Chair–chair ring inversion of d-fructose coupled to the mutarotation in aqueous-ethanolic solutions

Ultrasonic absorption spectra have been measured in the frequency range from 40 kHz to 2 GHz for solutions of d-fructose in water and in two water–ethanol mixtures as well. It is found that a low frequency relaxation with relaxation time around 1 μs exists in the aqueous solution. The amplitude of that relaxation term is considerably smaller in the aqueous-ethanolic systems, in conformity with a reduction of the content of the α-d-fructopyranose tautomer. Our measurements confirm the assignment of the low frequency acoustical relaxation to a 2C5⇌5C2 ring inversion of α-fructopyranose in solution.