i m working on making a programme, in that i need to separate ligand and receptor coordinate from pdb file via c programe , but i m unable to find a specific demarcation between coordinates of above two in file please help
Well, if you are lucky, you just need to distinguish between ATOM and HETATM lines, and then throw out water, metal ions, etc. This however only works if you have a cleanly tagged PDB file - many are not, and you can code an open-ended amount of additional program intelligence in sorting out the role of an atom from its context if all are just ATOMs.
I wonder why you need to write a new program, though. There are plenty of PDB handler applications available which already do what you need.