Probing the Effects of Cholesterol on Pyrene-Functionalized Interfacial Adlayers

Abstract

We have synthesized a novel set of pyrene-functionalized, covalently bound surface adlayers with and without cholesterol derivatives coadded to the adlayer. We have deposited these adlayers on quartz, oxidized silicon wafers, and indium-doped tin oxide coated substrates. The addition of tethered cholesterol to the adlayer creates a hydrophobic, likely disordered, microenvironment in which the surface-bound pyrene resides. X-ray photoelectron spectroscopy measurements demonstrate the covalent attachment of both cholesterol and pyrene in our adlayers. The presence of the cholesterol moieties gives rise to a reduction in film thickness, as measured ellipsometrically, and contact angle data indicate significant surface heterogeneity. Steady-state fluorescence data show that the presence of cholesterol moieties reduces the extent of pyrene excimer formation and provides a less polar environment for the chromophore. Fluorescence lifetime measurements on surface-bound pyrene were biexponential, consistent with multiple local environments, regardless of whether tethered cholesterol was present or not. Cyclic voltammetry reveals competition between the pyrene and cholesterol moieties for binding to available surface sites on the epoxide-terminated surface-binding layer we use.

Fluorescence and electrochemistry studies of pyrene-functionalized surface adlayers to probe the microenvironment formed by cholesterol

“…When both adlayer constituents were present in the surface-modification reaction vessel, both Chol-NH 2 and PSEDA reacted with the epoxide terminal groups yielding a lower PSEDA surface density. This result was consistent with our previous report where cholesterol was co-adsorbed with pyrenemethylamine on quartz [27]. In addition to this dilution argument, there is another factor that gives rise to our experimental observation of less efficient excimer formation for Qz/PSEDAChol.…”

“…The first point to note is that there can be significant variation in the thickness of GPTS adlayers on oxidized silicon, and the values recovered are consistent with the deposition of multilayers rather than monolayers. This finding is entirely consistent with our previous work and can be understood in the context of the trialkoxysilane GPTS reagent exhibiting significant siloxane polymerization during the deposition reaction [27]. Organosilane adlayers formed from silanes containing more than one alkoxy functionality that are prepared in the presence of ambient water are known to yield variable thicknesses due to the polymeric network of hydrolyzed alkoxy groups [31].…”

“…Cholesterol-ethylenediamine (Chol-NH 2 ) was prepared by using the same method as described in a previous study [27]. The preparation of N-1-pyrenesulfonyl-ethylenediamine (PSEDA) was based on a similar synthetic route, described elsewhere, by reacting ethylenediamine and pyrenesulfonyl chloride [3].…”

“…The schematic structure of the corresponding adlayers with (right) and without (left) cholesterol moieties. [27]. The gold ball electrode was cleaned by first immersing it in fresh piranha solution (Caution!…”

“…We have used cholesterol as a co-adsorbate with pyrene because of its important role in lipid bilayer membrane composition and function. Some of our recent work on pyrene-functionalized adlayers with co-immobilized cholesterol molecules [27] has shown that the surface-bound cholesterol plays an important role in modulating the local environment of the pyrene chromophore. In that work, it was clear that pyrene-cholesterol interactions were important, but one issue that remained to be considered is the role of the chromophore structure and how it was bound to the adlayer in the context of the interactions we sensed.…”