( First ( , ( ( the ( six-sided rings ) ) ( ( can ( include ( elements ( besides ( ( ( carbon , ) ( most commonly ) ) ( ( nitrogen and ) sulfur ) ) ) ) ) ) . ) ) ) )
(ROOT (S (ADVP (RB First)) (, ,) (NP (DT the) (JJ six-sided) (NNS rings)) (VP (MD can) (VP (VB include) (NP (NP (NNS elements)) (PP (IN besides) (NP (NP (NN carbon)) (, ,) (ADVP (RBS most) (RB commonly)) (NP (NN nitrogen) (CC and) (NN sulfur))))))) (. .)))
First, the six-sided rings can include elements besides carbon, most commonly nitrogen and sulfur.
(ROOT (S (NP (NNP Carbon)) (VP (VBZ is) (NP (NP (DT the) (ADJP (RBS most) (VBN found)) (NN element)) (PP (IN in) (NP (VBG living) (NNS organisms))))) (. .)))