Why is cysteine considered polar?

Answered by Stephen Mosley

Cysteine is considered polar because of the presence of a sulfur-hydrogen (S-H) bond in its side chain. This S-H bond is a polar covalent bond due to the difference in electronegativity between sulfur and hydrogen.

Electronegativity is a measure of an atom’s ability to attract electrons towards itself in a chemical bond. Sulfur has an electronegativity of 2.58, while hydrogen has an electronegativity of 2.2. The difference in electronegativity between sulfur and hydrogen is 0.38 (2.58 – 2.2), which is relatively small.

A polar covalent bond is formed when there is a difference in electronegativity between the atoms involved in the bond. In the case of the S-H bond in cysteine, the sulfur atom has a slightly higher electronegativity than the hydrogen atom. This causes the sulfur atom to attract the shared electrons in the bond more strongly, creating a partial negative charge on the sulfur atom and a partial positive charge on the hydrogen atom.

As a result, the S-H bond in cysteine has a dipole moment, with the sulfur end being slightly negative and the hydrogen end being slightly positive. This creates a polarity in the cysteine molecule, making it polar overall.

The polarity of cysteine’s side chain has important implications for its role in protein structure and function. The polar nature of cysteine allows it to form disulfide bonds with other cysteine residues in proteins, contributing to the stabilization of protein structures. Disulfide bonds are formed when two cysteine residues undergo an oxidation reaction, resulting in the formation of a covalent bond between their sulfur atoms.

In addition, the polar nature of cysteine’s side chain allows it to participate in various interactions within proteins and with other molecules. The partial positive charge on the hydrogen end of the S-H bond can form hydrogen bonds with other polar molecules or functional groups, while the partial negative charge on the sulfur end can interact with positively charged ions or polar molecules.

The polar nature of cysteine’s side chain, primarily due to the S-H bond, plays a crucial role in the structure, stability, and function of proteins. Understanding the polarity of cysteine is important in studying and manipulating protein systems, as well as in understanding the chemical properties and behavior of this amino acid.