Using the Intrinsic Chirality of a Molecule as a Label-Free Probe to Detect Molecular Adsorption to a Surface by Second Harmonic Generation

Abstract

Chiral second harmonic generation (C-SHG) has been used for the label-free detection of (R)-(+)-1,1′-bi-2-naphthol (RBN) and (S)-(+)-1,1′-bi-2-naphthol (SBN) binding to planar-supported lipid bilayers of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphotidylcholine (POPC) based on the intrinsic chirality of the molecules. C-SHG adsorption isotherms of RBN and SBN reveal Langmuir adsorption behavior with binding constants of 2.7 ± 0.2 × 10⁵ M⁻¹ and 3.0 ± 0.1 × 10⁵ M⁻¹, respectively. The kinetics of RBN binding to a POPC bilayer was also measured. It was determined that the adsorption rate for RBN was 5.7 ± 0.4 × 10³ s⁻¹M⁻¹ and the desorption rate was 2.1 ± 0.8 × 10⁻² s⁻¹. From the kinetic data a binding constant of 2.7 ± 1.0 × 10⁵ M⁻¹ was calculated, which agrees well with the thermodynamic measurement. The C-SHG technique was correlated with surface tension measurements in order to determine the RBN surface excess within the POPC membrane. The maximum surface excess of RBN in a monolayer of POPC was 4.3 ± 0.5 × 10⁻¹¹ mol cm². Using the maximum surface excess in conjunction with the C-SHG binding data a lower limit of detection of 1.5 ±0.1 × 10⁻¹³ mols cm⁻² was calculated. The results of these studies show that C-SHG is a powerful tool for the study of chiral molecular interactions at surfaces.