Mass Spectrometry Of Thermal Transformations
Mass spectrometry of thermal transformations
of oligosaccharides in condensed state and adsorbed by the surface of ultrafine silica
T.V. Kulik, N.P. Galagan, V.A. Pokrovskiy
Institute of Surface Chemistry, Ukrainian National Academy of Sciences, General Naumov st. 17, 03164 Kiev, Ukraine
e-mail: borbor@gala.net
Ultrafine silica (UFS) is a promising carrier for producing and stabilizing the medical forms of plant preparations, the active components of which are preferentially glycosides. Adsorption of biomolecules on the UFS surface may effect the biological activity of final medical preparation. It is known [1] that adsorption of flavonoids on the silica surface takes place through glycone, the carbohydrate fragment which is present in the structure of glycoside. That is why we were interested in effect of binding of the oligosaccarides by UFS on their surface, on thermal stability and mechanism of thermal transformations. In our previous studies we have already investigated main parameters of their adsorption on hydroxilated and modified silica surface [2] and found out the peculiarities of thermal decomposition of oligosaccarides in condensed state [3]. But mechanisms of carbohydrate decomposition in adsorbed state on the silica surface were not studied yet.
The species of oligosaccharides adsorbed by UFS surface from 10 % aquatic solutions of carbohydrares lactose, tregalose, raffinose were synthesised. Those species have been studied by means of temperature-programmed desorption with mass spectrometric registration of volatile products.
It was found out previously that during the thermal decomposition of mono- and oligosaccharides including hexacyclic pyranose ring, the molecule undergoes elimination of three water molecules from one pyranose cycle resulting in elimination of substituted γ–pyrone in molecular form. Necessary attention was not paid previously to the mechanism of its formation. We assume that the process of γ–pyrone formation from glucose molecule may occur as a series of three consequent reactions. At the first stage, alcohol dehydratation takes place resulting in elimination of two water molecules.
Each elimination occurs in the framework of mechanism E1and is catalysed by acids. Carben cation is an intermediate product of these reactions. The third water molecule is eliminated in accordance with mechanism of pinacoline rearrangement. It is probable that in conditions of thermal desorption experiment (high temperature, absence of oxygen) all those reactions may occur as monomolecular thermal reactions of elimination. Such reactions, as a rule, take place in coordination and it was proved that they may be guided by orbital symmetry [4]. For cyclic systems the most important factors predetermining the composition of reaction products are conformation effects and steric requirements of cyclic transition state [4]. In particular, water elimination is promoted by formation of conjugated double bonds. Thus, it is probable that elimination of three water molecules from pyranose cycle takes place by coordinated mechanism through pseudo-aromatic transition state with low activation energy.
Analysis of mass spectrometric information obtained in the framework of temperature-programmed desorption experiment has shown that thermolysis of adsorbed oligosaccarides is also accompanied by formation of γ–pyrone. In other words, the same reactions take place on silica surface as it was observed in the process of thermolysis in condenced state. But it should be noted that the form of thermal desorption curves for adsorbed species is much more complicated. Besides, the thermal stability of adsorbed species decreased as a result of adsorption on UFS surface. This effect is undoubtedly caused by interaction of saccaride molecules with UFS surface active sites. It means that active sites of silica possessing proton-donor and dehydroxillating properties, promote (catalyse) the reaction of water β-elimination. The initial stage of thermal decomposition of lactose in adsorbed state is displaced approximately by 100° С to low temperatures in comparison with condensed state, and displacement is about 70° С for tregalose and raffinose. Among species under investigation, lactose is expected to be the most unstable, for it includes free glycoside hydroxile and should be classified as reducting oligosaccaride. Tregalose and raffinose do not posess free glycoside hydroxils and should be classified as non-reducting oligosaccharides which may be the reason of their higher thermal stability in adsorbed state, in comparison with lactose.
References
1. Kulik T.V., Galagan N.P., Pogoreliy V.K., Pokrovskiy V.A. TPD MS studies of several polyketides in solid state and adsorbed on silica surface.//15th International Mass Spectrometry Conference, Barselona, Aug. 28-Sept.1., 2000; Book Abstr. P.105.
2. Galagan N.P., Kulik T.V., Dembnovetskaya L.I. Adsorption of some carbohydrates on ultrafine silica surface. Theoretical and experimental studies of interfacial phenomena and technological applications: Collection of abstracts / SCSEIO, Odessa, 2000.-P.54.
3. Кулик Т.В. Мас-спектрометрія вуглеводних фрагментів – термінальних груп рецепторних молекул в адсорбованому на поверхні високодисперсного кремнезему та конденсованому станах // Дисертація … канд.хім.наук. – Київ, 2000.-135 с.
4. Кери Ф., Сандберг Р. Углубленный курс органической химии: Под ред. В.М. Потапова. Книга первая. Структура и механизмы.-– М.: Химия, 1981. – 520 с.
Related articles::