PRE-RESONANCE RAMAN-SPECTRA AND CONFORMATIONS OF NYSTATIN IN POWDER, SOLUTION AND PHOSPHOLIPID-CHOLESTEROL MULTILAYERS

A Raman scattering study of the channel-forming polyene antibiotic nystatin, is reported in the solid state, in organic and aqueous solutions as well as in phospholipid and phospholipid-cholesterol multilayers. Measurements of the solid and solution spectra as a function of excitation wavelengths in the 459.7-514.5 nm range, and the phospholipid spectra as a function of temperature in the 10-60°C range, have also been made. The spectral features indicate a pre-resonance-enhanced Raman spectrum in which the CC and CC stretching modes of the polyene segment of nystatin are dominant. However, in contrast to previously published results on the nearly isostructural polyene antibiotic amphotericin B, a line at 1610 cm-1 assignable to the CO stretching mode is also observed to be strongly resonance enhanced. This is explained by a postulated ground-state conformation model in which a twisting of the molecule is facilitated by the break in the polyene chain. This allows the CO group at one end of the molecule to be aligned along the polyene unit at the other end, and the CC stretching vibration, which is strongly modulated by the polyene π → π* excited state, to mix with the CO stretching vibration. The peak frequencies and intensities of the CC and CC stretching modes in nystatin, however, remain essentially unchanged compared with amphotericin B, indicating that the polyene units in nystatin remain planar and trans both in the ground and excited states. The intensity of the CO mode with respect to the CC stretching mode was observed to vary appreciably with nystatin environment, indicating a. © 1979.