Setup

I am reporting a problem with Biopython 1.83, Python 3.11.7, and Linux-6.1.64-09049-g010fe86d9eae-x86_64-with-glibc2.36 as follows:

When running _assign_element in PDB/Atom.py, elements are not correctly assigned for atoms with 4 characters long names such as HNZ1 and HNE2 in the lysine residue.

I fixed the code and proposed it as a PR.

Expected behaviour

"H" is assigned for atoms with 4 characters long names such as HNZ1 and HNE2.

Actual behaviour

"X" is assigned for atoms with 4 characters long names such as HNZ1 and HNE2.

Steps to reproduce

To reproduce, construct Atom instance without designating elements for atoms using PDB file containing HNX(X = A-Z) group in the lysine residue.
(_assign_element is automatically run.)