Karle I L, Flippen-Anderson J L, Uma K, Balaram P
Laboratory for the Structure of Matter, Naval Research Laboratory, Washington, D.C.
Int J Pept Protein Res. 1993 Nov;42(5):401-10.
The crystal structure determination of three heptapeptides containing alpha-aminoisobutyryl (Aib) residues as a means of helix stabilization provides a high-resolution characterization of 6-->1 hydrogen-bonded conformations, reminiscent of helix-terminating structural features in proteins. The crystal parameters for the three peptides, Boc-Val-Aib-X-Aib-Ala-Aib-Y-OMe, where X and Y are Phe, Leu (I), Leu, Phe (II) and Leu, Leu (III) are: (I) space group P1, Z = 1, a = 9.903 A, b = 10.709 A, c = 11.969 A, alpha = 102.94 degrees, beta = 103.41 degrees, gamma = 92.72 degrees, R = 4.55%; (II) space group P21, Z = 2, a = 10.052 A, b = 17.653 A, c = 13.510 A, beta = 108.45 degrees, R = 4.49%; (III) space group P1, Z = 2 (two independent molecules IIIa and IIIb in the asymmetric unit), a = 10.833 A, b = 13.850 A, c = 16.928 A, alpha = 99.77 degrees, beta = 105.90 degrees, gamma = 90.64 degrees, R = 8.54%. In all cases the helices form 3(10)/alpha-helical (or 3(10)helical) structures, with helical columns formed by head-to-tail hydrogen bonding. The helices assemble in an all-parallel motif in crystals I and III and in an antiparallel motif in II. In the four crystallographically characterized molecules, I, II, IIIa and IIIb, Aib(6) adopts a left-handed helical (hL) conformation with positive phi, psi values, resulting in 6-->1 hydrogen-bond formation between Aib(2) CO and Leu(7)/Phe(7) NH groups. In addition a 4-->1 hydrogen bond is seen between Aib(3) CO and Aib(6) NH groups. This pattern of hydrogen bonding is often observed at the C-terminus of helices proteins, with the terminal pi-type turn being formed by four residues adopting the hRhRhRhL conformation.
通过确定三种含有α-氨基异丁酰基（Aib）残基以稳定螺旋结构的七肽的晶体结构，对6→1氢键连接的构象进行了高分辨率表征，这让人联想到蛋白质中螺旋终止的结构特征。三种肽Boc-Val-Aib-X-Aib-Ala-Aib-Y-OMe（其中X和Y分别为苯丙氨酸、亮氨酸（I）；亮氨酸、苯丙氨酸（II）；亮氨酸、亮氨酸（III））的晶体参数如下：（I）空间群P1，Z = 1，a = 9.903 Å，b = 10.709 Å，c = 11.969 Å，α = 102.94°，β = 103.41°，γ = 92.72°，R = 4.55%；（II）空间群P21，Z = 2，a = 10.052 Å，b = 17.653 Å，c = 13.510 Å，β = 108.45°，R = 4.49%；（III）空间群P1，Z = 2（不对称单元中有两个独立分子IIIa和IIIb），a = 10.833 Å，b = 13.850 Å，c = 16.928 Å，α = 99.77°，β = 105.90°，γ = 90.64°，R = 8.54%。在所有情况下，螺旋均形成3(10)/α-螺旋（或3(10)螺旋）结构，通过头对头氢键形成螺旋柱。在晶体I和III中，螺旋以全平行基序组装，在晶体II中以反平行基序组装。在四个经晶体学表征的分子I、II、IIIa和IIIb中，Aib(6)采取左手螺旋（hL）构象，具有正的φ、ψ值，导致Aib(2)羰基与Leu(7)/Phe(7)氨基之间形成6→1氢键。此外，还观察到Aib(3)羰基与Aib(6)氨基之间形成4→1氢键。这种氢键模式在螺旋状蛋白质的C末端经常观察到，末端的π型转角由四个采取hRhRhRhL构象的残基形成。
