At first I assumed that PDB chain identifiers are just uppercase letters (A, B, C,..) but found out there are protein chain ids that are numbers (0-9) and lowercase letters as well.
- 1NAL only has chain ids (1 - 4). It starts with chain '1' (number).
- 103L has chain identfier 'A'. It starts with chain 'A' (uppercase letter).
- 5afi's chain id's range from [a-zA-Z0-6] since it has 52 unique protein chains. It starts with chain 'a' (lowercase letter)
Can someone please explain? Or are id's just arbitrarily assigned?