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Research Article

Determining the Conformation of an Adsorbed Br−PEG−Peptide by Long Period X-Ray Standing Wave Fluorescence

Carrie A. Crot,^† Chunping Wu,^† Mark L. Schlossman,*^†^‡ Thomas P. Trainor,^§ Peter J. Eng,^£ and Luke Hanley*^†

Department of Chemistry, University of Illinois at Chicago, Chicago, Illinois 60607-7061, Department of Physics, University of Illinois at Chicago, Chicago, Illinois 60607, Department of Chemistry and Biochemistry, University of Alaska Fairbanks, Fairbanks, Alaska 99775, and Center for Advanced Radiation Sources, The University of Chicago, Chicago, Illinois 60603

Langmuir, 2005, 21 (17), pp 7899–7906

DOI: 10.1021/la0505115

Publication Date (Web): July 13, 2005

†

Department of Chemistry, University of Illinois at Chicago.

To whom correspondence should be addressed. E-mail: lhanley@uic.edu (L.H.).

‡

Department of Physics, University of Illinois at Chicago.

University of Alaska.

The University of Chicago.

Abstract

Long-period X-ray standing wave fluorescence (XSW) and X-ray reflectivity techniques are employed to probe the conformation of a Br−poly(ethylene glycol) (PEG)−peptide adsorbate at the hydrated interface of a polystyrene substrate. The Br atom on this Br−PEG−peptide construct serves as a marker atom allowing determination by XSW of its position and distribution with respect to the adsorption surface with angstrom resolution. Adsorption occurs on native or ion-beam-modified polystyrene films that are spin-coated onto a Si substrate and display either nonpolar or polar surfaces, respectively. A compact, oriented monolayer of Br−PEG−peptide can be formed with the peptide end adsorbed onto the polar surface and the PEG end terminating with the Br tag extending into the aqueous phase. The 108−141 Å distance of the Br atom from the polystyrene surface in this oriented monolayer is similar to the estimated 150 Å length of the extended Br−PEG−peptide. This Br−polystyrene distance depends on adsorption time and surface properties prior to adsorption. Incomplete multilayers form on the polar surface after sufficient adsorption time elapses. By contrast, adsorption onto the nonpolar surface is submonolayer, patchy, and highly disordered with an isotropic Br distribution. Overall, this combination of X-ray surface scattering techniques with a novel sample preparation strategy has several advantages as a real space probe of adsorbed or covalently bound biomolecules at the liquid−solid interface.