Based on an American proposal from the previous year, the 2010 Turkey-Brazil proposal would have allowed Iran to send large quantities of its low-enriched uranium to Turkey to be refined to medical reactor-grade purity, for the production of medical isotopes but not the development of a nuclear weapon.
It was a good idea in principle, and was too easily dismissed by the P5+1 negotiators when it was brought back to the table by a joint effort by Turkey and Brazil. One of the key reasons the Americans mentioned for rejecting the deal was that numbers for the amount of enriched uranium to be transferred out that were discussed in 2009 were already obsolete by 2010, as Iran had continued enriching uranium over the course of the negotiations.
As negotiators approach talks this time around, the specifics of an updated proposal along the lines of the Turkey-Brazil deal would have to be reworked, but the original approach was sound, and it is worth another try.
Nuclear negotiations with Iran have been consistently undermined by the adversarial relationship between Iran and the West. Decades of mutual distrust have made it nearly impossible to broker an agreement. The Turkey-Brazil negotiation circumvented this tension, and Turkish and Brazilian diplomats leveraged their warm relations with Iran, the United States, and European countries and their own status as rising powers in the international community to present themselves as honest brokers.
The need for credible intermediaries has not abated in the past year and a half. In fact, if there is any hope that a deal can still be reached, the chances of success will be best if both Iran and the P5+1 can work with intermediary countries that are trusted on both sides.