Early peptides could have been formed from amino acids in volcanic gas

In trying to work out how life on Earth began, the difficulty has not been so much in finding ways to create the initial building blocks but in joining the blocks together into something more complex. Now a team of US biologists and chemists from the Scripps Research Institute, La Jolla, and the Salk Institute for Biological Studies, San Diego, has discovered that a simple chemical component of volcanic gas may have been responsible for linking amino acids together to form the first peptides.

Research over the past 50 years has shown that a variety of amino acids were probably present on the early Earth, synthesised via prebiotic chemical reactions and delivered by meteorites and comets. ’There are lots of ways to make amino acids,’ says lead researcher M. Reza Ghadiri of the Scripps Research Institute. ’But the question is "How do you couple them together"?’

The researchers spotted a potential answer in studies carried out by other research groups indicating that carbonyl sulfide (COS) might play a role in the formation of peptides. On today’s Earth, COS is readily found in volcanic gases and deep sea vent emissions, and is likely to have been present on the early Earth.

To test the ability of COS to promote the formation of peptides, the researchers bubbled it through a solution containing the amino acid phenylalanine. After two days, they discovered that around seven per cent of the phenylalanine amino acids had linked together into dipeptides. Carrying out further experiments, the researchers discovered that the efficiency of the reaction could be greatly increased by adding stoichiometric amounts of metal ions, such as lead, iron and cadmium. The researchers also conducted reactions with equimolar mixtures of phenylalanine and various other amino acids, including tyrosine, leucine, alanine and serine, which produced a variety of different dipeptides and tripeptides.

The researchers admit that it is unlikely that high concentrations of COS were present in the atmosphere of the early Earth and therefore suggest that peptide chains could have gradually built up on the rocks near volcanic emissions, with the reactions catalysed by metal ions in the rocks. The researchers are now investigating whether COS could have participated in other prebiotic reactions, such as the reduction of carbon dioxide.

Jon Evans