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The D-peptide is a peptide using D-amino acids instead of L-amino acids (with the same AA sequence). I would expect the D-peptide to always have a mirror image conformation compared to the original. Indeed, Wikipedia reports:

If an L-protein does not require a chaperone or a structural cofactor to fold, its D-enantiomer protein should have a mirror image conformation with respect to the L-protein (Figure 2)

Many D-proteins and other D-peptides when placed in a nonchiral solvent like water, assume the mirror image conformation of their L-peptide counterpart.

However, doi:10.1021/acs.inorgchem.5c00438 reports different binding affinities for the achiral Cu(II) between original (L1) and the D-amino acid (L2). From Figure 6, even the conformations seem non-mirrored. The solvent also seems to be achiral.

I am puzzled by how it's possible for the L-peptide and D-peptide to not have mirror image conformations.

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Well, while I'm somewhat sceptical about this paper, a scenario like this is possible.

While Anfinsen's dogma says you should have just one structure, it doesn't necessarily have to be like that - proteins aren't always in global minimum of energy, even if properly folded - it could be another, local, minimum. And if protein is denatured by heat, you go so far from minimum there may not be much chance to get back.

So, if this paper is really legit, they got a native L-peptide and a D-peptide sitting in a different minimum, an isomer, you could say. An isomeric structure like this, but mirrored, should be available also for the original peptide, but apparently it's not preferred, perhaps because of higher energy.

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    $\begingroup$ Sharing derogatory opinions about an entire paper published in a respected journal, in just a few lines and without any convincing evidence, is a major disservice to science and to all readers. Where is the evidence that the paper cited by OP is not "really legit"? $\endgroup$ Commented 10 hours ago
  • $\begingroup$ @ACR There are a couple of weaknesses in the paper. The alphafold prediction (fig 1) shows a largely unfolded protein but this is not discussed, the mass spec and the HPLC retention times in the supplementary show difference between enantiomers but this is not discussed. They show computed structures without giving sufficient detail, and they don’t show chirality in the skeletal structures. But you are right, it is in a traditional peer-reviewed journal. $\endgroup$ Commented 11 mins ago

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