Vibrational and electronic optical activity of amide and disulfide groups in neurohypophyseal hormones and their models
P. Maloň1, L. Bednárová1, M. Pazderková1,2, M. Flegel1, J. Hlaváček1, V. Baumruk2
1Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, Flemingovo nám. 2, 166 10, Prague 6, Czech Republic
2Institute of Physics, Faculty of Mathematics and Physics, Charles University in Prague, Ke Karlovu 5, Prague 2, 121 16, Czech Republic
Vibrational and electronic circular dichroism spectra of neurohypophyseal hormones oxytocin, lysine-vasopressin, arginine-vasopressin, several of their analogs and structural models were measured in order to isolate chiroptical manifestation of their peptide backbone from the signals of aromatic chains of tyrosine and phenylalanine chromophores. We have also observed electronic disulfide-related ECD in the long wavelength spectral region and identified features in the spectra of Raman optical activity (ROA) which are associated with the S-S and C-S stretching vibrations. These allow us to infer the sense of disulfide twist in the particular compounds when signs of ROA features are compared to ROA calculations on simple model disulfides  and verified by a comparison with spectra of disulfide-bridged cyclodextrins.
The results indicate that (a) there is a remarkable similarity and consistent spectral behavior of the whole family of neurohypophyseal hormones; (b) vibrational optical activity allows to observe chiroptical manifestation of neurohypophyseal hormone backbone without the harmful interference of aromatic groups; (c) vibrational optical activity indicates that the prevailing solution conformation of neurohypophyseal hormones is a short segment of left handed helix. This is in accord with analogous ECD result, however interference with the bands due to side chains of Tyr and Phe makes this observation less discernible;(d) the ROA signals in the S-S and C-S stretching vibration regions are observable also with neurohypophyseal
Supported by Grant Agency of the Czech Republic, project no. P205-10-1276 .
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