Folding and unfolding of the tryptophan zipper in the presence of two thioamide substitutions

We studied the stability and folding and unfolding kinetics of the tryptophan zipper, containing dierent double thioamide subsitutions. Conformation change was triggered by photoisomerization of an integrated AMPP photoswitch in the turn region of the hairpin, and transient spectra were recorded in the deep UV and the mid-IR, covering the time window of the (un)folding transition from picoseconds to tens of microseconds. Thio-substitution of inward-pointing backbone carbonyls was found to strongly destabilize the β-hairpin structures, whereas molecules with two outward pointing thio-carbonyls showed similar or enhanced stability with respect to the unsubstituted sequence, which we attribute to stronger interstrand hydrogen bonding. Thiolation of the two Trp residues closest to the turn can even prevent the opening of the hairpin after cis-trans isomerization of the switch. The circular dichroism due to the two thioamide ππ∗ transitions is spectrally well-separated from the aromatic tryp...

本研究针对带有不同双硫代酰胺（thioamide）取代修饰的色氨酸拉链（tryptophan zipper）体系的稳定性、折叠与解折叠动力学展开了探究。该β发夹结构转折区整合的AMPP光开关（AMPP photoswitch）发生顺反光异构化，以此触发构象变化；实验通过深紫外（deep UV）与中红外（mid-IR）波段的瞬态光谱，覆盖了从皮秒至数十微秒的（解）折叠过渡时间窗口。研究发现，指向内侧的主链羰基发生硫代取代后，会显著破坏β发夹结构的稳定性；而带有两个指向外侧的硫代羰基的体系，相较于未取代的原始序列，稳定性反而相似甚至有所提升，我们将这一现象归因于更强的链间氢键（interstrand hydrogen bonding）作用。紧邻转折区的两个色氨酸（Trp）残基发生巯基化修饰后，甚至可在光开关发生顺反异构后，阻止发夹结构的解折叠。由两个硫代酰胺ππ*跃迁产生的圆二色性（circular dichroism）信号，在光谱上与芳香族色氨酸残基的相关信号实现了良好分离——