Question

When amino acids are synthesized in a test tube, \(\mathrm{D}\) and \(\mathrm{L}\) forms occur in equal amounts. However, cells use only L forms in their proteins. Occasionally, meteorites are found to contain amino acids. Based on these facts, how could NASA scientists determine whether the amino acids recovered from space are evidence of Earth-like extraterrestrial life rather than the result of nonmetabolic processes?

Short answer

NASA scientists can analyze the distribution of D and L forms of amino acids in meteorites. If there's an equal distribution of both forms, it suggests a nonmetabolic origin. However, if L-forms dominate, it could indicate the presence of Earth-like extraterrestrial life, as this form is exclusively used in terrestrial living organisms.

Step-by-step solution

Recognize The Amino Acids Synthesis

Generally, amino acids synthesized in a lab (or a test tube) will result in an equal mixture of \(\mathrm{D}\)-amino acids and \(\mathrm{L}\)-amino acids. This is because the synthesis process doesn't favor one form over the other. Therefore, a 50/50 mix of \(\mathrm{D}\) and \(\mathrm{L}\) forms is a signature of nonmetabolic (or non-biological) synthesis.

Consider The Preference of Life Forms

When it comes to life (at least on Earth), organisms are selective. It's known that Earth organisms only utilize the \(\mathrm{L}\) form in their proteins. This suggests that if all or majority of the amino acids present are of the \(\mathrm{L}\) form, it could indicate that the amino acids were produced by biological organisms, similar to those on Earth.

Apply the Information to Extraterrestrial Scenario

Finally, for NASA scientists investigating the origin of amino acids in meteorites, recognizing these differences becomes crucial. If the amino acids from a meteorite consist exclusively or largely of the \(\mathrm{L}\) form, it would suggest that they might be the product of Earth-like extraterrestrial life. However, if there's an equal distribution of \(\mathrm{D}\) and \(\mathrm{L}\) forms, it would suggest that the amino acids were likely produced by nonmetabolic processes.

Key concepts

Chirality

Chirality is a fundamental concept in chemistry and biology, relating to the spatial arrangement of atoms in molecules. An object is described as chiral if it cannot be superimposed on its mirror image. This often resembles a "handedness", where the right and left hand are mirror images but not identical. Amino acids, the building blocks of proteins, display chirality. Each amino acid can exist in two forms, known as enantiomers: one is the L-form (levo) and the other is the D-form (dextro).
A practical example of chirality is that, when synthesized in the lab, both forms appear in equal proportions, creating a racemic mixture. Life on Earth, however, exhibits a unique chiral preference for the L-form of amino acids. This asymmetry is critical for the functionality of biological proteins, which, if made from a mixture of D and L forms, wouldn't function properly. Chirality thus offers essential clues in the search for life because a bias toward one enantiomer suggests biological origin rather than chemical random chance.

Extraterrestrial Life

The search for extraterrestrial life is a topic that captivates scientists and the public alike. Amino acids could serve as potential indicators of life, as they are fundamental to the biological processes. When examining amino acids in space samples, the form and distribution provide clues about their origins.
If space-retrieved amino acids lean toward the L-form, similar to Earth's life, it might imply life beyond Earth with biological processes akin to ours. Such a finding would imply not just the presence of similar elemental chemistry, but possible life that follows comparable biological rules. On the contrary, a balanced mix of D and L forms suggests that the amino acids formed through random, non-biological processes. This understanding of amino acid chirality becomes a crucial tool in evaluating whether discovered molecular traces can be considered evidence of life in the cosmos.
In decoding these mysteries, scientists apply techniques of astrobiology to compare, contrast, and efficiently determine the likelihood of biological activity. The data serves as a puzzle piece in answering the age-old question of whether we are alone in the universe.

Astrobiology

Astrobiology is an interdisciplinary field that combines astronomy, biology, and geology to understand life's potential in the universe. It seeks to uncover the origin, evolution, distribution, and future of life, both on Earth and beyond. Central to this study is the analysis of extraterrestrial materials, like meteorites, for traces of organic compounds, including amino acids.
A vital part of this investigation is the identification of chiral signatures, which can distinguish between non-biological and potentially biological origins. For astrobiologists, finding predominantly L-amino acids in extraterrestrial samples suggests biological activity that mirrors life on Earth. This provides a baseline for hypotheses about life elsewhere. Meanwhile, equal distributions of D and L amino acids indicate abiotic or non-life processes.
Astrobiological research seeks to expand our knowledge of life's boundaries by exploring extreme environments on Earth, which can simulate extraterrestrial conditions. These studies provide insights into how life might adapt and thrive elsewhere, offering models to identify life signatures on distant worlds. Through these investigations, astrobiology not only enhances our understanding of life in the cosmos but also sheds light on the conditions necessary for life to flourish.