def extract_CA_coords(helices, model, verbose=False):
	"""get CA coordinates for rach residue in each helix
	input: helix list, each helix is list of its residues in format (chain, ('', resnum, ''))
	output: list of helices, each helix is list of CA coords (Biopython Vector class) of its residues
	"""

	print('Extracting CA coordinates...')
	helices_CA = []
	for helix in enumerate(helices):

		#ignore KeyError structures (i.e. those with HETATM)
		try:
			helix_CA = [ model[res[0]][res[1]]['CA'].get_vector() for res in helix[1] ]
		except KeyError:
			return KeyError

		if verbose:
			print('CA COORDS: HELIX ' + str(helix[0]))
			for h in helix_CA:
				print(h)
		helices_CA.append(helix_CA)

	return helices_CA