(a) Glucose with HI
On prolonged heating with hydrogen iodide, glucose is completely reduced to n-hexane :
$$C_6H_{12}O_6 \xrightarrow[\Delta,\ \text{long time}]{HI} CH_3-CH_2-CH_2-CH_2-CH_2-CH_3$$
What this proves : all six carbon atoms of glucose lie in a straight, unbranched chain. Every oxygen-containing group — the aldehyde and all five hydroxyls — is stripped away by this drastic reduction, leaving only the carbon skeleton.
This was one of the key pieces of evidence used in establishing the open-chain structure of glucose.
(b) Bonds holding the DNA double helix together
Hydrogen bonds between the complementary nitrogenous bases on the two strands.
The pairing is strictly specific :
- Adenine pairs with Thymine through two hydrogen bonds $(A = T)$
- Guanine pairs with Cytosine through three hydrogen bonds $(G \equiv C)$
Why this matters : because the pairing is fixed, the sequence of one strand automatically determines the sequence of the other. The two strands are said to be complementary, and this is what makes accurate self-replication of DNA possible.
(The sugar-phosphate backbone within each individual strand, by contrast, is held by strong covalent phosphodiester linkages.)
