Left guessing

日期:2019-03-07 12:14:04 作者:杨礻 阅读:

By Marcus Chown THE dominance of left-handed molecules among the building blocks of life might not be a trick of the light. Researchers in Belgium have found that the structure of two of the amino acids that make up proteins cannot be explained by a rare form of ultraviolet light in the interstellar cloud where Earth was born, as some astronomers had thought. Amino acids come in two mirror-image forms, but life opts almost exclusively for the so-called left-handed form. Last year, astronomers showed that the Orion Molecular Cloud—adjoining the Orion Nebula—contained circularly polarised light that preferentially destroyed right-handed amino acids (New Scientist, 8 August 1998, p 11). Now two Belgian scientists have cast doubt on this theory. In a paper to be published in the journal Space Science Reviews, biochemist Corinne Cerf and astrophysicist Alain Jorissen of the Free University of Brussels say that the cosmic mechanism for destroying right-handed amino acids might not work for tryptophan and proline, two of the twenty amino acids commonly used by living organisms. The electric field of circularly polarised light rotates one of two ways around its direction of motion. Each of the right and left-handed forms of the amino acid molecules absorbs one type of light better than the other. If only one type is present, that light can break down the mirror form that absorbs it more efficiently. Astronomers suggested that this might have happened in the interstellar cloud from which the Earth congealed, having found only one type of circularly polarised light in the Orion Nebula. But Cerf and Jorissen found two exceptions to the theory when they looked at how amino acids absorb ultraviolet light. “The data show that tryptophan and proline do not behave like other amino acids,” says Jorissen. As a result, both their mirror forms might survive in the presence of just one type of light. The researchers think this may happen because of differences in the structure of the side chains of these molecules. According to Stephen Mason of King’s College London, Cerf and Jorissen are correct that circularly polarised light cannot select for one form of tryptophan and proline. But he argues that although some wavelengths preferentially destroy right-handed amino acids, other wavelengths destroy left-handed ones. As the light from stars covers a wide range of wavelengths,