Fig.1

The molecular structure of NAA

Fig.2

(a) Optimal control (OC/OC′) pulse sequence for preparing and converting nuclear spin singlet state. The combination of CW and gradient field can remove spin coherent-states and retain nuclear spin singlet state. (b) The amplitude of OCI pulse versus time while OCI works; (c) The phase of OCI pulse versus time while OCI works

Fig.3

(a) The evolution trajectory of the spin states in a two-spin system (assume no J-coupling between Hb and Ha, Ha') operated by the pulse OCI; (b) The evolution trajectory of the spin states in an actual three-spin system operated by the pulse OCI'; (c) The evolution trajectory of the spin states in a three-spin system operated by the pulse OCI

Fig.4

(a) Singlet-state-COSY pulse sequence; (b) Singlet-state-NOESY pulse sequence. The pulse sequence OC′$ $before t1 evolution is for preparing and transferring the singlet state, and the following pulse sequence corresponds to the traditional COSY/NOESY

Fig.5

(a) Simulated singlet state spectra of NAA solution with pH of 2.3 obtained by (1) OC pulse sequence, (2) OC$ \mathrm{\text{'}} $ pulse sequence, and its (3) simulated and (4) experimental single pulse 1H NMR spectra; (b) Four simulated singlet state spectra using OC$ \mathrm{\text{'}} $ pulse sequence with offsetting the radio frequency center O1 of (1) 10 Hz, (2) 5 Hz, (3) ?5 Hz, (4) ?10 Hz, respectively; (c) Simulated (1) singlet state spectrum with OC′′ pulse and (2) single pulse 1H NMR spectrum of NAA solution with pH of 5.5

Fig.6

(a) The TS decay curve of the singlet state of Ha, Ha′ in NAA, TS=4.81±0.23 s; (b) The T1 decay curve of Ha、Ha′ in NAA, T1=0.73±0.03 s; (c) Experimental 1D NMR spectrum (the sample pH=2.3) of the nuclear spin singlet state. The gradient pulse and CW pulse have the effect of signal selection. The signals of Ha and Ha′ are reserved, while the other signals are suppressed

Fig.7

(a) Traditional COSY spectrum of the NAA and glucose mixture in D2O; (b) Singlet-state-COSY spectrum of the NAA and glucose mixture in D2O; (c) 1H spectrum obtained by one FID of the singlet-state-COSY experiment

Fig.8

(a) Traditional and (b) singlet-state-NOESY spectra of the NAA and glucose mixture in D2O