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Z.S. MARKOVIĆ et al.: CHARACTERIZATION OF PURPURIN ISOLATED FROM R.tinctorum Hem. ind. 67 (1) 77–88 (2013) Figure 3. Experimental (top) and calculated (bottom) UV spectra for purpurin. nm. This is essentially a HOMO–LUMO transition, involving excitation from π to π*. The shapes of the orbitals are shown in Figure 4. They indicate that the transition is associated with significant charge-transfer form the ring B to the ring A, similar to the situation characterizing the color bands in related compounds, such as chrysazin and anthralin, and the first strong UV–vis band of the parent anthraquinone chromophore [54]. 82 Similar charge transfer from the ring A to the ring B is observed for peak B, whose theoretical maximum is predicted at 305 nm. This band is in good agreement with the experimental polarization value of 291 nm. This transition involves excitation from HOMO-4 to LUMO. The experimental value for peak C at 252 nm is in excellent agreement with the calculated electronic transition of 254 nm. This peak mainly represents excitation from HOMO-1 to LUMO+1.
Z.S. MARKOVIĆ et al.: CHARACTERIZATION OF PURPURIN ISOLATED FROM R.tinctorum Hem. ind. 67 (1) 77–88 (2013) Table 4. Experimental and calculated electronic transitions for purpurin; λ – wave numbers in nm, f – oscillator strength Case Experimental TD- B3LYP/6-311+G(d,p) λ Λ f Leading configurations A 482 466.9 0.2028 H→L(98%) B 291 305.3 0.1675 H-4→L(81%) C 252 254.2 0.6015 H-1→L+1(69%) H→L+2(17%) Figure 4. Selected molecular orbitals for purpurin. The experimental and calculated 1 H- and 13 C-NMR spectra of purpurin are presented in Table 5. Obviously, there is a very good agreement between the experimental and theoretical values for the chemical shifts of the proton spectrum. Scaled theoretical values (scaling factor 0.953) for 13 C chemical shifts are also in accord with the experimental data. The agreement between the experimental and theoretical results indicates that the structure of purpurin used for the UV–vis and NMR measurements corresponds to that of 1 (Figure 2). It is well known that some anthraquinones are very good antioxidants. The structure of purpurin indicates a possibility of conjugation and antioxidant properties. This assumption is based on the suggestion of van Acker et al. [55] and Rice-Evans et al. [56], who supposed that the antioxidant properties of flavonoids could be derived just from their good delocalization possibilities. The same consideration can be applied to anthraquinones. Bearing this in mind, purpurin was screened for its antioxidant potential using the relative ability to scavenge the ABTS +• radical cation generated in the aqueous phase, expressed as the Trolox equivalent antioxidant capacity (TEAC). Purpurin was found Table 5. Experimental and theoretical chemical shifts (in ppm) for purpurin molecule Atom Exp. Calcd. Atom Exp. Calcd. C1 157.1 152.8 C12 109.7 111.3 C2 149.3 152.3 C13 133.3 141.6 C3 126.6 116.1 C14 134.5 138.9 C4 160.3 169.2 H1 13.2 13.5 C5 126.2 133.3 H2 11.6 11.3 C6 134.1 140.8 H3 6.5 6.4 C7 134.9 139.1 H4 12.9 13.4 C8 126.5 133.1 H5 8.0 8.6 C9 183.3 194.5 H6 7.8 7.5 C10 186.6 190.0 H8 7.8 7.9 C11 112.4 118.4 H8 8.1 8.7 83
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