Amide-, amine-, and hydroxyl-water proton exchange can generate MRI contrast through chemical exchange saturation transfer (CEST). In this study, we show that thiol-water proton exchange can also generate quantifiable CEST effects under near-physiological conditions (pH = 7.2 and 37°C) through the characterization of the pH dependence of thiol proton exchange in phosphate-buffered solutions of glutathione, cysteine, and N-acetylcysteine. The spontaneous, base-catalyzed, and buffer-catalyzed exchange contributions to the thiol exchange were analyzed. The thiol-water proton exchange of glutathione and cysteine was found to be too fast to generate a CEST effect around neutral pH due to significant base catalysis. The thiol-water proton exchange of N-acetylcysteine was found to be much slower, yet still in the fast-exchange regime with significant base and buffer catalysis, resulting in a 9.5% attenuation of the water signal at pH 7.2 in a slice-selective CEST NMR experiment. Furthermore, the N-acetylcysteine thiol CEST was also detectable in human serum albumin and agarose phantoms.

中文翻译：

---

谷胱甘肽，半胱氨酸和N-乙酰半胱氨酸的巯基-水质子交换：对CEST MRI的影响。

酰胺，胺和羟基水质子交换可通过化学交换饱和转移（CEST）产生MRI对比。在这项研究中，我们表明巯基水质子交换还可以通过表征谷胱甘肽磷酸盐缓冲溶液中巯基质子交换的pH依赖性来表征在接近生理条件（pH = 7.2和37°C）下可量化的CEST效应。 ，半胱氨酸和N-乙酰半胱氨酸。分析了自发的，碱催化的和缓冲催化的交换对硫醇交换的贡献。由于明显的碱催化作用，谷胱甘肽和半胱氨酸的硫醇-水质子交换太快而无法在中性pH左右产生CEST效应。N-乙酰半胱氨酸的硫醇-水质子交换被发现要慢得多，但在切片选择性CEST NMR实验中，在碱性和缓冲液催化作用下仍处于快速交换状态，导致pH值为7.2时水信号衰减9.5％。此外，在人血清白蛋白和琼脂糖体模中也可检测到N-乙酰半胱氨酸硫醇CEST。