Page 40 - PG 504-theoretical notes-phyto-1-2024-2025..

Clinical pharmacy PharmD program Third level Phytochemistry-1 (PG-504)

Ring structure of glucose

Necessity of ring structure: The open chain structure of glucose could not

explain the majority of its properties:

1) IR and UV examination of glucose indicates absence of a carbonyl-group.

2) Glucose does not give a red color with Schiff’s reagent. This indicates that

the aldehydo-group is in some way masked.

3) Mutarotation: It can be defined as the change of optical rotation with time.

Glucose exists in two forms:

1) - Form, specific rotation = + 113 o.

2) - Form, specific rotation = + 19 .
o

In aqueous solutions, the specific rotation of a freshly prepared -form

o
o
(+113 ), decreases with time until it reaches a constant value (+ 52 ). Similarly
o
specific rotation of -form (+ 19 ) increases with time until it reaches to same
o
equilibrium value (+ 52 )

+ 113 o + 52 o + 19 o

Mutarotation could not be explained by the open chain structure.

Cyclization takes place when the hydroxyl groups of C 4 or C 5 are in close

position with the carbonyl group, whereby an intramolecular nucleophilic
attack of the OH to the aldehydic group of the same molecule takes place with

the formation of an internal hemiacetal. As the union occurs through oxygen,

when it is (1→ 5), a six membered ring termed “pyranose ring” is formed, while

(1 → 4) union gives five membered ring known as furanose ring. These

formulae were created by Haworth and known as “Haworth formulae”.

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