Saccharides

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Saccharide is a synonyme for carbohydate which we all know as sugar. The simplest form of sugar is a monosaccharide, they have the general formula \begin{align} (CH_2O)_n \hspace{2cm}n=3,4,5,6,7,8 \tag{1} \end{align} The monosaccharides exist as chains and rings. For example glucose $C_6H_{12}O_6$

Glucose Fisher to Haworth.gif

Since the formula (1) does not uniquely define the structre, different structures carry different names, depending on where the $OH$ groups sit. Switching the $OH$ group e.g. glucose can be converted into mannose or galactose, thus they are called an epimer of glucose.

$\alpha$-D-mannose
$\alpha$-D-glucose


Ther exist two different forms of Glucose, that can be easily distinguished by looking at the projection of open-chain molecules on a plane, this results in the two different Fischer projections D and L.

L-D-Glucose.jpg

They differ by switchted $OH$ and $H$ groups. Besides the states belonging to the L- and D-projection, cyclic molecules can also exist in two isomeric states that differ by the position of the $1'$ $OH$ group, whereby the carbon atoms are numbered $1',2',3', \dots$. This pair of stereoisomers are called anomers.

$\alpha$-anomer
$\beta$-anomer

Monosaccharides can react to form disacchrarides. In this condensation reaction water is produced. The same reaction can occur in the reverse direction, whereby water $H_2O$ hydrolyses the bond.

Two monosaccharides react to form a disaccharide

Depending on which $OH$ groups (blonging to $1',2',\dots$ carbon) are involved in the condensation reaction disaccharides are distinguished. Examples are

Disaccharide Unit 1 Unit 2 Bond
Sucrose (table sugar, cane sugar, beet sugar, or saccharose) Glucose Fructose α1→β2
Lactulose Galactose Fructose β(1→4)
Lactose (milk sugar) Galactose Glucose β(1→4)
Maltose Glucose Glucose α(1→4)
Trehalose Glucose Glucose α1→α1
Cellobiose Glucose Glucose β(1→4)

Also oligosacharides with two up to ten monosaccharides and polysuccharides with more than ten monosaccharides can form. Glucose for example can form long polysaccharides called glucogen (or starch in plants) that serve as energysource. The DNA doublehelix consists of a sugar-phosephate backbone built with deoxy ribose, that is essentially the monosaccheride ribose, but without an oxigen (deoxy) at the $2'$ carbon.

D-ribose (with red oxygen) and 2-deoxyribose (without red oxygen) at the $2'$ carbon