A thiol group is a functional group in organic chemistry consisting of a sulfur atom bonded to a hydrogen atom (-SH). It is the sulfur analog of an alcohol group (-OH). Thiols are also known as mercaptans.

Let's analyze the given options:

Methionine: This is an amino acid with the structure containing a thioether group (-S-), not a thiol group (-SH).

Cytosine: This is a nitrogenous base (pyrimidine) found in DNA and RNA. Its structure contains amino and keto groups but no sulfur atom at all.

Cystine: This is formed from the oxidation of two cysteine molecules, creating a disulfide bond (-S-S-). It does not contain a free thiol group.

Cysteine: This is an amino acid that contains a thiol group (-SH) in its side chain. Its structure is: $HS - {CH}_{2} - CH ({NH}_{2}) - COOH$

Therefore, among the options, Cysteine is the correct answer as it is the only compound that contains a free thiol (-SH) group.

Related Topics & Formulae

Important Functional Groups in Organic Chemistry:

Thiol: -SH
Alcohol: -OH
Thioether: -S-
Disulfide: -S-S-

Key Points about Thiols:

Thiols are known for their strong, often unpleasant odors (e.g., smell of garlic).
They can be easily oxidized to form disulfides: $2 R - SH \to R - S - S - R$
Thiol groups are crucial in biological systems, especially in the structure of proteins (forming disulfide bridges that stabilize protein structure).