1H NMR spectrum and computational study of firefly luciferin in dimethyl sulfoxide

Experimental and theoretical 1H NMR chemical shift values (δs) of firefly luciferin (Ln) in dimethyl sulfoxide (DMSO) were studied for the estimation of the detailed structure of Ln and the characteristics of the interaction between Ln and DMSO. The 1H chemical shifts were recorded in DMSO-d6. The temperature dependence of δobs.s of Ln of the 1H NMR chemical shifts was also measured. The theoretical NMR chemical shifts (δcalc.s) were calculated based on the GIAO DFT approach, using the 6-311+G(2d,p) basis set (a total of 544 contracted basis functions), implemented in the Gaussian software package. The solvent effect was obtained by means of the Onsager reaction field model or an Ln···DMSO hydrogen-bonded complex model. The calculated results of the Ln···DMSO complex model are in good agreement with the observed chemical shifts, but all observed 1H peaks could not be reproduced for the optimized structure of Ln using the Onsager reaction field model. The considerably large δobs.s of -H and -OOH protons of Ln in the observed 1H NMR spectra are probably explained in terms of hydrogen-bonded complex model between protons in Ln and oxygen atoms in DMSO molecules, rather than in the Onsager reaction field model.