 He grimaced at his own doubts.
( He ( ( grimaced ( at ( his ( own doubts ) ) ) ) . ) )
(ROOT (S (NP (PRP He)) (VP (VBD grimaced) (PP (IN at) (NP (PRP$ his) (JJ own) (NNS doubts)))) (. .)))
He had no doubts. 
( He ( ( had ( no doubts ) ) . ) )
(ROOT (S (NP (PRP He)) (VP (VBD had) (NP (DT no) (NNS doubts))) (. .)))