Abstract
BackgroundIt 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.Methodology/Principal FindingsCircular 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/H2O 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/SignificanceOur 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.
Highlights
Circular dichroism (CD) is a powerful technique to deal with structural problems for both the small organic molecule and the biopolymer
For the bands at higher energy, in particular those involved in exciton couplets, the number, wavelength features, and polarization direction of the transitions need to be accurately known in order that the circular dichroism (CD) spectrum can be predicted in the frame of DeVoe’s model
This would require a reliable deconvolution of the absorption spectrum for the moieties that constitute the coupling products 2 and 5 into the component bands, as well as the measurement of polarization directions of related transitions
Summary
Circular dichroism (CD) is a powerful technique to deal with structural problems for both the small organic molecule and the biopolymer. Under favorable circumstances, when two or more equivalent chromophores - which absorb light strongly in the same spectral region are present in a molecular frame at suitable mutual distance and orientation, the CD spectral features (e.g. exciton couplets) may offer a clue as to the stereochemistry of the molecule [1], [2] Models of such systems, built from either classical electrodynamics [3], [4] or quantum mechanics [5], can allow a quantitative interpretation of the spectroscopic and structural terms that give rise to the exciton couplet [6]–[8]. 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
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