In fact, du is not an interrogative word, but the third person singular form of the negative emphatic conjugation, which expresses general negation.
in questions containing an interrogative word ending in -an (wh questions).
An intonational contour combining a high target on the sentence-initial interrogative word and a final H% interrogative boundary tone is what confers an interrogative sense on such sentences, distinguishing questions from statements (which have a final low L% boundary tone).
The linkage of morphosyntax and intonation is also apparent from the fact that no intonational substructuring can split the constituent extending from the interrogative word to the end of the sentence, whatever its length, since the final boundary tone confers an interrogative sense on the initial morpheme, and the contour as a whole shows that the scope of this morpheme is the entire sentence.
Even in this case, however, there can be no subdivision between the interrogative word and the final edge L tone (cf.