Nesmelova I, Krushelnitsky A, Idiyatullin D, Blanco F, Ramirez-Alvarado M, Daragan V A, Serrano L, Mayo K H
Department of Biochemistry, Molecular Biology, and Biophysics, University of Minnesota Health Science Center, 321 Church Street, Minneapolis, Minnesota 55455, USA.
Biochemistry. 2001 Mar 6;40(9):2844-53. doi: 10.1021/bi001293b.
13C-NMR relaxation experiments (T(1), T(2), T(1)(rho), and NOE) were performed on selectively enriched residues in two peptides, one hydrophobic staple alpha-helix-forming peptide GFSKAELAKARAAKRGGY and one beta-hairpin-forming peptide RGITVNGKTYGR, in water and in water/trifluoroethanol (TFE). Exchange contributions, R(ex), to spin-spin relaxation rates for (13)C(alpha) and (13)C(beta) groups were derived and were ascribed to be mainly due to peptide folding-unfolding. To evaluate the exchange time, tau(ex), from R(ex), the chemical shift difference between folded and unfolded states, Deltadelta, and the populations of these states, p(i), were determined from the temperature dependence of (13)C chemical shifts. For both peptides, values for tau(ex) fell in the 1 micros to 10 micros range. Under conditions where the peptides are most folded (water/TFE, 5 degrees C), tau(ex) values for all residues in each respective peptide were essentially the same, supporting the presence of a global folding-unfolding exchange process. Rounded-up average tau(ex) values were 4 micros for the helix peptide and 9 micros for the hairpin peptide. This 2-3-fold difference in exchange times between helix and hairpin peptides is consistent with that observed for folding-unfolding of other small peptides.
对两种肽中选择性富集的残基进行了¹³C核磁共振弛豫实验（T₁、T₂、T₁ρ和NOE），一种是疏水钉状α-螺旋形成肽GFSKAELAKARAAKRGGY，另一种是β-发夹形成肽RGITVNGKTYGR，实验分别在水中以及水/三氟乙醇（TFE）中进行。推导了¹³Cα和¹³Cβ基团自旋-自旋弛豫速率的交换贡献R(ex)，并认为其主要归因于肽的折叠-去折叠。为了从R(ex)评估交换时间τ(ex)，根据¹³C化学位移的温度依赖性确定了折叠态和未折叠态之间的化学位移差Δδ以及这些状态的丰度p(i)。对于这两种肽，τ(ex)值都在1微秒到10微秒的范围内。在肽折叠程度最高的条件下（水/TFE，5℃），每种肽中所有残基的τ(ex)值基本相同，这支持了全局折叠-去折叠交换过程的存在。向上取整后，螺旋肽的平均τ(ex)值为4微秒，发夹肽的平均τ(ex)值为9微秒。螺旋肽和发夹肽在交换时间上2至3倍的差异与其他小肽折叠-去折叠的情况一致。
