein to study its refolding process since it is more stable and facile.With the powerful tools like solid state NMR, EPR and Raman spectroscopy, we have investigated the refolding process of silk fibroin under the influence of ferric and ferrous ions. NMR and Raman results show that difference between ferric and ferrous systems is quite obvious since the ferric ions make considerable promotion on the conformational changes of silk fibroin while ferrous ions do not. EPR results indicate ferric ions bind with silk fibroin in a disordered octahedral form at high-spin state （S=5/2） and the ’link’ region, i.e., motif GTGSSGFGPYVAN（H）GGYSGYEYAWSS ESDFGT is very likely to be involved. And histidine and tyrosine residues are considered to be the possible binding sites.

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