Preferred conformation of the tert-butoxycarbonyl-amino group in peptides.
Preferred conformation of the tert-butoxycarbonyl-amino group in peptides.
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肽中叔丁氧基羰基氨基的优选构象。
DOI：
10.1111/j.1399-3011.1980.tb02949.x
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发表时间：
1980
期刊：
影响因子：
--
通讯作者：
Scheraga,HA
中科院分区：
文献类型：
--
作者：
Benedetti,E;Pedone,C;Toniolo,C;Némethy,G;Pottle,MS;Scheraga,HA
根据X射线结晶学数据，编制了氨基酸和线型多肽衍生物的结构参数，这些氨基酸和线型多肽衍生物含有当时终止的叔丁氧羰基(Boc)或其下一个更高的同系物，即叔丁氧羰基。BOC-衍生物中氨基甲酸酯基团的几何构型与多肽基团的几何结构比较表明，由于多肽单元的C-αH基团被酯氧取代时，相互作用发生了变化，因此在三角碳的键角上有很小的差异。与多肽键强烈地优先于转变相反(除非它先于脯氨酸)，氨基甲酸胺键在晶体中既采用这种构象，也采用转变构象。在含有叔氮的化合物(如Boc-Pro)的晶体中或在分子间强相互作用稳定的结构中，顺式氨基甲酸酯构象是首选的。对BOC-氨基酸N‘-甲酰胺的构象能计算表明，氨酯酰胺键的反式构象和顺式构象的能量几乎相等(即使是对氨基酸的反式构象也是如此)，而反式构象的能量要低得多。计算的BOC-氨基酸衍生物中顺式含量的增加(与相应的N-乙酰衍生物相比)与观察到的晶体结构中的构象分布和n.m.r.一致。在溶液中进行研究。通常，当酰胺键反式时，BOC取代N-乙酰端基不会改变端基后面的氨基酸残基的构象偏好(如φ，Ψ值和相对能量所示)。然而，某些侧链(例如苯丙氨酸的侧链)和庞大的Boc端基之间强烈的吸引人的相互作用可以稳定特定的构象。
Structural parameters, derived from X‐ray crystallographic data, have been compiled for amino acid and linear peptide derivatives which contain theN‐terminaltert‐butoxycarbonyl (Boc) group or its next higher homolog, thetert‐amyloxycarbonyl group. The comparison of the geometry of the urethane group in Boc‐derivatives with that of the peptide group shows small differences in bond angles about the trigonal carbon, because of altered interactions when a CαH group of a peptide unit is replaced by an ester oxygen. In contrast to the strong preference of the peptide bond for thetransform (except when it precedes proline), the urethane amide bond adopts both thecisandtransconformations in crystals. The cis urethane conformation is preferred in crystals of compounds with a tertiary nitrogen (such as Boc‐Pro) or in structures stabilized by strong intermolecular interactions. Conformational energy computations on Boc‐amino acid N'‐methylamides indicate that the trans and cis conformations of the urethane amide bond have nearly equal energies (even for amino acids other than proline), in contrast to the peptide bond, for which the trans conformation has a much lower energy. The computed increase of the cis content in Boc‐amino acid derivatives (as compared with the corresponding N‐acetyl derivatives) is consistent with the observed distributions of conformations in crystal structures and with n.m.r. studies in solution. Usually, the substitution of a Boc for an N‐acetyl end group does not alter the conformational preferences (as indicated by φ, Ψ values and relative energies) of the amino acid residue which follows the end group when the amide bond is trans. Particular conformations, however, can be stabilized by strong attractive interactions between some side chains (e.g. that of phenylalanine) and the bulky Boc end group.