Estimation of the 2.0₅ helix type i鈫?span style='font-style: italic'>i hydrogen bond energy at Aib^{鈭?/sup>-Oxa motif: an isodesmic approach}

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• 作者：Ravula Thirupathi^b ; Erode N. Prabhakaran^a ; ^{erodeprabhakaran@gmail.com" class="auth_mail}
• 关键词：2.05 Helix ; Hydrogen bond ; Isodesmic reaction ; Oxazine ; 伪-Aminoisobutyric acid
• 刊名：Tetrahedron Letters
• 出版年：4 June 2014
• 年：2014
• 卷：55
• 期：23
• 页码：3418-3421
• 全文大小：1158 K

文摘

Bending at the valence angle N–C^伪–C′ (蟿) is a known control feature for attenuating the stability of the rare intramolecular i→i hydrogen bonded pseudo five-membered ring C₅ structures, the so called 2.0₅ helices, at Aib. The competitive 3₁₀-helical structures still predominate over the C₅ structures at Aib for most values of 蟿. However at Aib^∗, a mimic of Aib where the carbonyl O of Aib is replaced with an imidate N (in 5,6-dihydro-4H-1,3-oxazine = Oxa), in the peptidomimic Piv-Pro-Aib^∗-Oxa (1), the C₅i structure is persistent in both crystals and in solution. Here we show that the i→i hydrogen bond energy is a more determinant control for the relative stability of the C₅ structure and estimate its value to be 18.5 ± 0.7 kJ/mol at Aib^∗ in 1, through the computational isodesmic reaction approach, using two independent sets of theoretical isodesmic reactions.