They were brought together on what looked like mission impossible to seal a landmark deal with Iran on its suspect nuclear programme.

Despite their different outlooks, backgrounds and personalities, these globe-trotting diplomats admit they have spent more time locked in negotiations in recent months than with their families.

On a final day of talks in Switzerland before a midnight deadline to agree the outlines of a potentially historic deal, here are pen portraits of some of the major players:

US Secretary of State John Kerry

The lanky 71-year-old with a shock of grey hair has become an instantly recognisable figure during the 18 months that the talks have toured the globe. During breaks from negotiations, he has often headed out of his hotel for walks or bike rides, accompanied by his posse of security guards. Reaching the long-elusive deal is the pinnacle of Kerry's long career, which saw him make a tilt for the White House in 2004. He moved into the State Department in 2013, joking he was stepping into the "heels" of Hillary Clinton after serving 29 years as a senator for Massachusetts. The son of a diplomat, Kerry spent much of his childhood in Europe, speaks French fluently and a smattering of German and Italian.

Iranian Foreign Minister Mohammad Javad Zarif

After the rather austere image of Iranian leaders shown to the world in past years, the US-educated Zarif has become the smiling face of the new leadership of President Hassan Rouhani. Zarif, 55, is fluent in English and was the first minister to have official accounts on Facebook and Twitter, both banned in Iran. A veteran loyalist of the Islamic revolution that toppled Iran's US-backed monarchy in 1979, he believes in the country's pursuit of nuclear power. But his moderate views have clashed with ultraconservative factions in the regime.

Former EU High Representative Catherine Ashton

A British life peer with the title of Baroness Ashton of Upholland, Ashton rose from obscurity in 2009 to become the EU's foreign policy chief having made a virtue of not being an "ego on legs." While critics initially questioned whether she had the clout to do the job, she won admiration for her deft handling of years of dragging Iran nuclear talks among other burning issues. A former top treasurer with the lobbying group the Campaign for Nuclear Disarmament, she has described her style as "quiet diplomacy." Ashton, 59, stepped down in late 2014.

French Foreign Minister Laurent Fabius

Fabius, 68, has revelled in his role as the most hawkish member of the P5+1 powers negotiating with Iran. The tall, elegant veteran politician has managed on several occasions to throw a spanner in the works, notably in November 2013 when he arrived hot on Kerry's heels at Geneva talks publicly slamming a deal on the table as too weak. Diplomatic sources say he was marked by "disastrous" past experiences with Iran. While he was prime minister from 1984-1986, France was rocked by a series of bomb attacks blamed on Tehran and the kidnapping of French diplomats in Beirut by Iran-backed Hezbollah militia.

Russian Foreign Minister Sergei Lavrov

The wily Lavrov, known for his inscrutable face, has steered Russian diplomacy for more than a decade. He embodies the geopolitical muscle and defiance his country often exercises in the UN Security Council. The chain smoker, who served for years as Russia's envoy to the UN before becoming foreign minister, petitioned against the smoking ban in UN buildings. Lavrov enjoys Scottish whisky and is not immune to wry irony, setting him apart from the humourless apparatchik style of other Russian leaders. With his strong personality, he toes Moscow's line. He had developed strong working ties with Kerry, but that has soured in recent months amid the Ukraine crisis. The American has even openly accused Russian officials of lying to his face.

Chief US negotiator Wendy Sherman

Once described as a diplomat with "superhuman abilities," Sherman has impressed with her steely, yet elegant manner over the years of negotiations. She has become steeped in the complex technicalities of the nuclear industry including proliferation and weaponisation having also served as the special coordinator for North Korea policy from 1997 to 2001. Sherman even organised a landmark trip to the reclusive state by then secretary of state Madeleine Albright. A Democratic Party stalwart, Sherman, who has said she wakes up optimistic every day, has gained a reputation as an indefatigable negotiator with a razor-sharp intellect. But even her immense wells of patience had begun to run dry -- after long weeks in Vienna she confessed to growing tired of schnitzel -- as well as suffering "a twisted ankle, a broken nose, and a ruptured pinkie finger."

Chinese head of arms control department Wang Qun

China's point man in the talks, Wang is doubtless a serious negotiator behind closed doors. But for long-suffering journalists who have been covering the talks he has provided some light relief with his impressive collection of bow ties. "I have an arsenal of them," Wang, cutting a dashing figure of diplomatic sophistication, quipped to reporters in Lausanne. "The IAEA is more than welcome to come and inspect them."

What an Iran nuclear deal could look like
Lausanne (AFP) March 31, 2015 - Global powers were racing Tuesday to reach the outlines of a deal to stop any Iranian bid to develop nuclear weapons.

The "framework" accord is meant to be fleshed out into a comprehensive agreement packed with complicated technical annexes by June 30 to end more than a decade of tensions with the Islamic republic.

Here are the possible contours of such an agreement, which Iran and the so-called P5+1 -- Britain, China, France, Germany, Russia and the United States -- have been negotiating since late 2013.

The goal:

To reach a verifiable comprehensive agreement that limits Iran's ability to harness enough fissile material to build a nuclear bomb.

Breakout time:

World powers want to cut Iran's ability to build an atomic weapon to a "one-year breakout time." That would mean Tehran would need at least 12 months to be able to produce enough material for a nuclear bomb.

The assumption is that the international community would have enough time to detect such a move -- and could seek to strike or destroy the facilities.

This year-long breakout time would stay in place for the length of the deal. Officials say that they were homing in on a 10- to 15-year duration for any deal, but stressed that different elements would be subject to a variety of deadlines.

Enrichment:

This is one of the trickiest issues. Several years ago, the international community wanted to deny Iran any capability to enrich uranium.

In April 2006, Iran launched a process to enrich uranium to 3.5 percent. By February 2010, Iran had the ability to enrich to 20 percent, giving it the possibility of moving quickly to 90 percent -- the level needed for a bomb.

Iran currently has about 19,000 centrifuges. About 10,200 centrifuges are in operation, used for spinning uranium gas at supersonic speeds to make it suitable for power generation and medical uses but also, at high purities, for a bomb.

Diplomats told AFP there had been tentative agreement that Iran would slash the total number of its centrifuges by about two-thirds, to about 6,000.

Sanctions:

Iran has publicly said it wants all sanctions imposed by the US, European Union and United Nations to be lifted. But world powers have refused, talking instead about a phased, gradual easing of the measures.

Experts say untangling the sanctions -- from those also imposed for Tehran's alleged terror activities for example -- has proved to be one of the most difficult tasks.

A senior US official said all parties, including Iran, had agreed there should be a phased, step-by-step, reciprocal approach. But there is still disagreement on the formula to be adopted.

Sanctions relief would be staggered and linked to certain "milestones" by Iran over the lifetime of the deal.

Research and development:

Certain Western negotiators say the limits on highly enriched uranium mean nothing if the agreement does not take into account the technological progress made by Iran. A US official said research and development remained one of the biggest areas of contention.

Iran's nuclear chief Ali Akbar Salehi has said the deal would not stop Iran "continuing with force" the development of more powerful and modern centrifuges.

Nuclear plants:

Any deal will have to lay out what nuclear sites Iran would be allowed to maintain. The US does not want Iran to be allowed to develop weapons-grade plutonium at its unfinished Arak reactor.

Plutonium can be used as an alternative fissile material to highly-enriched uranium.

Iran should also not use its Fordo nuclear plant to enrich uranium, which would leave only its Natanz plant capable of enriching uranium.