Why is there a carbon atom without a greek letter in protein residues?

Question

For protein residues I know that $C\alpha$ denotes the first carbon atom attached to a functional group. Each subsequent carbon is given a corresponding greek letter ($\beta, \gamma, \delta$) except for one. For example, each residue in a PDB file has a list of atoms that looks something like this:

• ATOM 32 N AARG
• ATOM 33 CA AARG
• ATOM 34 C AARG
• ATOM 35 O AARG
• ATOM 36 CB AARG
• ATOM 37 CG AARG
• ATOM 38 CD AARG

(A,B,G,D are stand-ins for the greek letters). Why is there just a plain C in this list? What is significant about that carbon?

Answer 1

In your context, the "plain-C" labelled carbon is the carbon atom in the carbonyl ($\ce{C=O}$) group. In organic chemistry, the alpha carbon is the carbon immediately adjacent to a functional group. In general, the greek letters in alphabetical order denote the distance in increasing order from a functional group. The $\ce{C_\alpha}$ in the case of amino acids is alpha carbon with respect to both the amine and carbonyl.