O=C1C[C@@H](C(=O)OCc2ccccc2)NC(=O)C[C@@H](C(=O)OCc2ccccc2)NC(=O)C[C@@H](C(=O)OCc2ccccc2)N1
The electronegative atoms usually involved in hydrogen bonding are oxygen, nitrogen and fluorine. How many hydrogen bond acceptors does the molecule contain?