N₂₁,N₂₂-carbonyl-bridged biliverdin. red-blue color change effected by conformation

Abstract

An N 21 ,N 22 -carbonyl-bridged mesobiliverdin, prepared in high yield by reaction of the unbridged parent (λ max 639 nm, ε 15,700, chloroform) with 1,1 '-carbonyldiimidazole and 1,8-diazabicyclo[5.4.0]undec-7-ene, gave magenta-colored solutions in chloroform that absorb strongly in the visible spectrum (λ max 534 nm, ε 27,700) and shifted to bright blue (λ max 669 nm, ε 35,300) upon addition of trifluoroacetic acid.J. Heterocyclic Chem., 42, 161 (2005).Reaction of dipyrrinones, such as methyl xanthobilirubinate (Scheme 1A) with 1,1'-carbonyldiimidazole (CDI) in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) has been shown to give the N,N'-carbonyl-bridged dipyrrinones in excellent yield [1]. The latter are highly fluorescent (φ F~0 .7-0.8) [2] but the long wavelength absorption λ max (428 nm, chloroform) shows a 20 nm bathochromic shift from that of the parent (λ max 406 nm, chloroform) [1,3]. A similar reaction with blue-colored (λ max 639 nm, chloroform) mesobiliverdin-XIIIα dimethyl ester (Scheme 1B) afforded the N 21 ,N 22 -bridged verdin (1) in 76% yield. The new "verdin" was neither blue nor green, but exhibited an intense magenta color in chloroform solution (λ max 534 nm) associated with a strong hypsochromic shift relative to the parent. It was also non-fluorescent to the eye. The absence of visible fluorescence at room temperature was confirmed by instrumental measurements in chloroform and trifluoroacetic acid following independent excitation of the three uv-visible absorption bands.The structure of 1 follows from that of its symmetric verdin precursor [4]. The 13 C-nmr (Table 1) (Table 1), due to a rapid prototropic shift, N 22 -H to N 23 , tautomerism.) The 13 C nmr assignments and structure were further confirmed by long range heteronuclear correlation experiment (gHMBC). In particular, the hydrogen at C(5) appeared deshielded to 6.49 ppm as it does in xanthoglow methyl ester (Scheme 1A) and is correlated ( 2 J) to the carbon signals at 132.5 ppm C(4) and 130.2 ppm C(6) and ( 3 J) to C(3) at 146.6 ppm and C(7) at 123.0 ppm. In the opposite half of the molecule, the hydrogen at C(15) is with normal chemical shift at 5.84 ppm and correlated to C(13), C(14), C(16) and C(17). The 2 J correlation C(15)-H to C(14) and the unique 3 J correlation C(13 1 )-CH 3 to C(14) is interesting because it reveals that C(14) is strongly deshielded (to 169.7 ppm) relative to C(6) (130.2 ppm). The carbon and proton nmr spectral data are thus consistent with structure 1 for the new bridged verdin.A striking color change, from magenta to bright blue was seen when solutions of 1 were exposed to acid, e.g., chloroform and dimethyl-sulfoxide solutions plus trifluoroacetic acid. The color change is readily measured spectrophotometrically, as seen in the uv-visible spectra of 1 (Figure 1) in a variety of solvents, although solvents such as tetrahydrofuran, dimethylformamide and dimethylsulfoxide require larger excess of trifluoroacetic acid than do benzene, chloroform, etc. No similar large wavelength changes are det...