Interesting recent paper discussing the possiblity that proteins made of a mix of L and D amino acids could still adopt functional structures:
Some times going back and checking old assumptions is worth it. Why was it assumed that heterochirality would be an insurmountable barrier to functional biological macromolecules? We have life as we know it, we by definition don’t have life as we don’t know it.
If functional heterochiral protein catalysts are possible, but which nevertheless exhibit chirally selective catalysis, this could provide for a feedback loop of increasingly homochiral reproduction of polymers.
I haven’t come across something that explicitly argued it, as much as it sort of just being implicitly taken for granted in experiments involving (for example) polymerization of amino acids by wet-dry cycling.
For some reason all experiments I’ve ever heard of on that topic have worked exclusively with some purely homochical solution of amino acids or their derivatives. That just doesn’t seem like a plausible starting point to me, and I some times wonder if they’re actually missing important properties that might be conducive to the origin of life, by using chemical mixtures that look more like life as we know it.
But it’s entirely conceivable that life can’t begin by a mix of chemicals taken from life as we know it, but that it can begin from a more prebiotically plausible, impure mix. Rather than think these dirty chemistries with many stereoisomers and racemix mixtures are somehow barriers to the emergence of functional peptides, they could actually help it instead. Someone has to do those experiments and run those simulations. Throw away the assumption that if it doesn’t look like life as we know it, then it’s not relevant or conducive to the origin of something living.