| Summary: | 碩士 === 國立成功大學 === 化學系 === 103 === The electron potential plots are informative. Firstly, the most acidic enolic proton of curcumin is buried in the negatively charged electric field resulting from lone-paired electrons on the two oxygen atoms. Curcumin (CCM) can thus chelate the attracted metal cations through its conjugate base, CCM anion, and lead to the coexistence of free CCM and metal-binded CCM. This conclusion is consistent with the reported NMR data. Furthermore, this prevents the acid-base interaction between selenite and enolic hydrogen of CCM. This is also true for the phenolic hydrogen. On the other hand, the positively charged hydrogen atoms, including sp2-tpye hydrogen atoms and methyl hydrogen atoms, can form H-bonding with the attracted selenite oxygen atom(s). The phenolic hydrogen at both ends of CCM is protected by intramolecular H-bonding and, therefore, allow the methyl hydrogen atom to form improper (or un-conventional) H-bonding with selenite. In the closely contacted solid state, this phenolic hydrogen is forced to form H-bonding with the oxygen atom of selenite. This explains the intensity enhancement of the O-H stretching peak in IR spectra.
In the didemethyl CCM (DCCM), one of the two adjacent hydroxyl groups (in each end of DCCM molecule) reveal acidic nature. This enables the acid-base interaction of DCCM and selenite to generate more stabilized adduct as compared to CCM. Unlike CCM, furthermore, one DCCM molecule can bind two selenites through each end of phenolic fragment. This explains that DCCM exhibited (1) better activity than CCM observed in preventing selenite induced lens crystallin aggregation and (2) larger binding affinity than CCM observed by ITC experiments. The values of soichiometry, 1.22 for CCM and 1.90 for DCCM are also understood through the senelie-binding mechanisms: acid-base interaction for DCCM and H-bonding for CCM.
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