Electrochemical assay for highly charged polyamino acids: Application to polyamino acid functionalized microfiltration membranes

A simple and sensitive assay for polyamino acids (e.g., poly-L-glutamic and poly-L-aspartic acids) is described. The assay is performed by titrating anionic polyamino acids with a poly-L-arginine titrant. Earlier reported lower molecular weight titrants, such as protamine, did not interact with the highly negatively charged polyamino acids, and thus were not suitable for this analysis. The increase of poly-L-arginine concentration at the end point is monitored by utilizing a polycation-sensitive membrane sensor as an indicator electrode in a potentiometric set-up. Polyamino acids such as poly-L-glutamic and poly-L-aspartic acids are titrated down to 5 μg/mL with large potential changes at the end point (ca. 160 mV). The effect of the polyamino acid molecular weight and pH of the sample solution are also described. The method is applied for the evaluation of poly-L-glutamic acid functionalized cellulosic membranes.