Infrared spectroscopy of proteins in reverse micelles

Abstract

Reverse micelles are a versatile model system for the study of crowded microenvironments containing limited water, such as those found in various tissue spaces or endosomes. They also preclude protein aggregation. Reverse micelles are amenable to study by linear and nonlinear infrared spectroscopies, which have demonstrated that the encapsulation of polypeptides and enzymatically active proteins into reverse micelles leads to conformational changes not seen in bulk solution. The potential value of this model system for understanding the folding and kinetic behavior of polypeptides and proteins in biologically important circumstances warrants increased study of reverse micelle systems by infrared spectroscopy. This article is part of a Special Issue entitled: FTIR in membrane proteins and peptide studies.

Keywords: Aggregation; Amyloid; Confinement; Crowding; Encapsulation.

Fig. 1

(Left) Snapshot of a RM with w_o = 11.4 from a molecular dynamics simulation. Blue spheres are AOT, and white/red spheres are water molecules in the interior. The organic phase (isooctane) is not explicitly represented. (Middle) Thin cross-section showing AOT molecules on the periphery and sodium ions as turquoise spheres in the interior. (Right) Thin cross-section showing AOT molecules on the periphery, and the 40-residue amyloid β protein as a random-coil in the interior.

Fig. 2

(Upper) Mid-IR spectra of empty RM (i.e. containing only water with w_o = 11.40) and RM with the amyloid β protein in approximately 1 out of 75 RM, (i.e. 74 out of 75 RM are empty). The difference is imperceptible. (Middle) The result of interactively subtracting the two spectra in the upper panel to minimize the low-frequency edge of the ester C=O stretching band. (Lower) Zoomed-in view of the amide I’ region of the spectrum in the middle panel.