Returns the sequence of the `Polymer` as a string.

        Returns
        -------
        sequence : str
            String of the `Residue` sequence of the `Polypeptide`.

def sequence(self):
        """Returns the sequence of the `Polymer` as a string.

        Returns
        -------
        sequence : str
            String of the `Residue` sequence of the `Polypeptide`.
        """
        seq = [x.mol_letter for x in self._monomers]
        return ''.join(seq)