Hartings Matthew R, Kinnear Brian S, Jarrold Martin F
Chemistry Department, Indiana University, 800 East Kirkwood Avenue, Bloomington, Indiana 47405-7102, USA.
J Am Chem Soc. 2003 Apr 2;125(13):3941-7. doi: 10.1021/ja020610u.
Ion mobility measurements have been used to examine the conformations present for unsolvated Ac-(AG)(7)A+H(+) and (AG)(7)A+H(+) peptides (Ac = acetyl, A = alanine, and G = glycine) over a broad temperature range (100-410 K). The results are compared to those recently reported for Ac-A(4)G(7)A(4)+H(+) and A(4)G(7)A(4)+H(+), which have the same compositions but different sequences. Ac-(AG)(7)A+H(+) shows less conformational diversity than Ac-A(4)G(7)A(4)+H(+); it is much less helical than Ac-A(4)G(7)A(4)+H(+) at the upper end of the temperature range studied, and at low temperatures, one of the two Ac-A(4)G(7)A(4)+H(+) features assigned to helical conformations is missing for Ac-(AG)(7)A+H(+). Molecular dynamics simulations suggest that the different conformational preferences are not due to differences in the stabilities of the helical states, but differences in the nonhelical states: it appears that Ac-(AG)(7)A+H(+) is more flexible and able to adopt lower energy globular conformations (compact random looking three-dimensional structures) than Ac-A(4)G(7)A(4)+H(+). The helix to globule transition that occurs for Ac-(AG)(7)A+H(+) at around 250-350 K is not a direct (two-state) process, but a creeping transition that takes place through at least one and probably several intermediates.
离子迁移率测量已被用于研究在较宽温度范围（100 - 410 K）内未溶剂化的Ac-(AG)₇A + H⁺和(AG)₇A + H⁺肽（Ac = 乙酰基，A = 丙氨酸，G = 甘氨酸）所呈现的构象。将这些结果与最近报道的Ac-A₄G₇A₄ + H⁺和A₄G₇A₄ + H⁺的结果进行比较，这两种肽具有相同的组成但不同的序列。Ac-(AG)₇A + H⁺的构象多样性比Ac-A₄G₇A₄ + H⁺少；在所研究温度范围的上限，它的螺旋结构比Ac-A₄G₇A₄ + H⁺少得多，并且在低温下，Ac-(AG)₇A + H⁺缺少了Ac-A₄G₇A₄ + H⁺中被指定为螺旋构象的两个特征之一。分子动力学模拟表明，不同的构象偏好不是由于螺旋态稳定性的差异，而是由于非螺旋态的差异：似乎Ac-(AG)₇A + H⁺比Ac-A₄G₇A₄ + H⁺更灵活，能够采用能量更低的球状构象（紧凑的随机三维结构）。Ac-(AG)₇A + H⁺在约250 - 350 K发生的螺旋到球状转变不是一个直接的（两态）过程，而是一个通过至少一个且可能几个中间体发生的渐进转变。
