Abstract: 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 pK(a) 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 (pK(a) approximate to 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 pK(a)(0)'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 pK(a) shifts, Delta pK(a)(Pol), 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, Delta pK(a)(el) = 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.

Cite: Voinov M.A. , Kirilyuk I.A. , Smirnov A.I.
Spin-Labeled pH-Sensitive Phospholipids for Interfacial pK(a) Determination: Synthesis and Characterization in Aqueous and Micellar Solutions
Journal of Physical Chemistry B. 2009. V.113. N11. P.3453-3460. DOI: 10.1021/jp810993s WOS Scopus РИНЦ

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Dates:

Published online:	Feb 23, 2009
Published print:	Mar 19, 2009

Identifiers:

Web of science	WOS:000264111200026
Scopus	2-s2.0-65249175181
Elibrary	13601112
OpenAlex	W2013517198

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