Scientists have developed a new peptide-based catalyst that enables more efficient production of macrocycles – large molecular rings with 12 to 18 atoms[1]. This method uses a bifunctional peptide catalyst to direct the ends of a linear precursor, favoring ring formation instead of unwanted side reactions[1]. The catalyst controls the stereoselectivity – that is, which of the two possible mirror configurations is formed – even in cases where the linear precursor contains pre-existing stereochemical properties[1]. Scientists have synthesized diverse macrocyclic lactones and lactams from achiral linear precursors[1]. This new method provides a more practical approach to the production of chiral macrocycles with predictable stereochemical results[1]. Macrocycles have potential in new drug discovery because they can bind to large, flat or flexible protein surfaces that are inaccessible to traditional small molecules[5].