This map is from a BBC news article on the German elections which notes that:
the former route of the Berlin Wall divides the city into voting choices. In the constituencies of the East, voters chose Die Linke (The Left party), descended from the old communist party.
In the West, they voted for the Social Democrats and the Christian Democrats (CDU), both formerly West German parties.
In a few locales in the centre of Berlin, on either side of what was the Wall, the Greens came out on top – and closer examination reveals these to be areas which have been gentrified heavily, with large numbers of young, professional incomers.
The author suggests this is because:
There was no great cross-border migration in the city after 1989. People had security of tenure in their flats, and they stayed put. Berlin had a large concentration of members of the Socialist Unity Party (as the communist party in East Germany was called), as well as the civil servants and Stasi operatives who kept the communist state running, and they have remained in their areas and transferred their loyalty to Die Linke.
Though he also notes that in general the CDU performed reasonably well in the old GDR perhaps because many voters their have a personal loyalty to Ms Merkel, who also grew up in East Germany.