Retro-D-Relaxin

When biochemists use abbreviations such as this title, the mean-ing is usually discoverable by contemplation (as opposed to the meaning of abbreviations in molecular biology). ‘Retro’ means the reverse and it is occasionally used exchangeably with ‘inverto’, and both refer to the direction into which the amino acid backbone is pointing. Peptides are always written with the amino group on the left side and the carboxyl group on the right side, and each single amino acid is incorporated into this peptide in the same direction. The retro- or inverto peptide has each amino acid remaining in the same spot relative to the others, but flipped, so that the carboxyl group now points to the left. Thus, the inverto of Leu→Ala→Gly→Ile would be Leu←Ala←Gly←Ile where the arrows point from the amino terminus to the carboxyl terminus. Using the same example and the conventional way of writing the peptide sequence from the N to the C terminus the retro-form of the original peptide Leu-Ala-Gly-Ile would be Ile-Gly-Ala-Leu. Figure 15.1 explains the concept. The reader will notice that ‘retro’ does not refer to a natural property of a peptide but rather to one sequence relative to another, i.e., there is no retro peptide without the reference peptide to which it is ‘retro’.