Interaction of heme proteins with poly(propyleneimine) dendrimers in layer-by-layer assembly films under different pH conditions

With the isoelectric point at pH 7.4, hemoglobin (Hb) has net positive surface charges at pH 5.0 and overall negative charges at pH 9.0, and is essentially neutral at pH 7.0. The fifth-generation poly(propyleneimine) (PPI) dendrimer is usually positively charged in aqueous solution. The {PPI/Hb}n films under different pH conditions have been successfully fabricated on various solid surfaces by the layer-by-layer assembly technique, and the growth of films was monitored by ultraviolet-visible (UV-vis) spectroscopy, quartz crystal microbalance (QCM), and cyclic voltammetry (CV). Not only was the negatively charged Hb at pH 9.0 alternately adsorbed with positively charged PPI onto solid substrates by electrostatic attraction between them, but the positively charged Hb at pH 5.0 was also successfully assembled with like charged PPI into layer-by-layer {PPI/Hb(pH 5.0)}n films. For the latter, the localized electrostatic interaction or the charge reversal of proteins on PPI surface may be the main driving force. For {PPI/Hb(pH 7.0)}n films, however, the hydrophobic/hydrophilic interaction may play a more important role in the assembly, making the amount of adsorbed Hb even less than that of {PPI/Hb(pH 5.0)}n films. For comparison, negatively charged catalase (Cat) at pH 8.0 was used to assemble layer-by-layer films with positive PPI, but {PPI/Cat}n films showed quite different properties from {PPI/Hb}n films. UV-vis and infrared (IR) spectroscopy, QCM, ellipsometry, and voltammetry were utilized to characterize the {PPI/protein}n films. The results suggest that the proteins in the multilayer films retain their near-native structure and display good voltammetric response for heme Fe(III)/Fe(II) redox couples at underlying pyrolytic graphite (PG) electrodes. Electrocatalysis of oxygen and hydrogen peroxide based on direct electrochemistry of heme proteins at {PPI/protein}n film electrodes was also demonstrated.