+aa_3letter = [
+ 'DPR','DLY','DAR','DHI','DAS','DGL','DSG','DGN','DSN','DTH',
+ 'DAL','DTY','DTR','DVA','DLE','DIL','DPN','MED','DCY',
+ 'CYS','MET','PHE','ILE','LEU','VAL','TRP','TYR','ALA','GLY','THR',
+ 'SER','GLN','ASN','GLU','ASP','HIS','ARG','LYS','PRO',
+ 'SME','AIB','ABU','DBZ']
+
+
+def pdb_code_chain(pdbid):
+
+ msg=''
+ chain=''
+ if len(pdbid)>4:
+ if pdbid[4]!=':':
+ return('use : between pdb code and chain id, like 5G3Q:B to select single chain')
+ chain=pdbid[5]
+ pdbid=pdbid[:4]
+
+ test=urllib.urlopen('http://files.rcsb.org/download/'+pdbid+'.pdb')
+ if test.code != 200:
+ msg = 'wrong pdb code'
+ else:
+ msg=pdb_missing_res_chain(test,chain)
+ test.close()
+ return(msg)
+
+def pdb_missing_res_chain(file,chain):