ABSTRACT
Hydroquinone-based derivatization reagents have also been suggested for reaction with primary and/or secondary amines to form electroactive products. Recently two new reagents for this purpose were developed [141]. The most promising is NDTE. The reaction scheme for the NDTE derivatization of peptides is shown in Figure 9 [141]. Maximum yield for step 1 is achieved in approximately 30 min at 25°C; 3 hours is required for maximum yield to be achieved for step 2. Once formed, the derivative is stable for one day when stored in an acidic environment. One drawback associated with NDTE is that it is converted to electroactive homogentisic acid during the derivatization process, which may cause interference in trace chromatographic analyses. Nonetheless, oxidation at +0.30 V versus Ag/AgCl allows for determination of Ile-Leu-OMe with detection limits in the 250 nM range [141]. NDTE has also been used as a derivatization reagent for analysis of angiotensin II [142]. Using a radial-flow thin-layer electrochemical detection cell at a potential of +0.25 V versus. Ag/AgCl, detection of angiotensin II is accomplished with a detection limit of 62.5 nM. A wide separation window between the derivatization agent and the analyte is achieved using gradient elution, thereby eliminating interference from NDTE. Determination of angiotensin II transport across an in vitro cell culture model of the blood-brain barrier is possible, with mean concentrations over 2 hours in the range of 80 to 495 nM.
