Reducing sugars and redox potential

I am wondering if reducing sugars are those that can act as reducing agents, and they act as such with Tollens and Fehling reaction, then what's the redox potential here? And is it a reversible reaction like with for example redox cofactors, or once the sugar is reduced it cannot go back?




It is known that the reducing cofactor of dopamine-$\beta$-monooxigenase is a sugar, L-ascorbic acid. That means, the oxidized dehydroascorbate has to be reduced back to ascorbate for this system to work. Thus, it is safe to say oxidation of reducing sugar is reversible process depending on the conditions used.


Nonetheless, although all redox half-reaction systems are shown reversible, the reversibility is based on the conditions. For example, it is shown that the redox reduction half-reaction of silver is reversible:
$$\ce{Ag+ + e <=> Ag } \qquad E^\circ = \pu{0.7996 V} \tag1$$
It doesn't mean all the condition this reaction is reversible. For instance, suppose you used a weaker reducing agent (stronger oxidizing agent) than silver:


$$\ce{Au+ + e <=> Au } \qquad E^\circ = \pu{1.692 V} \tag2$$


Then, the dominant reaction will be the backward reaction as shown in the equation $(4)$:
$$\ce{Au+ + e <=> Au } \qquad E^\circ = \pu{1.692 V} \tag2$$
$$\ce{Ag <=> Ag+ + e } \qquad E^\circ = \pu{-0.7996 V} \tag3$$
$$\ce{Ag + Au+ <=> Ag+ + Au } \qquad E^\circ\_\mathrm{rxn} = \pu{0.8924 V} \tag4$$


On the other hand, suppose you have used a stronger reducing agent (weaker oxidizing agent) than silver such as [glucose](http://butane.chem.uiuc.edu/pshapley/genchem2/b9/2.html):
$$\ce{glucose + H2O <=> gluconic acid + 2H+ + 2e} \qquad E^\circ = \pu{-0.050 V} \tag5$$


Then, the dominant reaction would be the forward reaction as shown in the equation $(6)$:


$$\ce{2Ag+ + glucose + H2O <=> 2Ag + gluconic acid + 2H+} \qquad E^\circ\_\mathrm{rxn} = \pu{0.7496 V} \tag6$$


As shown for the silver, gluconic acid can be reduced to glucose (Ref.1) similar to glucose been oxidized to gluconic acid.


**References:**


1. J. W. E. Glattfeld and Edna H. Shaver, "The catalytic reduction of *d*-gluconic acid to *d*-glucose," *J. Am. Chem. Soc.* **1927**, *49(9)*, 2305–2308 (DOI: <https://doi.org/10.1021/ja01408a026>).


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