# What does the following diagram represent?

Suppose, the following is a diagram of a protein's polypeptide chain:

What does this diagram represent?

1. What are the letters A, E, M, W, L, N, S, etc. represent? (I suppose these are amino acid codes. Right?)
2. Which one is the c-alpha atom here?
3. What are the sequence numbers i-2, i-1, i, i+1, i+2, etc. represent? Why there are negative and positive increments from i?

The following is a related diagram:

• Note the schematic is from chemistry.stackexchange.com/questions/153935/… Aug 23 '21 at 13:27
• What is the origin of the diagram? Aug 23 '21 at 14:08
• @BuckThorn, some book. i don't know the exact source. Aug 23 '21 at 16:08