Just as your right hand and your left hand are mirror images of each other, in similar fashion a carbon atom which is bonded to four different substituents also exhibits the characteristic of “handedness”. A molecule that is not identical to its mirror image is technically called an enantiomer meaning opposite. Two otherwise chemically identical molecules possessing mirror-image structures exhibit chirality.
One way to detect if one has enantiomers is to pass polarized light through a solution of your material. If there is a mixture of the two forms (racemic mixture) the light does not pass through the solution because one form of the material rotates light to the right (right-handed or dextrorotary of d-isomer), and the opposite form rotates light to the left (left-handed or levorotary or l-isomer), cancelling each other out. It was Louis Pasteur in 1849 who discovered this phenomenon.
Long chains of molecules which contain a mixture of enantiomeric forms, will exhibit a different shape from those which are completely the l form, or the d form. The physical shape is different while the chemistry is the same, but their biological function is not the same.
The amazing thing is that living cells do not deal with racemic mixtures of biological molecules. It has been found that the proteins which contribute to living cell contents, are, with very few exceptions, levorotary. Often proteins consisting of only dextrorotary proteins are lethal. Thus, levorotary chirality in the amino acids is an absolute necessity for life.
All nucleic acids on the other hand exhibit an exclusive dextro configuration.
Thus, it is evident that for origin of life chemistry (abiogenesis) to take place, all the amino acids involved must be the l form. If even small amounts of the d-amino acids are present, different three-dimensional structures will be formed which are unsuitable for life. All amino acids, the building blocks of life, that are formed by lightning (chance) or other natural, non-biological processes, produce racemic (50-50) mixtures of l and d forms. Such mixtures are completely unsuitable for life.
The Miller-Urey experiment did produce some amino acids, but always in a racemic mixture. Since about 1900, generations of scientists have attempted in vain to produce optically pure amino acids by random inorganic methods without involving living cells or other previously preset optically active compounds from the laboratory.
Without optically pure compounds, origin of life research cannot even get past the need to form protein polymers as an essential basis for the living cell.