Fabri Deborah, Williams Martin A K, Halstead Thomas K
Department of Chemistry, University of York, Heslington, York YO19 6AL, UK.
Carbohydr Res. 2005 Apr 11;340(5):889-905. doi: 10.1016/j.carres.2005.01.034.
1H spin-spin relaxation times of water were measured with the CPMG sequence in dilute aqueous solutions of glucitol, mannitol, glycerol, glycol, the methyl D-pyranosides of alpha-glucose, beta-glucose, alpha-galactose, beta-galactose, alpha-xylose, beta-xylose, beta-arabinose and sucrose, alpha,alpha-trehalose, beta-maltose, maltotriose and maltoheptaose. The relaxation-time dispersion was measured by varying the CPMG pulse spacing, tau. These data were interpreted by means of the Carver-Richards model in which exchange between water protons and labile solute hydroxyl protons provides a significant contribution to the relaxation. From the dependences on temperature and tau, parameters characteristic of the pool of hydroxyls belonging to a given solute were extracted by nonlinear regression, including: the fraction of exchangeable protons, P, the chemical-shift difference between water protons and hydroxyl protons, deltaomega, the intrinsic spin-spin relaxation time, T2, and the chemical exchange rate, k. These solute-specific parameters are related, respectively, to the concentration, identity, mobility and exchange life-time of the hydroxyl site. At 298 K, values of deltaomega, T2 and k were found to be of the order of 1 ppm, 100 ms and 1000 s(-1), respectively. Effects of molecular size, conformation and solute concentration were investigated. The exchange mechanism was characterised by Eyring activation enthalpies and entropies with values in the ranges 50-70 kJ mol(-1) and -10 to 60 J K(-1)mol(-1), respectively.
在葡糖醇、甘露醇、甘油、二醇、α-葡萄糖、β-葡萄糖、α-半乳糖、β-半乳糖、α-木糖、β-木糖、β-阿拉伯糖和蔗糖、α,α-海藻糖、β-麦芽糖、麦芽三糖和麦芽七糖的稀水溶液中，使用CPMG序列测量了水的1H自旋-自旋弛豫时间。通过改变CPMG脉冲间隔τ来测量弛豫时间色散。这些数据通过Carver-Richards模型进行解释，其中水质子与不稳定溶质羟基质子之间 的交换对弛豫有显著贡献。根据对温度和τ的依赖性，通过非线性回归提取了属于给定溶质的羟基池的特征参数，包括：可交换质子的分数P、水质子与羟基质子之间的化学位移差δω、固有自旋-自旋弛豫时间T2和化学交换速率k。这些溶质特异性参数分别与羟基位点的浓度、特性、迁移率和交换寿命相关。在298K时，发现δω、T2和k的值分别约为1ppm、100ms和1000s-1。研究了分子大小、构象和溶质浓度的影响。交换机制由Eyring活化焓和熵表征，其值分别在50-70kJ mol-¹和-10至60J K-¹mol-¹范围内。
