Complexity in Exobiology ( theoretical ) , Complexity in

Glossary Chiral, achiral and racemic A molecule is chiral if its three-dimensional structure is different from its mirror image. Such molecules tend to be optically active and turn the polarization plane of linearly polarized light in the right-or left-handed sense. Correspondingly , they are referred to as D-and L-forms, which stand for dextrorotatory and levorotatory molecules. An achiral molecule is mirror-symmetric and does not have this property. A substance is racemic if it consists of equally many left-and right-handed molecules. A polymer is said to be isotactic if all its elements have the same chirality. Enantiomers and enantiomeric excess Enantiomers are a pair of chiral molecules that have opposite handed-ness, but are otherwise identical. Enantiomeric excess, usually abbreviated as e.e., is a normalized measure of the degree by which one handedness dominates over the other one. It is defined as the ratio of the difference to the sum of the two concentrations, so e.e. always falls between 1 and +1. Epimerization and racemization Epimerization is a spontaneous change of handedness of one sub-unit in a polymer. Racemization indicates the loss of a preferred handedness in a substance. Catalysis and auto-catalysis Catalysts are agents that lower the reaction barrier. A molecule reacts with the catalyst, but at the end of the reaction, the catalyst emerges unchanged. This is called catalysis. In auto-catalysis the catalyst is a target molecule itself, so this process leads to exponential amplification of the concentration of this molecule by using some substrate. Biological catalysts are referred to as enzymes. Nucleotides and nucleic acids Nucleotides are mono-mers of nucleic acids, e. g., of RNA (ribonucleic acid) or DNA (deoxyribonucleic acid). They contain one of four nucleobases (often just called bases) that can pair in a specific way. Nucleotides can form polymers, and their sequence carries genetic information. One speaks about a polycondensation reaction instead of polymer-ization because one water molecule is removed in this step. Other nucleotides of interest include peptide nucleic acid or PNA. Here the backbone is made of peptides instead of sugar phosphate. Peptides and amino acids Amino acids are molecules of the general form NH 3-CHR-COOH, where R stands for the rest, which makes the difference between different amino acids. For glycine, the simplest amino acid, we have R=H, so two of the bonds on the central C atom are the same and the molecule is, therefore, chiral. For alanine, R=CH 3 , so …