... but most people wouldn't know who he is. Anna Fazackerley meets Salvador Moncada, a living legend among scientists

I am just working away in a little corner. I'm not interested in the limelight," Salvador Moncada says. His "little corner" is actually a large, quietly luxurious office in University College London's beautiful old Cruciform building. And, unlike many in research, he has the look of a man who dresses carefully - his beard is stylishly trimmed and his suit says "success".

But ask someone outside academia to list some really important scientists and Moncada's name is unlikely to come up. He doesn't make television appearances and he chooses not to offer his opinions in newspapers.

This raises questions about scientific fame, because Moncada is big - seriously big.

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According to citation specialist Thomson ISI, other scientists have cited Moncada's work 68,889 times over the past 20 years - putting him second in the world in terms of scientific impact. No other scientist working in Britain makes it into the top 20 over the same period and there are only five UK names in the top 50.

Perhaps because he is South American rather than British, Moncada is manifestly proud of this achievement. "I would like to believe that citations are a sign of peer recognition," he says. "That is disputed by some people, as everything is disputed, but I feel that it is the least biased way. If someone feels they need to quote your work when writing about your field, that is recognition."

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Nancy Rothwell, Medical Research Council research professor at Manchester University, agrees. "I think Moncada is held in huge esteem in international science. He has made a great contribution and he is thought of as very charismatic," she says. "I think he is delightful."

For Hugh Watkins, professor of cardiovascular medicine at Oxford University, Moncada stands out because he has made not one but several discoveries of lasting importance to science, "each of which would have denoted a remarkable career".

Moncada, director of the Wolfson Institute for Biomedical Research at UCL, has certainly been churning out papers - well over 500 in the past 20 years. But he isn't interested in volume.

"We all publish papers of varying significance," he says. "Not everything Mozart composed was at the highest level. The number of publications doesn't mean very much - what means a lot is the few that make a difference."

Moncada has published five or six such papers. In the Seventies he led the discovery of prostacyclin, a natural substance that dilates blood vessels and inhibits blood clotting.

Then, in the Eighties, he created even bigger waves, identifying the unknown substance that makes blood vessels relax as nitric oxide - a gas that had previously been regarded only as an environmental pollutant.

This discovery was crucial to understanding cardiovascular disease. By making the blood vessels relax, nitric oxide helps blood to flow more easily and prevents blood cells sticking to the walls of blood vessels, which can cause fatal blockages.

Peter Weissberg, medical director of the British Heart Foundation, describes this discovery as "truly revolutionary". He explains: "Nitric oxide has turned out to be the 'essential ingredient' in maintaining normal function of our blood vessels." Moncada's work still centres on the gas that made his career. For the past few years his team has been looking at the way in which nitric oxide regulates oxygen consumption in cells.

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He hopes that this will lead to a deeper understanding of the origins of major diseases such as Parkinson's and cancer.

"It has been known for many years that cancer cells, unlike normal cells, do not consume oxygen very effectively. Instead they use glucose. Nobody knows exactly why this happens, although we do know this is a fundamental change in cancer," he explains.

This desire to cure disease is key to all Moncada's work. In the late Sixties, he trained as a doctor in El Salvador and, although he says he never seriously considered a clinical career, this experience has clearly stayed with him. But his time at medical school in the Central American republic also shaped his career for more unpleasant reasons. While he was studying he grew increasingly angry about the political abuses he saw in his country and decided that he had to speak out. As a result, he was imprisoned, persecuted and, finally, expelled.

"I have a basic disgust for injustice. The worst form of injustice - from one human directly to another - is what I cannot bear. That was what moved me to do things in a society that was so corrupt," he says. And although the consequences were severe, he is unrepentant. "I'm not sorry, not at all. My views haven't changed - I've mellowed with age but I still feel the same way."

After this, his arrival in the UK was an accident of fate. Banished to Honduras, where he had been born, the young Moncada was separated from his first wife who remained in El Salvador. They used to meet in neighbouring Guatemala, and it was here that a friend and professor of physiology pulled strings to get him to the UK. The friend, Fernando Molina, wrote a letter of recommendation to a colleague from his student days at the Royal College of Surgeons in London, the pharmacologist (and later Nobel laureate) John Vane.

An answer came almost immediately. Vane did not know who this chap Moncada was, but if his friend Molina rated him that was qualification enough.

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In February 1971, Moncada, who barely spoke any English, joined Vane's team in London. Within months his name was on one of three now classic papers describing how aspirin-like drugs worked. His stellar research career had begun.

"The majority of my classmates went to the US for training and several have had very successful careers there," he says. "But I can safely say that, for me, coming to the UK was absolutely the right choice. I've been treated extremely well and have been given all sorts of opportunities."

This sort of gratitude crops up a lot in Moncada's conversation - as do references to good fortune. He explains that he spends his life asking questions, that often open up new areas of research, adding modestly: "I've been lucky enough to strike on interesting things two or three times."

His other explanation for his success is an elementary one - and ironically one that would make him an ideal candidate for media attention.

"I am obsessed with trying to write things in the simplest possible way," he says. "There is a tendency towards obscure writing in science that I don't think is necessary. I believe if you're communicating - communicate."

When they write their results up, his research group is always careful to make sure that anyone - whether or not they are expert in the field - can understand them.

"There is an interesting consequence to this approach," he chuckles. "If you make things seem simple and write about them clearly, people tend to think they have always been there."

The trouble with things always having been there, of course, is that we might overlook the person who discovered them.

Moncada was spectacularly overlooked in 1998, when the Nobel prize was awarded to three American scientists for their uncovering of the role of nitric oxide as a biological mediator.

The science community was outraged and many scientists still refer to this as the one event that shook their faith in this most coveted of awards.

Moncada is not short of prizes. He has received an impressive array of distinctions and awards from all over the world for his science, including the Atomo d'Oro prize from the Roman Academy of Medical and Biological Sciences, and the Royal Society's Royal medal. But, like it or not, nothing has the stature of the Nobel.

Moncada won't be drawn on the impact this blow had on him, although colleagues say that it affected him deeply. "I really don't think about it," he insists. "It's not for me to say whether this was just or unjust."

And perhaps all of this is incidental to the business of doing research.

Looking to the future, Moncada is writing a book - his large desk is covered with piles of neatly bookmarked scientific papers - and getting on with his life in private. One colleague comments that he is famous for his parties and his cooking. "I don't work all the time," he says, smiling. "I enjoy working and certainly work is a very significant part of my life, but I have time for my family, for travel, for going to the theatre."

If the average person in the street does not know who he is, that does not bother him. "I've always concentrated on doing my work," he explains. "If, at the end of the day, I do anything that is significant, it will show.

That, however, is not the objective."

Salvador Moncada will deliver the Croonian prize lecture at the Royal Society on May 10 at 6pm.

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