“…Such behavior by 1 is found in a wide variety of solvents of wide-ranging polarity and hydrogen bonding ability, from benzene to methanol to N -methylformamide . Only in dimethyl sulfoxide are the CD Cotton effects greatly diminished, an observation attributed to intercalation of solvent molecules into the matrix of intramolecular hydrogen bonds (Figure B). ,,, 2 Circular dichroism spectra of ∼10 -5 M (β R ,β‘ R )-dimethylmesobilirubin-XIIIα ( 1 ) in chloroform (spectrum 1, Δ +337, Δ −186), methanol (spectrum 2, Δ +285, Δ −177), and dimethyl sulfoxide (spectrum 3, Δ −23, Δ +6) solvents at 23 °C.…”
The bichromophoric pigment bilirubin acts as a molecular exciton in its UV−visible and circular
dichroism (CD) spectroscopy. The optically active analogue, (βR,β‘R)-dimethylmesobilirubin-XIIIα exhibits
intense bisignate CD Cotton effects in the region of its long wavelength UV−vis absorption near 400 nm,
with Δ
+337, Δ
−186 in the nonpolar solvent CHCl3, and nearly as intense Cotton effects in the
polar, hydrogen bonding solvent CH3OH: Δ
+285, Δ
−177. Addition of amines leads to Cotton
effect sign inversions: in isopropylamine Δ
−605, Δ
+375, due to an inversion of molecular
chirality.
“…Such behavior by 1 is found in a wide variety of solvents of wide-ranging polarity and hydrogen bonding ability, from benzene to methanol to N -methylformamide . Only in dimethyl sulfoxide are the CD Cotton effects greatly diminished, an observation attributed to intercalation of solvent molecules into the matrix of intramolecular hydrogen bonds (Figure B). ,,, 2 Circular dichroism spectra of ∼10 -5 M (β R ,β‘ R )-dimethylmesobilirubin-XIIIα ( 1 ) in chloroform (spectrum 1, Δ +337, Δ −186), methanol (spectrum 2, Δ +285, Δ −177), and dimethyl sulfoxide (spectrum 3, Δ −23, Δ +6) solvents at 23 °C.…”
The bichromophoric pigment bilirubin acts as a molecular exciton in its UV−visible and circular
dichroism (CD) spectroscopy. The optically active analogue, (βR,β‘R)-dimethylmesobilirubin-XIIIα exhibits
intense bisignate CD Cotton effects in the region of its long wavelength UV−vis absorption near 400 nm,
with Δ
+337, Δ
−186 in the nonpolar solvent CHCl3, and nearly as intense Cotton effects in the
polar, hydrogen bonding solvent CH3OH: Δ
+285, Δ
−177. Addition of amines leads to Cotton
effect sign inversions: in isopropylamine Δ
−605, Δ
+375, due to an inversion of molecular
chirality.
“…However, when the polar aprotic solvent is capable of participating in hydrogen bonding as in (CH 3 ) 2 SO, the Cotton effect intensities of 1 − 12 drop to <10% of the values in nonpolar solvents, and in 1 − 10 the signs are reversed. Solvent molecules are apparently involved in the network of intramolecular hydrogen bonds, and NMR and CD studies suggest that the propionic acids are linked to the dipyrrinones by (CH 3 ) 2 SO. It seems probable that the favored folded conformation has become somewhat more open (larger θ angle in Figure ) to accommodate attachment of the solvent molecules.…”
The relative steric size of methyl, ethyl, isopropyl, tert-butyl, phenyl, and benzyl groups has been
determined from a sensitive tetrapyrrole model and exciton coupling circular dichroism (CD) measurements.
Unlike the classical cyclohexane model, from which the relative steric demand of functional groups has been
assessed quantitatively (A-values) and is based on the preference for equatorial vs axial orientations, the bilirubin
model assesses substituent size from head-to-head steric compression. Thus, exciton CD amplitudes of a set
of sensitive anti-chiral α(R/S)-substituted-β‘(S)-methylmesobilirubins-XIIIα (1−6) suggests an apparent relative
steric size: tert-butyl ∼ isopropyl > phenyl ∼ ethyl > benzyl > methyl. The order is somewhat different
from that obtained by measuring equatorial vs axial configuration preferences on substituted cyclohexanes by
NMR spectroscopy: tert-butyl ≫ phenyl > isopropyl > ethyl ∼ benzyl ∼ methyl.
“…Analysis of 3 J HH in the propionic acid chains clearly shows the ABCX pattern of the ®xed staggered propionic acid geometry in 1±4, thus providing strong supporting experimental evidence for folded ridge-tile structures. On the other hand, the less complicated A 2 B 2 pattern found in (CD 3 ) 2 SO (Table 2) indicates more motional freedom in the propionic acid segment, whose CO 2 H groups are thought to be linked to the dipyrrinones via bound solvent molecules [17,19,21,22]. Taken collectively, the NMR data support intramolecularly hydrogen-bonded ridge-tiles for 1±4 ( Figure 3) and a ®xed staggered conformation of the propionic acid segments.…”
Section: Analysis Of Hydrogen Bonding and Conformation By 1 H Nmrmentioning
Analogs of bilirubin with vinyl groups replaced by symmetrically-disposed o-¯uoro-phenyls (1, bis-exo, and 2, bis-endo) were synthesized and characterized spectroscopically. Their 1 H NMR spectra and NOE data are consistent with an intramolecularly hydrogen-bonded ridge-tile conformation where each propionic acid group embraces an opposing dipyrrinone. Like bilirubin, 1 and 2 exhibit negative chirality induced circular dichroism (ICD) Cotton effects in chloroform containing quinine. Unlike bilirubin, however, in aqueous buffer containing human serum albumin, 2 exhibits a negative exciton chirality ICD, whereas that of 1 is positive.
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