Deuterium isotope effects in a - pyranose and in pyranose-furanose interconversions

Abstract

(Decem be r 15, 1966) Rates of Illutarota tion cata lytic coe ffi c ie nt s, a nd isotope e ffec ts a re re port ed for th e mut arot a ti o ns o(a-D.xylose, a-D·glu cose, 'and ,B-D·fruc tose in H 20 and in D20 at 20°C. The isotope effects (kH /k/J) for the muta rotation of ,B-D·fru c tose (a pyranose· fur anose inte rconve rs ion) parallel in striking manne r the isotope e ffect s for the mutarotation of a-D·glucose (an a-,B pyranose anomerization). For each sugar, the isoto pe e ffect is lowes t for the acid·catalyzed reactio n , a nd highest for the wate r-catalyzed reaction. Th e pa ra ll e li sm of th e valu es o bt a in ed for th e isotope e ffec ts und e r variou s co nditi ons s hows that the rat e· d e te rminin g s te ps in the tw o reac tions a re s imilar. Pres umabl y, in both in s ta nces, the ove~all muta rotati on ari ses from conc urre nt reacti ons o pe ratin g o n diffe re nt s pec ies of th e s ugar and s hoWIng s ub st a nti a ll y diffe re nt iso to pe effec ts. Th e g radu a l c hange in th e isotope e ffec t indi c at es that , und e r th e co ndition s s tudi e d , three re action s take p lace co nc urrently. The follo wing isotope e ffec ts we re fo und for the mutarotation s at 20 °C: For a-D·g1ucose, kH"o+/kn"o+ = 1.39: kH,O/ko,o = 3.87; and kB/k~ = 1.83. For ,B-D·fruc tose, kH ,o +/k'hO + = 1.39; kH,o/kD,o = 3.87; and kB/k~ = 1.92. Mechani sms are prese nted for the se ve ral conc urre nt ac id · a nd base·catalyzed mutaro· tation re ac tio ns. Key Words: Ac id-b ase cata lys is in D2 0, deut e rium isotope e ffect s, D·fru c tose, D·glu cose, isotope effec ts, mec ha ni s m of mutarotati on , mutarotat ion , pyranose·furano se int e rco nve r· s ions, s uga rs in so luti on.