def rxn_query_str(reactant, product, rxn_id):
    """
    Generate cypher MERGE query for reactant and product node.
    """
    return "MERGE (r: Molecule {smiles_str:\""+ reactant +"\"}) MERGE (p: Molecule {smiles_str:\""+ product +"\"}) MERGE (r)-[:FORMS {rxn_id: "+ str(rxn_id) +"}]->(p)"