Dave, thank you so very much. I understood that almost completely. Certainly well enough to feel I understand what chirality is in this regard.
Now, for two questions:
In what you wrote was the statement, "Looking at a mirror image of a sugar or amino acid chain will not give you the same version of the molecule." By "will not give you the same version of the molecule" do we mean that the mirror image is not a sugar or amino acid, or that it is a sugar or amino acid but functions differently than the other-handed sugar or amino acid?
Second question, at the end you wrote, "Life prefers L-amino acids, not D-versions." I feel that this is getting us into the discussion of how life arose. But I need you to explain the relationship between "life preferring L-amino acids" and why this presents a difficultly for explaining how life arose.
Thank you again.
It's still the same type of molecule in a chemical sense. Leucine, for example, has the chemical formula C6H13NO2. That is true whether it takes the L- form or the D- form. Only the L- form is used in the body for general metabolic functions, such as triggering the protein synthesis response, being converted to enzymes, or being incorporated into the protein structural chain. That doesn't mean that the D- enantiomer has no biologic response (it is found in trace amounts, and it has been found to counteract seizures in mice), but it's not involved in the general cell machinery.
For your second question, the problem is that when amino acids are produced in laboratory conditions, the quantities tend to be racemic, including equal numbers of the L- and D- enantiomers. A functional protein cannot form when a D- form of an amino acid gets taken up by a growing chain. This is certainly a problem in origin-of-life research, because there needs to be a way for living molecules to filter out the D- versions and find adequate numbers of the L- versions. It's not a problem now, because D- amino acids and L- sugars are very rare in nature. During the pre-biotic phase of Earth's history, however, the two versions would have been much more equal in numbers.
Some consider this an insurmountable problem, but all too often what seems insurmountable with limited information becomes "merely" difficult as knowledge increases. And there do appear to be ways in nature that the discrepancy can be potentially overcome. Whether that potential can be realized is still to be shaken out.