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Glucose in water solution is mainly in cyclic form. Both base and acid can catalize formation of hemiacetal, but in distinct mechanisms. I found information that monosacharides eg. glucose exist mainly in cyclic form under neutral pH eg. human organism pH~7,40, but in alkaline and acidic environment chain becomes open and open chain is the form that is most abundant under this conditions. Is that true and if it is how it can be explained?

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I would be curious to know if you still have that source claiming the glucose becomes uncyclized. In any case, I asked my biochem prof your question and he said nothing really changes especially for cyclized glucose. Here's why nothing would change: the pKa for a primary alcohol is around 15. This means you're going to have to increase the pH to at least 15 if you want the alcohol to be deprotonated. Also, that pKa value only increases as you switch from primary to secondary, to tertiary alcohols (here's a site with a lot more pKa values: https://www.engineeringtoolbox.com/paraffinic-benzoic-hydroxy-dioic-acids-structure-pka-carboxylic-dissociation-constant-alcohol-phenol-d_1948.html). Recall that the majority of hydroxyl groups on glucose are secondary alcohols. In other words, you're going to be needing a solution that is so powerfully alkaline if you want to see any changes to a glucose compound. Further, it seems unlikely that increasing pH (even extreme increases) would uncyclize glucose and even more unlikely that it would uncyclize polymerized glucose. If you wanted to open a glucose ring, you're looking at something along the lines of an SN2 reaction, not merely an increase in pH.

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