title: On the chiroptical behavior of conjugated multichromophoric compounds of a new pseudoaromatic class: bicolchicides and biisocolchicides.
abstract: BACKGROUND
  It is well known that, stemming from the mutual interplay between chromophores, circular dichroism (CD) is a powerful technique to deal with structural problems for both the small organic molecule and the biopolymer. However, quantitative interpretations of the spectroscopic and structural terms that give rise to the exciton couplet are usually presented for ideal cases, or a few CD bands only are taken into account, overlooking the role of the solvent medium.
  
  
  RESULTS
  Circular dichroism and UV absorption spectra were carried out for colchicide (3) and isocolchicide (6), as well as their coupling products, 10,10'-bicolchicide (2) and 9,9'-biisocolchicide (5), in both hydrogen bonding and non hydrogen bonding solvents, as well as MeCN/H(2)O mixtures. A dramatic control by the solvent emerged, as even tiny changes in the composition of solvent mixtures, at ca 1 water molar fraction, induced a dramatic modification of their CD bands. A mutarotation phenomenon--long known for isocolchicine (8)--was also observed for 5, and can be attributed to the interconversion between atropisomers (R(a),7S),(R(a),7'S)-5a and (R(a),7S),(S(a),7'S)-5b.
  
  
  CONCLUSIONS
  Our data show that with molecules built on two structurally identical moieties which embody both hydrophilic and hydrophobic groups, even tiny changes in the composition of solvent mixtures cause a dramatic modification of the CD bands. Their analysis arrives at a qualitative rationalization of the observed CD couplets from the coupling of high energy transitions, while attempts at a quantitative interpretation of these phenomena through time-dependent density functional theory allowed to reproduce satisfactorily the CD spectrum in the 300-450 nm region only. Failure with higher energies probably reflects currently inadequate specific theoretical treatments of the solvent medium.
authors.affiliation: Dipartimento di Chimica e Chimica Industriale, Università di Pisa, Pisa, Italy. tiziana@dcci.unipi.it
authors.forename: Tiziana
authors.initials: T
authors.lastname: Funaioli
authors.forename: Marino
authors.initials: M
authors.lastname: Cavazza
authors.forename: Maurizio
authors.initials: M
authors.lastname: Zandomeneghi
authors.forename: Francesco
authors.initials: F
authors.lastname: Pietra
mesh_terms: D002851:Chromatography, High Pressure Liquid; D002942:Circular Dichroism; D003078:Colchicine / Q000031:analogs & derivatives* / Q000138:chemical synthesis / Q000737:chemistry / Q000302:isolation & purification; D006841:Hydrocarbons, Aromatic / Q000138:chemical synthesis / Q000737:chemistry* / Q000302:isolation & purification; D013056:Spectrophotometry, Ultraviolet; D013237:Stereoisomerism; D013696:Temperature
chemical_list: D006841:Hydrocarbons, Aromatic; C059352:isocolchicine; C111968:colchicide; D003078:Colchicine