Tetracycline and derivatives—assignment of IR and Raman spectra via DFT calculations

Abstract

IR and Raman spectra (λex = 1064, 400 and 280 nm) were recorded for solutions of tetracycline and several derivatives in acidified and alkaline H2O and D2O, respectively. Based on the observed resonance enhancement, an empirical assignment of most of the Raman bands in the wavenumber range 1000 < /cm−1 < 1600 to either the A- or the BCD-chromophore could be established. In addition, DFT/BP86 calculations yielded normal mode frequencies and coordinates, as well as IR intensities. By comparison of these with the information contained in IR and Raman spectra (especially the deuterium isotope shift), a fairly unique correlation between experimental and theoretical spectra could be achieved. Inspection of the normal coordinate pictures also provides a rationale for which of the bands are shifted upon partial or complete deprotonation or upon derivatization of tetracycline. Altogether, the presented material provides strong evidence that the fully protonated tetracyclines investigated exhibit in aqueous environment a conformation which is very close to that found in the crystalline state (tetracycline hexahydrate).