Chiral Carbon Paste Electrode with (+)-TCA/Graphene for Voltammetric Discrimination of Tryptophan Isomers

We report for the first time the application of intrinsic chirality of RRRR (+) - (18-Crown-6) − 2, 3, 11, 12- tetra carboxylic acid ((+) TCA) along with graphene nanosheets (GN) in a chiral carbon paste electrode (CCPE) system for the voltammetric discrimination of tryptophan (TRY) isomers. Embedding (+) TCA in GN enhances rigidity and forms tripodal hydrogen bonds with amine groups of L-TRY and D-TRY, providing the sensor high stereoselectivity. Experimental and quantum chemical computational studies confirmed the stronger affinity of (+) TCA for L-TRY over D-TRY using the differential pulse voltammetry (DPV) and the counterpoise method. The stability constants (1:1 complex) of L-TRY and D-TRY with (+) TCA were 1191 M −1 and 588 M −1 , respectively. Additionally, the peak potential difference (ΔE p = E L —E D ) between enantiomers on CCPE was 129 mV. The lowest energy conformation of (+) TCA-L-TRY has a Gibbs free energy higher than (+) TCA-D-TRY by 7.08 kcal mol −1 (B3LYP) or 9.10 kcal mol −1 (M06–2X), indicating a preference for the L enantiomer. The detection limits for L-TRY and D-TRY are 0.023 μM and 0.075 μM, respectively, with a linear range from 1 μM to 120 μM. The sensor quantified TRY in blood serum and urine samples.