Microcrystals of l-Asn-d-Trp-l-Phe-NH₂ (NdWFamide), a tripeptide derived from Aplysia kurodai that exhibits invertebrate cardiac activity, were evaluated by vibrational circular dichroism (VCD). The chirality of the tryptophan residue at the second position in NdWFamide was associated with the conformation and biological characteristics. The VCD spectrum of NdWFamide was a mirror image of its enantiomer; however, it was significantly different from that of its diastereomer, NWFamide, which is its precursor. The obtained VCD signals of NdWFamide were in good agreement with the VCD signals that were calculated based on the optimized aggregates of NdWFamide, which formed a helical-like backbone conformation. The evaluation of the VCD results revealed the conformation of NdWFamide in the crystalline state and succeeded in distinguishing its stereoisomers. Therefore, this study demonstrates VCD as a useful method for the structural analysis of naturally occurring d-amino acid-containing peptides.

中文翻译：

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晶体形式的含d-氨基酸肽NdWFamide的振动圆二色性

l -Asn- d -Trp- l -Phe-NH ₂ (NdWFamide) 的微晶，一种来自海兔的三肽表现出无脊椎动物心脏活动的，通过振动圆二色性 (VCD) 进行评估。NdWFamide第二位色氨酸残基的手性与构象和生物学特性有关。NdWFamide的VCD光谱是其对映体的镜像；然而，它与其前体 NWFamide 的非对映体明显不同。得到的 NdWFamide 的 VCD 信号与基于 NdWFamide 的优化聚集体计算的 VCD 信号非常吻合, 形成了螺旋状的骨架构象。VCD 结果的评估揭示了 NdWFamide 在结晶状态下的构象，并成功区分了其立体异构体。所以，d-含氨基酸的肽。