[axelitis]

The attached image was submitted to me as part of a document for publication as a simple depiction of L- & D-forms of a tripeptide showing the D-form as 'mirror' and reversed order of the L-.  I'm a medical scientist, but my knowledge of enantiomers and peptide chemistry fails to provide me the confidence properly to judge the validity of the figure attached herewith.  I would very much appreciate input from experts who can enlighten me and (I hope) participants who may find the matter of interest.  Thank you!

[Babcock_Hall]

I am going to bend a rule of mine, which is not to comment until I have thought about something carefully.  The enantiomer of a tripeptide of L-aminoacids should be the same tripeptide with D-amino acids.  I am having a difficult time seeing why it should reverse which amino acid residue is the N-terminal one.

[axelitis]

Thank you very much for your reply.  As I should have mentioned, the actual (20+ AA peptide) was synthesized in reverse (using D-AAs) and was found to be both resistant to peptidases and as, or more, biologically/ therapeutically active than was the L-form. I look forward to yours and all considered opinions.

[kriggy]

Im not realy sure what exactly is your question.
But it seems there is nothing wrong with the picture, it is just analogue of the 1st peptide built from D-amino acids in  reverse order.

An L-peptide has three analogue sequences (Figure 2) built from L and D amino acids: the D-enantiomer or inverso-peptide with the same sequence, but composed of D-amino acids and a mirror conformation; the retro-peptide, consisting of the same sequence of L amino acids but in reverse order; and the retro-inverso or D-retro-enantiomer peptide, consisting of D-amino acids in the reversed sequence.[3] [4]
While the L-peptide and its D-enantiomer are mirror structures of each other, the L-retro-peptide is the mirror image of the D-retro-inverso-peptide. On the other hand, the L-peptide and the D-retro-inverso-peptide share a similar arrangement of side-chains, although their carboxyl and amino groups point in opposing directions. For small peptides that do not depend on a secondary structure for binding, an L-peptide and its D-retro-inverso-peptide is likely to have a similar binding affinity with a target L-protein.

figure 2:

1: L-peptide
2: D-peptide with reverse order structure
3: D-peptide
4: L-peptide with reverse order structure

[Babcock_Hall]

It seemed to me that the absolute configuration (L or D) of the alpha carbons was correctly portrayed.  Was that your question?  We can discuss how to check this if you like.

[axelitis]

Thank you both for your help.  One problem I have with the figure I submitted is that the labeling may be incorrect.  On the lower half, shouldn't the labeling show "Ser Ala Cys" rather than "Cys Ala Ser"?  Admittedly, I am having trouble interpreting the ball and stick diagrams.  My purpose here is to be certain that 'those skilled in the art' are satisfied (even if I'm not) that the example shown in the diagram is apt.

[Babcock_Hall]

The sequences of peptides are routinely described by starting at the N-terminus and concluding at the C-terminus.  There may be exceptions, but I am unaware of them if they exist.  In other words, the labeling looks correct to me.

[axelitis]

Thank you very much!