A mirror-image version of the normal amino acid — enzymes that break down standard amino acids typically cannot break down the mirror version, making D-amino acid-containing peptides much harder to degrade.All naturally occurring amino acids in human proteins are L-configured. Peptidases are stereospecific — they cleave L-peptide bonds efficiently but cannot process D-configured ones. Inserting D-amino acids at specific positions in synthetic peptides (D-Phe7 in Melanotan II and afamelanotide; D-amino acids in selank and semax) significantly extends plasma half-life by blocking proteolytic degradation at those positions. A key medicinal chemistry tool for improving peptide stability without changing receptor binding properties.