Abstract
We have used the orthogonal carbodiimide condensation and copper-catalyzed azide-alkyne "click" cycloaddition (CuAAC) reactions to prepare self-assembled monolayers that present distinct peptides to stem cells in a bioinert background. The approach involved first forming mixed SAMs with three components: (i) an azide-terminated hexaethylene glycol alkanethiolate (HS-EG6-N3), (ii) a carboxylate-terminated hexaethylene glycol alkanethiolate (HS-EG6-COOH), and (iii) a triethylene glycol alkanethiolate (HS-EG3). An acetylene-bearing peptide and an amine-terminated peptide were then immobilized to these substrates using a "click" CuAAC reaction and a carbodiimide condensation reaction, respectively. Polarization-modulated infrared reflectance-absorbance spectroscopic analysis demonstrated formation of well-ordered, close-packed self-assembled monolayers (SAMs), chemoselective conjugation of amine-terminated peptides to surface carboxylate groups, and subsequent conjugation of acetylene-terminated peptides to the azide groups on SAMs. Varying the mole fraction of HS-EG6-N3, HS-EG6-COOH, and HS-EG3 during SAM formation allowed for control over the densities of each peptide on the substrate. Substrates presenting varying surface densities of RGESP (a nonfunctional peptide), RGDSP (a cell adhesion peptide), or TYRSRKY (a heparin/heparan sulfate-binding peptide) were then used to characterize the relationship between peptide surface density and human mesenchymal stem cell (hMSC) adhesion. Results demonstrate that RGESP does not influence RGDSP-mediated adhesion of hMSCs, which indicates that a second peptide with distinct bioactivity can be immobilized alongside RGDSP to characterize the influence of two peptides on hMSC behavior. Our results also demonstrate that RGDSP and TYRSRKY act synergistically to promote hMSC adhesion in the absence of serum. Interestingly, heparin sequestered by TYRSRKY inhibits cell adhesion on substrates presenting RGDSP = 0.1% and > or = 0.1% TYRSRKY or RGDSP = 1% and > or = 0.5% TYRSRKY. Taken together, these results indicate that two peptides can be controllably presented to stem cells on the same otherwise bioinert SAM substrate, and that multiple, distinct extracellular moieties act in concert to regulate hMSC adhesion.