In Situ Infrared Attenuated Total Reflection (IR ATR) Spectroscopy: A Complementary Analytical Tool for Drug Design and Drug Delivery

Abstract

A comprehensive summary of basic relations for quantitative IR ATR spectroscopy of isotropie and oriented samples is given. Experimental requirements for detection of sub-monolayer quantities in aqueous environment are discussed. New instrumental developments such as a single-beam-sample-reference (SBSR) attachment, and FTIR modulation spectroscopy at low frequencies are presented. Examples of in situ experiments with tertiary-amine local anesthetics interacting with planar, immobilized lipid bilayers are discussed. Partition coefficients of the total amine, as well as of the protonated and deprotonated forms have been determined. Structural alterations, especially of lipids, were detected upon interaction. Adsorption isotherms revea1ed multilayer formation at the membrane surface. The onset of this process for dibucaine is at ca. 5 mm bulk concentration or even below. Preferential accumulation of dibucaine base is observed already at bulk pH 5.5 (pKa = 8.83). For peptide conformation analysis, FTIR temperature (T) modulation experiments were performed for the first time. A modulation amplitude of ΔT = ± 1° at a given mean temperature is shown to result in high-quality phase-resolved spectra enabling detailed insight into periodically induced changes of the state of the sample.