Typical linear and porphyrin-like structure representations of bilirubin give an incorrect impression of its actual shape and expected solution properties. Conformational analysis of bilirubin, assisted by molecular dynamics computations, indicates that (i) nonbonded intramolecular steric interactions are minimized in a ridge-tile shape conformation lying at a global energy minimum on the conformational energy map and (ii) considerable additional stabilization is achieved through a networkof intramolecular hydrogen bonds. The linear and porphyrin-like conformations arecomputed to lie some 37-48 kcal/mol above the isoenergetic global minimum energy conformations, which correspond to superimpsable (identical) or to nonsuperimposable (enantiomeric) mirror image intramolecularly hydrogen-bonded ridge-tile conformers. Two different low-energy (1 9.5 and 2 1.4 kcal/mol) transition states can be identified on pathways for interconverting these conformational enantiomers. The conformation of bilirubin may be determined experimentally by UV-visible and, especially, circular dichroism (CD) spectroscopy. Such conformation dependent spectra arise from exciton coupling between the two dipyrrinone chromophores of bilirubin. Theoretical analysis using the exciton coupled oscillator model allowed a mapping of CD Ae for each bilirubin conformation of the conformational energy surface. An intense bisignate CD spectrum is predicted for the global energy minimum conformation with Cotton effects A€ N 1200 L mol-' cm-1 for the long wavelength UV-visible absorption near 450 nm. Surprisingly, Cotton effect sign reversals without inversion of molecular absolute configuration are predicted when the ridge-tile conformations are flattened somewhat into higher energy structures.

Four different hexahelicenes, 5-aza-hexahelicene (1), hexahelicene (2), 2-methyl-hexahelicene (3), and 2-bromo-hexahelicene (4), were prepared and their enantiomers, which are stable at r.t., were separated. Vibrational circular dichroism (VCD) spectra were measured for compound 1; for all the compounds, electronic circular dichroism (ECD) and circularly polarized luminescence (CPL) spectra were recorded. Each type of experimental spectrum was compared with the corresponding theoretical spectrum, determined via Density Functional Theory (DFT). Following the recent papers by Nakai et al., this comparison allowed to identify some features related to the helicity and some other features typical of the substituent groups on the helical backbone. The Raman spectrum of compound 1 is also examined from this point of view

Blue light converts bilirubin in the skin of jaundiced rats to metastable geometric isomers that are transported in blood and excreted in bile. The same reaction probably occurs in jaundiced babies exposed to light, particularly during treatment with phototherapy. Excretion of unisomerized bilirubin is prevented by intramolecular hydrogen bonding, and the pigment has to be metabolized to more polar derivatives to be excreted efficiently.

No abstract

The absolute stereochemistry of hexahelicene (1) has been determined from an X-ray study of optically active 2-bromohexahelicene (2). Levorotatory 2-bromohexahelicene (2a) with a left-handed chirality was converted to levorotatory hexahelicene (la). Thus (-)-hexahelicene (la) also has a left-handed helical configuration. Hexahelicene(1 = la + lb) was first synthesized5 and resolved6•6 by Newman and Lednicer. Moscowitz later indicated7 that hexahelicene constitutes a prime example of a class of chromophores termed inherently dissymmetric.7'8 The dissymmetry of hexahelicene extends in a helical manner through the entire chromophore (molecule) as shown in structures 1 and 2.Although hexahelicene was resolved6 6 over 15 years ago, and calculations7•9-13 were performed to relate the sign of the optical activity and the stereochemistry of the molecule, until recently1 the absolute configuration remained undetermined.Detailed optical rotatory dispersion (ORD) and circular dichroism (CD) analyses of hexahelicene13-15 and sulfur heterohelicenes16 17 have appeared recently.(1) For a preliminary account of this work, see D. A. Lightner, D. T. Hefelfinger, G. W. Frank, T. W. Powers, and K. N. Trueblood, Nature (London), 233, 124 (1971).(2) Financial support by the National Science Foundation (Grant No. GP-9533 to D. A. L. and Grant No. GP-10949 to K. N. T.). Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research (ACS-PRF-4949 to D. A. L.),(3)

UV, circular dichroism (CD), fluorescence and circularly polarized luminescence (CPL) spectra were recorded for a set of four related [2.2.1] bicyclic compounds ((1S,4S)-and (1R,4R)-1,7,7-trimethylbicyclo[2.2.1]heptan-2-one, namely (1S)- and (1R)-camphor (), (1S,4R)-4,7,7-trimethylbicyclo[2.2.1]hept-5-en-2-one, (1S)-dehydro-epicamphor (), (1S,4S)-1,7,7-trimethylbicyclo[2.2.1]heptane-2,5-dione, (1S)-5-oxocamphor (), (1S,4R)- and (1R,4S)-1,7,7-trimethylbicyclo[2.2.1]heptane-2,3-dione, (1S)- and (1R)-camphorquinone ()) and a set of three related [2.2.2] bicyclic compounds (1S,4S)-bicyclo[2.2.2]octan-2,5-dione (saturated diketone ()), (1R,4R)-bicyclo[2.2.2]oct-7-en-2,5-dione (unsaturated diketone ()), ((1S,4S)-bicyclo[2.2.2]oct-7-en-5(S)-ol-2-one (which we refer to as unsaturated hydroxy-ketone ()). For the latter three compounds also mid-IR vibrational circular dichroism (VCD) spectra were recorded and are presented. Time-Dependent Density Functional (TD-DFT) calculations provide a satisfactory interpretation of both absorption and emission chiroptical spectra and permit insight into ground and excited state electronic properties. We discuss the applicability of the octant rule or of other approximated models to rationalize the observed sign of the CPL.

No abstract

Vibrational circular dichroism (VCD) and IR spectra have been recorded in the fingerprint and carbonyl stretching regions for endo,endo-bicamphor (1), exo,exo-bicamphor (2), endo,exo-bicamphor (3), 3,3'-bicamphorylidene (4), and exo,exo-bis-thiocamphor (5). The C2 symmetry possessed by these molecular systems (except in one case), as well as their limited conformational mobility associated with well-defined degrees of freedom, allow for optimal test of the vibrational circular dichroism exciton chirality (VCDEC) rule introduced by Taniguchi and Monde. Density functional theory calculations are employed not only to predict the entire aspect of the VCD and IR spectra but also to study how the VCDEC rule may be impacted by the coupling between C═O stretchings and from C═O stretchings with other vibrational modes and by the rotation about the C-C bond connecting the two camphors. Comments are provided about the limitations and potentialities of the VCDEC method and about the manifestation of different vibrational excitons in other regions of the VCD spectra, either in the mid-IR or in the CH-stretching regions.