Spin-labeled pH-sensitive phospholipids for interfacial pKa determination: synthesis and characterization in aqueous and micellar solutions.
The synthesis and characterization of spin-labeled phospholipids (SLP)--derivatives of 1,2-dipalmitoyl-sn-glycero-3-phosphothioethanol (PTE)--with pH-reporting nitroxides that are covalently attached to the lipid's polar headgroup are being reported. Two lipids were synthesized by reactions of PTE with thiol-specific, pH-sensitive methanethiosulfonate spin labels methanethiosulfonic acid S-(1-oxyl-2,2,3,5,5-pentamethylimidazolidin-4-ylmethyl) ester (IMTSL) and S-4-(4-(dimethylamino)-2-ethyl-5,5-dimethyl-1-oxyl-2,5-dihydro-1H-imidazol-2-yl)benzyl methanethiosulfonate (IKMTSL). The pKa values of the IMTSL-PTE lipid measured by EPR titration in aqueous buffer/isopropyl alcohol solutions of various compositions were found to be essentially the same (pKa approximately 2.35), indicating that in mixed aqueous/organic solvents, the amphiphilic lipid molecules could be shielded from changing bulk conditions by a local shell of solvent molecules. To overcome this problem, the spin-labeled lipids were modeled by synthesizing IMTSL- and IKMTSL-2-mercaptoethanol adducts. These model compounds yielded the intrinsic pKa0's for IMTSL-PTE and IKMTSL-PTE in aqueous buffers as 3.33 +/- 0.03 and 5.98 +/- 0.03, respectively. A series of EPR titrations of IMTSL-PTE in mixed water/isopropyl alcohol solution allowed for calibrating the polarity-induced pKa shifts, deltapKapol, vs bulk solvent dielectric permittivity. These calibration data allowed for estimating the local dielectric constant, epsilon(eff), experienced by the reporter nitroxide of the IMTSL-PTE lipid incorporated into the nonionic Triton X-100 micelles as 60 +/- 5 and 57 +/- 5 at 23 and 48 degrees C, respectively. For micelles formed from an anionic surfactant sodium dodecyl sulfate (SDS) the electrostatic-induced pKa shift, deltapKael = 2.06 +/- 0.04 units of pH, was obtained by subtracting the polarity-induced contribution. This shift yields psi = -121 mV electric potential of the SDS micelle surface.
Voinov
Maxim A
MA
Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204, USA. mvoynov@ncsu.edu
Kirilyuk
Igor A
IA
Smirnov
Alex I
AI
D004563:Electrochemistry; D004578:Electron Spin Resonance Spectroscopy; D006863:Hydrogen-Ion Concentration; D007202:Indicators and Reagents; D007700:Kinetics; D008623:Mercaptoethanol / Q000737:chemistry; D008823:Micelles; D008958:Models, Molecular; D009584:Nitrogen / Q000737:chemistry; D010743:Phospholipids / Q000737:chemistry*; D012996:Solutions; D013113:Spin Labels; D014867:Water
D007202:Indicators and Reagents; D008823:Micelles; D010743:Phospholipids; D012996:Solutions; D013113:Spin Labels; D014867:Water; D008623:Mercaptoethanol; D009584:Nitrogen