Invoking trigger when an answer is marked as Best Answer in Chatter

I am trying to invoke a trigger when an answer is marked as 'Best Answer' in Chatter Question. I tried a trigger on FeedItem and FeedComment objects, but the triggers were not invoked when I mark an answer as a 'Best Answer'. I am working on Chatter Questions and even tried creating a trigger on Question object, but get an error Invalid type: Schema.Question while trying to save. Can someone help?