Paper BI-MoM8
ToF-SIMS Study of Orientation of FnIII_9-10 Fibronectin Fragment on Self-Assembled Monolayers

Monday, October 31, 2011, 10:40 am, Room 108

Fibronectin (FN) is an extra cellular matrix protein that is involved in many cell processes such as adhesion, migration and growth. The orientation and conformation of FN adsorbed onto surfaces can therefore have a critical effect on cell-surface interactions. Experimental orientation studies of model systems also provide an important benchmark for molecular simulations and are of great value for further development of force fields used in many theoretical studies of protein adsorption. In this study the adsorbed orientation of the 9-10 fragment of FNIII was studied on three different model surfaces (self-assembled monolayers (SAM) of C₁₁ alkanethiols on Au, -CH₃, -NH₂, and -COOH terminated SAMs.) The FNIII_9-10 fragment includes the two binding sites for the FN protein, a RGD site on segment 10 (seg10) and a synergy site on segment 9 (seg9). This fragment also has a significant asymmetry in its amino acid distribution with His found exclusively on seg9, Lys only on seg10 and three times more Tyr on seg10 then seg9. Taking advantage of this asymmetry, we use time of flight secondary ion mass spectrometry (ToF-SIMS) to study the different orientation on different surfaces and X-ray photoelectron spectroscopy (XPS) to determine the difference in surface coverage. While CH₃ and NH₂ have similar full coverage the highest COOH coverage is about half the maximal coverage observed for the two other surfaces. A ToF-SIMS comparison of the FNIII_9-10 on COOH and NH₂ at similar coverages shows significantly more His on the COOH and more Tyr on the NH₂ surfaces suggesting an opposite orientation of the fragment on these two surfaces. Results indicate that on the COOH surface the fragment is oriented with seg10 down while on the NH₂ surface seg9 is closer to the surface. Comparing similar coverages of the FNIII_9-10 on NH₂ and CH₃, the later has less Tyr signal (from seg10) suggesting that the seg10 is closer to the surface on CH₃ than on NH₂. In contrast, when FNIII_9-10 on CH₃ and COOH results were compared, the fragment on the CH₃ sample has less His signal suggesting that seg9 is closer to the surface compared to COOH.