Roberts J K, Wade-Jardetzky N, Jardetzky O
Biochemistry. 1981 Sep 15;20(19):5389-94. doi: 10.1021/bi00522a006.
Titration curves plotting chemical shift vs. pH for inorganic phosphate and glucose 6-phosphate in solutions of various composition are presented. Physiological concentrations of K+ (0.1 M) and Mg2+ (5 mM) are shown to significantly shift the titration curve. The Mg2+ effect can be partly or completely reversed by addition of sufficient quantities of adenosine triphosphate or organic acids. The acidic protein bovine serum albumin and soluble maize root tip protein have no noticeable effect on the titration curves, whereas the basic protein protamine exerts a profound effect. The results clearly indicate that knowledge of intracellular ionic strength and free Mg2+ concentrations in the sample are required if the determination of intracellular pH by 31P NMR is to be considered accurate within +/- 0.05-01 pH unit.
给出了在不同组成溶液中，无机磷酸盐和6-磷酸葡萄糖的化学位移与pH值的滴定曲线。结果表明，生理浓度的K+（0.1M）和Mg2+（5mM）会显著改变滴定曲线。加入足量的三磷酸腺苷或有机酸可部分或完全逆转Mg2+的作用。酸性蛋白牛血清白蛋白和可溶性玉米根尖蛋白对滴定曲线无明显影响，而碱性蛋白鱼精蛋白则有显著影响。结果清楚地表明，如果要通过31P NMR准确测定细胞内pH值（误差在±0.05 - 0.1pH单位内），就需要了解样品中的细胞内离子强度和游离Mg2+浓度。
