Study On The Stability Of Vitamin A

There are five conjugated bonds and hydroxyl or ester group in vitamin A alcohol and its ester derivants which are sensitive to ultraviolet light, oxygen and free radical substances, which lead to reactions such as oxidation, dehydration, Diels-Alder reaction and so on. Those undesired reactions would affect vitamin A’s structure and decrease its retention rate and bioavailability. In order to isolate the vitamin A from the ultraviolet light, oxygen or free radical substances, some technologies like emulsification and embedding are used in vitamin A’s processing, storage and application. However, it is difficult to keep vitamin A structure stable even in the absence of light or oxygen. So it is important to study the reactions without light and oxygen.In this study, high performance liquid chromatography was used to keep track of contents of products with time when the all-trans vitamin A was heated into melting or dissolved in ODO and tributyrin. Isomerization and Diels-Alder reaction took place when the all-trans vitamin A was melted. Apart from isomerization and Diels-Alder reaction, dehydration occurred when the all-trans vitamin A was heated in ODO and tributyrin. Two dehydrated vitamin A isomers were detected as well as dimer isomers. There are as many as dozens of isomers because vitamin A isomers and dehydrated vitamin A isomers could react with each other.Quantum chemistry method was used to analyse structures of dimer isomers and the reaction mechanism of cyclodimerization with Gaussian software. According to the single point energy of seven vitamin A isomers under ideal condition, the stability of them are as following:all-trans isomer>13-cis isomer>9-cis isomer>9,13-cis isomer>9,11-cis isomer>11-cis isomer>11,13-cis isomer. The activation energy of vitamin A isomers transformed into dimers was calculated by using the transition state theory, which shows that the minimum activation energy occurred when the trans-vitamin A transformed into cis-vitamin A(acting as dienophile), followed by its reaction with another trans-vitamin A (acting as diene). The energy change was insvestigated in second-order perturbation when the Diels-Alder reaction took place. It was found that the two acetyl oxygen methyl groups were ortho-position in the dimmer structure. The produced six dimers could be divided into three groups according to UV analysis and these six dimers’structures were identified.