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).