Tuning the physicochemical properties of surface coatings from elastin-like recombinamers by tailoring their amino acid sequence

The surface of implantable devices is the first contact with the organism and dictates the subsequent biological events that lead to either biointegration or rejection. Nowadays, bioactive surfaces are often generated by functionalization with bioactive polypeptides mimicking the native proteins. However, protein adsorption is a complex phenomenon highly dependent on the protein intricate composition and structure. We hypothesize that this dependence can be explored to control the properties of surfaces obtained via adsorption of genetically engineered polypeptides in which the amino acids can be positioned with great precision. Elastin-like recombinamers (ELRs), a family of fibrillary polypeptides based on the sequence (VPGXG)_n (X being any natural or synthetic amino acid), offer this possibility. Two different ELRs were biosynthesized: a 35 kDa MGKKKPV(VPGVG)₈₄V hydrophobic monoblock and a 47 kDa MGKKKPV[(VPGVG)₂(VPGEG)(VPGVG)₂]₁₀[VGIPG]₆₀V diblock containing one hydrophilic segment (E-block) and one hydrophobic (I-block). Each ELR (referred to as V84 and EI, respectively) was adsorbed onto self-assembled monolayers (SAMs) exhibiting –CH₃, –OH, –COOH or –NH₂. Both variants were modified with tetrakis (hydroxymethyl) phosphonium chloride (THPC), an amine-reactive compound.

Quartz-crystal microbalance with dissipation monitoring (QCM-D) showed that the adsorption of EI reached an equilibrium within 20 minutes regardless the underlying SAM, owing to its more versatile block architecture, whereas V84 took 3 hours. We observed a time-delayed adsorption of about 2 minutes until maximum kinetics was reached for both ELRs. This delay was interpreted as the time needed for the polypeptides to adopt an optimal adsorption conformation. The thickness of the adsorbed ELRs increased in the following order: –OH<–COOH<–NH₂<–CH₃, showing high affinity of their hydrophobic segments with –CH₃ (e.g. 8.8 nm for V84). QCM-D coupled with surface plasmon resonance (SPR) showed hydration levels between 18-21% and 32‑64% for EI and V84, respectively. The relation between frequency and dissipation variations monitored by the QCM-D further revealed that V84 binds rigidly to the surface and reorganizes into an extended highly hydrated conformation as the mass increases. In contrast, EI rearranges into a less dissipative, more rigid layer.

This work showcases that small variations in the sequence of ELRs affect significantly the adsorption profile, rigidity and topography. These conclusions will be crucial for the proper design of protein coatings for implantable medical devices.