Finally the black rook reaches the open e file.

At first I was thinking about Qa5 looking at the dark squares when I realised that after Qa8+ the King can only go to c7. Because the pawns on b7 and c6 occupy potential escape squares for the king it would be mate if the Queen can reach d8. This move protects d1 to allow the rook force the queen off the d file and set up the mate on d8.

Black frees another square for his king in the event of 27.Qa8+ but the move does not defend the Rd1 threat.

27 ... Qxd1 28. Qxd1 stops the mate leaves black hopelessly behind on material.

Mate as visualized when playing Qa4. White could have retained greater control by more accurate opening play, but black did not see a series of double attacks and pins in the early middle game that led to material disadvantage.