The letters ' $D$ ' or ' $L$ ' before the name of a stereoisomer of a compound indicate the correlation of configuration of that particular stereoisomer. This refers to their relation with one of the isomers of glyceraldehyde. Predict whether the following compound has 'D' or 'L' configuration.
Here, OH group present on second last carbon in at left side hence this has L configuration.
Aldopentoses named as ribose and 2-deoxyribose are found in nucleic acids. What is their relative configuration?
In case of cyclic structure of saccharide, if $-$OH group present at second last carbon is present at bottom side, then it is considered as D configuration (as shown above)
Which sugar is called invert sugar? Why is it called so?
Sucrose is dextrorotatory but sucrose on hydrolysis gives dextrorotatory glucose and laevorotatory fructose. Hence, mixture becomes laevorotatory.
This sugar which on hydrolysis changes its sign of rotation from dextro to laevo is known as invert sugar.
Amino acids can be classified as $\alpha-, \beta-, \gamma-, \delta$ - and so on depending upon the relative position of amino group with respect to carboxyl group. Which type of amino acids form polypeptide chain in proteins?
$\alpha$ amino acid forms polypeptide chain by elimination of water molecules.
$\alpha$-helix is a secondary structure of proteins formed by twisting of polypeptide chain into right handed screw like structure. Which type of interactions are responsible for making the $\alpha$-helix structure stable?
$\alpha$-helix is a secondary structure of proteins formed by twisting of polypeptide chain to right handed screw like structure. Hydrogen bonds formed between - NH- group of amino acids in one turn with the 
groups of amino acids belonging to adjacent turn is responsible for making the $\alpha$-helix structure stable.