Observation of a metal cation entering into a phospholipid bilayer by total internal reflection with thermal lens spectroscopy

Abstract

We monitored the diffusion of Cu^{2 +} into a phospholipid bilayer containing a metallochromic indicator, 1-(2-pyridylazo)-2-naphthol (PAN), by using total internal reflection with thermal lens spectroscopy (TIR-TLS), which is a non-destructive and surface-selective spectrometry. When Cu^{2 +} entered into the bilayer, a formation of Cu-PAN chelate resulted in decrease of the absorbance at the wavelength of the excitation beam. The decrease was detected by TIR-TLS. We measured five kinds of bilayer samples, whose molar ratios of two phospholipids, 1,2-dioleoyl-sn-glycero-3-phosphatidylcholine (DOPC) and 1-stearoyl-2-oreoyl-sn-glycero-3-phosphatidylserine (SOPS), were 10 : 0, 7 : 3, 5 : 5, 3 : 7, 0 : 10. In the case of a bilayer whose ratio of DOPC to SOPS was 7 to 3, the signal intensity showed an obvious decrease compared to a blank. On the other hand, there no significant change was observed in the case of other samples. Several attempts using a 7 : 3 bilayer revealed a non-reproducible incorporation of Cu^{2 +} into the lipid. We also proved the effect of a counter ion by comparing sulfate ion and chloride ion.