Derivative spectroscopy as a nonperturbative tool to detect the state of liposome-entrapped sulbactam

Sulbactam, a β-lactam antibiotic, absorbs uv light at 273 nm when in alkaline media, whereas at neutral or acidic pH this peak disappears. Sulbactam-loaded liposomes, prepared by reverse-phase evaporation, were spectroscopically analyzed by the derivative mode measuring the peak-through amplitude between +258 and -285 nm, so that the spectral interference of the sample is eliminated. Taking advantage of this experimental approach, we could study the influence of different parameters on the dissociation state of the drug when entrapped in dipalmitoyl phosphatidylcholine liposomes. In particular, following the time course of sulbactam peak disappearance, we found that: (i) the effectiveness of protonating the sulbactam of the chosen buffers is acetate/acetic acid > succinate/succinic acid > citrate/citric acid; (ii) the rate of signal disappearance is influenced by the externally imposed pH and can be somewhat related to the dissociation state of the organic acids; (iii) as expected, the whole phenomenon is temperature dependent. The observations reported here might be the basis for quantitative permeation studies in synthetic and/or natural membranes using this methodology.