Except for glycine, every amino acid exists in two configurations that are mirror images, like a left and a right hand: the L form and the D form. The body's proteases evolved to recognise chains made of L-amino acids and struggle to cut bonds that involve a D-amino acid.
Replacing one or two key positions with their D version is a classic strategy to extend the life of a peptide. It is seen in melanocortin analogues such as Melanotan II or bremelanotide (PT-141), and in the mitochondrial tetrapeptide SS-31, which starts with D-arginine.
The change is not neutral: the D form alters the local geometry and can modify receptor affinity. That is why it is introduced only at positions where activity is preserved, something determined through structure-activity relationship studies.
In the sequence they are marked with the “D-” prefix (D-Arg, D-Phe). Mass spectrometry cannot tell L from D, since they have the same mass, so stereochemical identity depends on the synthesis method and on specific chromatographic techniques.
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Definition for educational and scientific purposes. It is not medical advice or a recommendation for use. NeoPeptidos products are sold labelled for research use only (RUO).