85 minutes seems like enough time to me. I did that two years ago, where I was scheduled to arrive in FRA at 4 pm and depart FRA at 5:25 pm. The only challenge was that my IST-FRA flight was 50 minutes late, and we were parked out on the tarmac, then it took a mad rush to get to my new gate...and I made it just fine. I was one of the last people on the flight, but easily had a few minutes to spare.
The issue would be, what's the downside for you in case the IST-FRA flight is, say, 90 minutes delayed? It's unlikely, but sometimes happens.
Unless I absolutely, positively, had to be in the office the next day, I'd still be willing to risk the connection.