Single-Molecule Force Spectroscopy Reveals Multiple Binding Modes between DOPA and Different Rutile Surfaces

Inspired by marine mussel adhesive systems, numerous 3,4-dihydroxyphenylalanine (DOPA)-containing surface coating materials have been recently designed. It is well known that DOPA has a strong adhesion ability to different kinds of wet surfaces. However, the molecular mechanism of DOPA adhesion remains elusive. Recent biophysical studies of DOPA adhesion by both surface force apparatus (SFA) and atomic force microscopy (AFM) suggest that DOPA can bind to a wide range of surfaces exhibiting diverse chemical properties through different binding mechanisms. Here, using AFM-based single-molecule force spectroscopy, we show that even for chemically well-defined crystal surfaces, DOPA can bind to them by multiple binding modes. The binding forces between DOPA and different rutile TiO₂ surfaces can vary within a broad range from 40-800 pN at a pulling speed of 1000 nm s^-1 and are largely dependent on the surface properties. Our findings indicate that the local chemical environment can greatly affect DOPA adhesion, and that single-molecule force spectroscopy is a unique tool to reveal the heterogeneity of DOPA adhesion to the same surface.