Diffusion of 2,2,6,6-tetramethylpiperidine 1-oxyl derivatives of variable hydrophobicity in tropocollagen I solution

Electrochemical time-of-flight was used to measure the diffusion coefficients of 2,2,6,6-tetramethylpiperidine 1-oxyl derivatives, CnTPA, (3 to 7 CH2 groups), in tropocollagen I solution, as a function of the chain length and the cross-linking with glutaraldehyde. The values of the diffusion coefficient of CnTPA in pure aqueous electrolyte follow the Stokes–Einstein law, i.e. the diffusion coefficient is inversely proportional to the size of the redox probe. Different behavior is observed in 0.5% (w/v) tropocollagen solution where the molecules with longer alkyl chains show larger diffusion coefficients than the smaller molecules. This behavior is explained in terms of electrostatic interactions between tropocollagen chains and the CnTPA molecules. The measurements of the diffusion coefficients of CnTPA in 0.5% tropocollagen cross-linked with glutaraldehyde indicate that while the C7TPA and C5TPA probes exhibit lower diffusion coefficients upon addition of 0.05% GA and 0.1% (v/v) GA respectively, the other CnTPA molecules exhibit either unchanged or increased diffusion coefficients under the same conditions thus indicating the presence of hydrophobic pockets selectively interacting with CnTPAs. These results demonstrate the utility of electrochemical time-of-flight in measurements of diffusion coefficients in complex biopolymeric media.