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The Beta carbon of serine. In general, the beta carbon is the first "nub" of the sidechain. (EnzDes coloring.)

The beta carbon (β-carbon or Cβ) is the first atom of the sidechain in an amino acid.

In the "cartoon" protein views in Foldit, the beta carbon is the first "nub" of the sidechain.

When the "show stubs" view of sidechains is selected, all sidechains are shown as just a beta carbon. (The sidechains expand when you hover the mouse pointer over the stub.)

Glycine doesn't have a sidechain, which is what allows glycine to be the most flexible amino acid.

Alanine has a sidechain which contains only a beta carbon.

Some Foldit recipes offer a "band beta carbons" option. Banding the beta carbons may allow the protein to move more freely than banding the backbone.

The beta carbon is normally atom 5 of a segment. On the last segment, or C terminal of a protein chain, the beta carbon is atom 6.

In contrast, the alpha carbon (α-carbon) is part of the backbone. The alpha carbon is atom 2 in all segments. In a recipe, creating a band without specifying the atom number bands to the alpha carbon.