McEwen I
Department of Organic Chemistry, University of Gothenburg, Sweden.
Biopolymers. 1993 Jun;33(6):933-42. doi: 10.1002/bip.360330609.
The cyclic hexapeptide cyclo [-Pro1-Gly2-Glu3 (OBzl)-Pro4-Phe5-Leu6-] (1) was modeled and synthesized to be used for chiral discrimination studies. Total correlated spectroscopy and nuclear Overhauser effect spectroscopy spectra of the cyclic hexapeptide 1 in CDCl3 showed the presence of three stereoisomers: two dominant stereoisomers 1a and 1b that exchanged chemically with each other, and a minor stereoisomer 1c (4%) that exchanged exclusively with the stereoisomer 1b. Of the two dominant stereoisomers, only 1a interacted specifically with t-butyloxycarbonyl (Boc-) and 9-flourenylmethyloxycarbonyl (Fmoc-) amino acids in CDCl3. The interaction site of 1a when complexing with the derivatized amino acids was the chain segment Phe5-Leu6. The Phe5 NH and Leu6 NH protons are contiguous and solvent exposed. Their nmr signals shifted strongly downfield with the addition of Boc- or Fmoc- amino acids to the peptide solution. Thus, both NH protons hydrogen bond to the amino acids, forming a two-point hydrogen-bonding complex. The peptide stereoisomer 1b did not interact specifically with the Boc- and Fmoc-amino acids because of the lack of two contiguous and solvent-exposed peptidic NH protons that seem to be needed for specific interactions of the cyclic hexapeptide 1 with the Boc- and Fmoc-amino acids. A clear difference in the interaction of 1a with D- and L-enantiomers of Boc- Trp and Fmoc-Trp was observed with nmr spectroscopy. Docking models and molecular mechanics calculations together with nmr observations showed that the NH proton of the indole ring of the Boc-L-Trp and the Fmoc-L-Trp hydrogen bonded to the Pro1 carbonyl group. In this three-point hydrogen-bonding complex, the indole ring becomes locked underneath the Leu residue. The nmr signals of all the Leu6 protons (except for Leu NH) shifted strongly upfield owing to the shielding effect of the indole aromatic ring currents. The indole NH of the D-enantiomer did not hydrogen bond to the Pro1 carbonyl group because the formation of such a three-point hydrogen-bonding complex was thermodynamically unfavorable.
环六肽环-Pro1-Gly2-Glu3(OBzl)-Pro4-Phe5-Leu6-被进行建模并合成，用于手性识别研究。环六肽1在CDCl3中的全相关谱和核Overhauser效应谱表明存在三种立体异构体：两种主要的立体异构体1a和1b，它们相互之间发生化学交换，以及一种次要的立体异构体1c(4%)，它仅与立体异构体1b发生交换。在这两种主要的立体异构体中，只有1a在CDCl3中与叔丁氧羰基(Boc-)和9-芴甲氧羰基(Fmoc-)氨基酸发生特异性相互作用。1a与衍生化氨基酸络合时的相互作用位点是链段Phe5-Leu6。Phe5 NH和Leu6 NH质子相邻且暴露于溶剂中。向肽溶液中加入Boc-或Fmoc-氨基酸后，它们的核磁共振信号强烈向低场移动。因此，两个NH质子都与氨基酸形成氢键，形成两点氢键复合物。肽立体异构体1b没有与Boc-和Fmoc-氨基酸发生特异性相互作用，因为缺少两个相邻且暴露于溶剂中的肽NH质子，而这似乎是环六肽1与Boc-和Fmoc-氨基酸特异性相互作用所必需的。通过核磁共振光谱观察到1a与Boc-Trp和Fmoc-Trp的D-和L-对映体之间的相互作用存在明显差异。对接模型和分子力学计算以及核磁共振观察表明，Boc-L-Trp和Fmoc-L-Trp吲哚环的NH质子与Pro1羰基形成氢键。在这种三点氢键复合物中，吲哚环被锁定在Leu残基下方。由于吲哚芳香环电流的屏蔽作用，所有Leu6质子(Leu NH除外)的核磁共振信号强烈向高场移动。D-对映体的吲哚NH没有与Pro1羰基形成氢键，因为形成这样的三点氢键复合物在热力学上是不利的。
