Electronic and dielectric properties of protein--methylglyoxal complexes

Steady-state conduction and dielectric measurements over the frequency range 10(-5) to 10(5) Hz are reported for several proteins that have been complexed with methylglyoxal. Compared with the normal (white) proteins the brown protein--methylglyoxal complexes exhibit a marked increase in electronic conductivity and a pronounced low-frequency dielectric dispersion. The intensity of the brown colour and the electronic activity is directly related to the number of free lysine groups available to react with the methylglyoxal. It is proposed that the methylglyoxal molecules form Schiff bases with the epsilon-amino groups of lysine residues and that these Schiff bases then form a charge-transfer complex with a neighbouring peptide unit. For collagen, in particular, it is found that the electron 'holes' so formed in the polypeptide backbone are capable of long range motion in what can be interpreted as being the valence band of extended electronic states of the protein structure. The protein--methylglyoxal complexes have electronic and dielectric properties similar to those exhibited by the perylene-chloranil charge-transfer complex.