Isopedopeptins are antibiotic cyclic lipodepsipeptides containing the subsequence L-Thr—L-2,3-diaminopropanoic acid—D-Phe—L-Val/L-3-hydroxyvaline. Acidic hydrolysis of isopedopeptins in D₂O showed the D-Phe residues to racemize extensively in peptides with L-3-hydroxyvaline but not in peptides with L-Val. Similarly, one Leu residue in pedopeptins, which are related peptides containing the subsequence Leu—2,3-diaminopropanoic acid—Leu—L-Val/L-3-hydroxyvaline, was found to racemize in peptides with L-3-hydroxyvaline. Model tetrapeptides, L-Ala—L-Phe—L-Val/3-hydroxyvaline—L-Ala, gave the corresponding results, i.e. racemization of L-Phe only when linked to a L-3-hydroxyvaline. We propose the racemization to proceed via an oxazoline intermediate involving Phe/Leu and the L-3-hydroxyvaline residues. The 3-hydroxyvaline residue may form a stable tertiary carbocation by loss of the sidechain hydroxyl group as water after protonation. Elimination of the Phe/Leu H-2 and ring-closure from the carbonyl oxygen onto the carbocation results in the suggested oxazoline intermediate. The reversed reaction leads to either retained or inversed configuration of Phe/Leu. Such racemization during acidic hydrolysis may occur whenever a 3-hydroxyvaline residue or any amino acid that can form a stable carbocation on the C-3, is present in a peptide. The proposed mechanism for racemization was supported by incorporation of ¹⁸O in the 3-hydroxyvaline sidechain when the acidic hydrolysis was performed in H₂O/H₂¹⁸O (1:1). The 2,3-diaminopropanoic residues of isopedopeptins and pedopeptins were also found to racemize during acidic hydrolysis, as previously described. Based on the results, the configuration of the Leu and 2,3-diaminopropanoic acid residues of the pedopeptins were reassigned to be L-Leu and D-Leu, and 2 × L-2,3-diaminopropanoic acid.

Isopedopeptins 是抗生素环状脂缩肽，包含子序列 L-Thr-L-2,3-二氨基丙酸-D-Phe-L-Val/L-3-羟基缬氨酸。Isopedopeptins 在 D ₂ O 中的酸性水解表明 D-Phe 残基在含有L-3-羟基缬氨酸的肽中广泛外消旋，但在含有L-Val 的肽中没有。类似地，在pedopeptins，它们是含有所述亚序列LEU-2,3-二氨基丙酸-LEU-L-缬氨酸/ L-3-hydroxyvaline相关的肽一个Leu残基，发现外消旋化在用肽L-3-hydroxyvaline。模型四肽，L-Ala-L-Phe-L-Val/3-羟基缬氨酸-L-Ala，给出了相应的结果，即只有当 L-Phe 与L-3-羟基缬氨酸连接时才会发生外消旋化. 我们建议通过涉及 Phe/Leu 和 L-3-羟基缬氨酸残基的恶唑啉中间体进行外消旋化。3-羟基缬氨酸残基可通过质子化后侧链羟基损失为水而形成稳定的叔碳正离子。Phe/Leu H-2 的消除和从羰基氧到碳正离子的闭环导致建议的恶唑啉中间体。逆反应导致 Phe/Leu 的保留或反转构型。每当肽中存在 3-羟基缬氨酸残基或可在 C-3 上形成稳定碳正离子的任何氨基酸时，就会发生这种酸性水解过程中的外消旋化。建议的外消旋化机制得到了¹⁸当在 H ₂ O/H ₂ ¹⁸ O (1:1) 中进行酸性水解时，3-羟基缬氨酸侧链中的O。如前所述，还发现isopedopeptins 和pedopeptins 的2,3-二氨基丙酸残基在酸性水解期间外消旋化。根据结果​​，足肽素的 Leu 和 2,3-二氨基丙酸残基的构型被重新分配为 L-Leu 和 D-Leu，以及 2 × L-2,3-二氨基丙酸。