Disulphide chromophore and its optical activity

 

L.Bednárova1, P.Maloň1, H.Dlouhá1, M.Kubáňová2 and V.Baumruk2

 

1Institute of Organic Chemistry and Biochemistry, Flemingovo n. 2, Prague 6, 166 10, Czech Republic

2Charles University in Prague, Institute of Physics, Ke Karlovu 5, Praha 2, 121 165, Czech Republic

bednarova@uochb.cas.cz

 

 

The use of various spectroscopic methods and their chiroptical variants, for the determination of peptide/protein conformation is relatively well established.1-2 Although the information is of rather low resolution it can be obtained for samples in solution and therefore has a distinct advantage over more informative methods like NMR or X-ray crystallography. Electronic circular dichroism (ECD) measured in the visible and near UV spectral region carries majority of structural information via the amide group. Detailed analyses of ECD give also additional structural data about other functional groups existing in peptide/protein molecules. These involve aromatic chromophores of Phe, Tyr and Trp side chains, the not very well understood contribution of the imidazole ring of histidine and a contribution of cystine disulphide chromophore, which is sometimes detectable as the high wavelength tail of the CD spectrum. Disulphide group is the only chromophore in proteins and peptides, which by itself exhibits inherent chirality and therefore should give rise to substantial chiroptical manifestation in electronic spectra (the non-planar disulphide chromophore itself is of C2 symmetry). In practice, it is unfortunately not the case and especially the low energy CD bands of the disulphide group with the maximum at about 260 nm are low in intensity and rather broad. If we consider, in addition, the possible overlap with CD bands of aromatic chromophores of phenylalanine, tyrosine and tryptophan residues, it is not surprising that structure oriented application of electronic CD spectroscopy to a disulphide chromophore is quite difficult.

In this contribution we scrutinize chiral disulphides by other variants of chiroptical spectroscopy, namely vibrational optical activity measured in Raman scattering.3 Raman spectroscopy is for this purpose rather promising already in its non chiral variant (it gives information on the C-S bond conformation), but one should underline that the obtained information is not complete. In that way no information about ‘absolute’ conformation of the disulphide bridge can be acquired. According to theoretical calculations 4 Raman optical activity could provide this very specific information using the S-S (~500  cm-1) and C-S (~700 cm-1) stretching vibrations.

The ECD, IR and Raman spectra, VCD and ROA spectra of model systems are presented with the aim to cast light on this unresolved problem. The spectra are compared with theoretical predictions.

 

1.     Woody R.W., Dunker A.K., in Circular Dichroism: Conformational Analysis of Biopolymers, G.D. Fasman, p.109, ed., Plenum Press, New York 1996

2.     Havel, H. A. (ed.), in Spectroscopic Methods for Determining Protein Structure in Solution, VCH Publishers, New York, 1996..

3.     Kapitán J., Baumruk V., Hulačová H., Maloň P., Vib. Spectrosc. 42, 88-92 (2006)

4.     Bednárová L., Bouř P., Maloň P.: Chirality, submitted

 

Acknowledgements.

This work has been supported by the Grant Agency of the Czech Republic (project No. GA 203/07/1335).