Karle I L, Gopi H N, Balaram P
Laboratory for the Structure of Matter, Naval Research Laboratory, Washington, DC 20375-5341, USA.
Proc Natl Acad Sci U S A. 2001 Mar 27;98(7):3716-9. doi: 10.1073/pnas.071050198. Epub 2001 Mar 20.
A beta-hairpin conformation has been characterized in crystals of the decapeptide t-butoxycarbonyl-Leu-Val-beta Phe-Val-(D)Pro-Gly-Leu-beta Phe-Val-Val-methyl ester [beta Phe; (S)-beta(3) homophenylalanine] by x-ray diffraction. The polypeptide chain reversal is nucleated by the centrally positioned (D)Pro-Gly segment, which adopts a type-I' beta-turn conformation. Four intramolecular cross-strand hydrogen bonds stabilize the peptide fold. The beta Phe(3) and beta Phe(8) residues occupy facing positions on the hairpin, with the side chains projecting on opposite faces of the beta-sheet. At the site of insertion of beta-residues, the polarity of the peptide units along each strand reverses, as compared with the alpha-peptide segments. In this analog, a small segment of a polar sheet is observed, where adjacent CO and NH groups line up in opposite directions in each strand. In the crystal, an extended beta-sheet is formed by hydrogen bonding between strands of antiparallel pairs of beta-hairpins. The crystallographic parameters for C(65)H(102)N(10)O(13) x 3H(2)O are: space group P2(1)2(1)2(1); a = 19.059(8) A, b = 19.470(2) A, c = 21.077(2) A; Z = 4; agreement factor R(1) = 9.12% for 3,984 data observed >4 sigma(F) and a resolution of 0.90 A.
通过X射线衍射，在十肽叔丁氧羰基 - 亮氨酸 - 缬氨酸 - β苯丙氨酸 - 缬氨酸 - （D）脯氨酸 - 甘氨酸 - 亮氨酸 - β苯丙氨酸 - 缬氨酸 - 甲酯[β苯丙氨酸；（S）-β(3) 高苯丙氨酸]的晶体中表征了一种β发夹构象。多肽链的反转由位于中心位置的（D）脯氨酸 - 甘氨酸片段引发，该片段采用I'型β转角构象。四个分子内跨链氢键稳定了肽折叠。β苯丙氨酸(3)和β苯丙氨酸(8)残基占据发夹上相对的位置，侧链伸向β折叠的相反面。与α肽段相比，在β残基插入位点，每条链上肽单元的极性发生反转。在这个类似物中，观察到一小段极性片层，其中相邻的羰基和氨基在每条链中沿相反方向排列。在晶体中，反平行β发夹对的链之间通过氢键形成延伸的β折叠。C(65)H(102)N(10)O(13)·3H(2)O的晶体学参数为：空间群P2(1)2(1)2(1)；a = 19.059(8) Å，b = 19.470(2) Å，c = 21.077(2) Å；Z = 4；对于3984个观测值大于4σ(F)且分辨率为0.90 Å的数据，一致性因子R(1) = 9.12%。
