9128

646 دكتري

دكتري

شيمي آلي

اصفهان: دانشگاه صنعتي اصفهان، دانشكده شيمي

1393

پانزده،149ص.: مصور،جدول،نمودار

ص.ع.به فارسي و انگليسي

عبدالحسين دباغ

حسين فرخ پور

25/4/93

حسين لقماني، حسين توكل

شيمي

ID646 دكتري

153 Degradation of vitamin C in the presence of water and ZnO nanoparticles and computational study of isomerization Fatemeh Azami shahnaz azami@yahoo com f azami@ch iut ac ir May 20 2014 Department of Chemistry Isfahan University of Technology Isfahan 84156 83111 I R Iran Supervisor Prof Hossein A Dabbagh Email dabbagh@cc iut ac ir Advisor Assist Prof Hossein Farrokhpour Isfahan University of Technology 84156 83111 Isfahan Iran Isfahan University of Technology 84156 83111 Isfahan Iran Abstract The tautomers of four RS stereosiomers of ascorbic acid were investigated at B3LYP 6 311 G d p and MP2 6 311 G d p levels Calculated equilibrium constants of the selected isomerspredicted possible isomerization of tautomers which is the major cause of racemization Experimentally specific rotation of vitamin C were monitored in water at room temperature and at 80 90 oC during 122 days Optical activity of L ascorbic acid was reduced to zero after 6 days in water at 80 90 oC The optimizedgeometries and calculated vibrational frequencies were evaluated The latter was compare with those ofexperimental values The max values of L ascorbic acid were found 259 about pH 3 experimentally 238and 247 nm calculated at TD DFT methods respectively The structures stabilities conformational analysisand electronic transitions of L ascorbic acid anions four stereoisomers were studied theoretically Theseanions are produced from the de rotonation of C4 H and C5 H sites of L ascorbic acid stereoisomers It wasobserved that the de protonation at C5 site of two stereoisomers leads to the ring opening in bothphases Isomerization of the L form to one of the D form was observed during the optimization of the anionsat C5 Conformational analysis potential energy surface scan of the opened ring anions was performed insearch of energy minima and or maxima The absorption electronic transitions of the anions in the UV regionwere calculated using Time Dependent Density Functional TD DFT Key words Vitamin C Tautomers Anion Stereoisomers Optical activity Introduction Generally AA is used in pharmaceutical chemical cosmetic and food industry due toits bioactivity and as antioxidant 1 The lack of ascorbic acid leads to scurvy 2 VitaminC has been demonstrated to kill HIV positive cells and help HIV positive individuals byincreasing the immune system 3 Generally ascorbic acid is known as sugar acid lactone and ene diol As a weak di acid pKa1 4 25 and pKa2 11 79 monoanion is formedat pH 4 5 with deprotonation of O3 and dianion is formed at pH 11 12 with deprotonationof O2 The monoanionic form is more stable due to delocalization of the negative chargebetween oxygen atoms at 1st and 3rd positions 4 Y Dimitrova studied the hydrogen bonded system formed between L ascorbic acid and five water molecules 5 R A Yadavet al studied the neutral L ascorbic acid molecule and its cation and anion 6 J R Juhaszet al studied the effects of conformation on the acidity of L ascorbic acid 7 I Georgievaet al reported DFT study of four low energy neutral four anion and two dianionconformers of AA 8 Recently Dabbagh et al investigated four anions of AAstereoisomers theoretically They revealed that the lactone ring of L isomer 4R 5S andD isomer 4S 5R was opened during optimization of the geometry in vacuo and aqueous

عبدالحسين دباغ

حسين فرخ پور

حسين لقماني، حسين توكل