G-quadruplex-based ultrasensitive and selective detection of histidine and cysteine.

Histidine and cysteine detection is critically important since an abnormal level of histidine or cysteine is an indicator for many diseases. In this paper, we demonstrated a novel label-free, G-quadruplex-based approach for simultaneous detection of histidine and cysteine. The present assay is based on the highly specific interaction among amino acids (histidine or cysteine), Cu(2+) and NMM/G-4 (NMM: N-methylmesoporphyrin IX; G-4: G-quadruplex). The fluorescence intensity of NMM was dramatically enhanced in the presence of G-quadruplex formed from 24GT, which can be effectively quenched by cupric ion (Cu(2+)) due to the chelation of Cu(2+) by NMM as well as the unfolding of G-quadruplex by Cu(2+). The presence of histidine or cysteine will disturb the interaction between Cu(2+) and NMM/G-4 because of the strong binding affinity of Cu(2+) to the imidazole group of histidine or the interaction of Cu(2+) with thiol group in cysteine, leading to distinct fluorescence emission intensity. High selectivity is conferred by the use of cysteine-masking agent N-ethylmaleimide (NEM), which helps to discriminate histidine from cysteine. At last, a novel and simple approach was developed to determine each precise concentration of histidine and cysteine according to the different response of the system with and without NEM. Importantly, histidine can be also detected even in the presence of a large amount of other amino acids. A detection limit as low as 3 nM for histidine and 5 nM for cysteine was obtained by practical measurement rather than conventional calculation (S/N=3), confirming the high sensitivity of the present approach. Meanwhile, this sensing protocol can determine histidine and cysteine in diluted biological samples such as urine, exhibiting great potential to meet the need of practical application.