This property lists the entities that participate in this interaction. For example, in a biochemical reaction, the participants are the union of the reactants and the products of the reaction. This property has a number of sub-properties, such as LEFT and RIGHT in the biochemicalInteraction class. Any participant listed in a sub-property will automatically be assumed to also be in PARTICIPANTS by a number of software systems, including Protégé, so this property should not contain any instances if there are instances contained in a sub-property.
Source:http://www.biopax.org/release/biopax-level2.owl#PARTICIPANTS