Ten-membered cyclotripeptides. III. Synthesis and conformation of cyclo(-Me beta Ala-Phe-Pro-) and cyclo(-Me beta Ala-Phe-DPro-).

S Cerrini, E Gavuzzo, G Lucente, G Luisi, F Pinnen, L Radics
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Abstract

The 10-membered cyclotripeptide cyclo(-Me beta Ala-Phe-Pro) 3 and its diastereoisomer cyclo(-Me beta Ala-Phe-DPro-) 4 have been synthesized under mild cyclization conditions starting from linear precursors containing C-terminal proline. The crystal and molecular structure of the two models has been determined by X-ray crystallography. Analysis of the NMR spectra supported by NOE data clearly indicates that the conformations found in the crystals are retained in solution. Both cyclotripeptides exhibit a cis-cis-trans backbone conformation. The two tertiary peptide bonds, at the proline and Me beta Ala nitrogen atoms, adopt a cis conformation whereas the CO-NH junctions are trans in both the models. The deviations from planarity of the peptide units vary from delta omega values of ca. 18 degrees for the Pro-Me beta Ala and DPro-Me beta Ala bonds to ca. 7 degrees for Phe-Pro and Phe-DPro bonds. Relevant conformational details of 3 and 4, as revealed by X-ray and NMR analysis, are reported. Crystals of 3 are monoclinic: P2(1), a = 5.317(2), b = 17.059(6), c = 9.514(3) A, beta = 99.18(3), Z = 2. The final R and Rw are 0.054 and 0.071 respectively. Crystals of 4 are orthorhombic: P2(1)2(1)2(1), a = 8.797(2), b = 19.440(9), c = 21.605(10) A, Z = 8. The final R and Rw are 0.069 and 0.104 respectively.

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拥有cyclotripeptides。3环(- me β ala - ph - pro -)和环(- me β ala - ph - dpro -)的合成和构象。
以含c端脯氨酸的线性前体为原料,在温和的环化条件下合成了10元环三肽cyclo(- me β ala - ph - pro) 3及其非对映异构体cyclo(- me β ala - ph - dpro -) 4。用x射线晶体学测定了两种模型的晶体和分子结构。NOE数据支持的核磁共振谱分析清楚地表明,在晶体中发现的构象在溶液中被保留。两种环三肽均呈顺-顺-反骨架构象。两个三级肽键,在脯氨酸和Me β - Ala氮原子上,采用顺式构象,而CO-NH连接在两种模型中都是反式的。肽单元与平面度的偏差从Pro-Me β Ala和DPro-Me β Ala键的δ ω值约18度到ph - pro和ph - dpro键的δ ω值约7度不等。通过x射线和核磁共振分析,报道了3和4的相关构象细节。3个晶体为单斜晶:P2(1), a = 5.317(2), b = 17.059(6), c = 9.514(3) a, β = 99.18(3), Z = 2。最终R和Rw分别为0.054和0.071。4个晶体为正交晶型:P2(1)2(1)2(1), a = 8.797(2), b = 19.440(9), c = 21.605(10) a, Z = 8。最终R和Rw分别为0.069和0.104。
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