Molecular orientation distributions in protein films. 1. Cytochrome c adsorbed to substrates of variable surface chemistry

Molecular orientation in hydrated cytochrome c (cyt c) films formed by adsorption to substrates of differing surface chemistry was investigated. The orientation distribution of the heme groups in the protein films was determined using a combination of two techniques: absorption linear dichroism, measured in a planar integrated optical waveguide attenuated total reflection geometry, and emission anisotropy, measured in a total internal reflection fluorescence geometry. The mean heme tilt angle and angular distribution about the mean were recovered using a Gaussian model for the orientation distribution. These data are the first orientation distribution measurements reported for protein film assemblies. The results show that a macroscopically ordered film of adsorbed cyt c molecules is produced when a single, high-affinity type of noncovalent binding occurs between the surface of the protein and the substrate surface. For example, electrostatic adsorption of the positively charged protein to the negatively charged head groups of a Langmuir-Blodgett film of arachidic acid produces a narrow orientation distribution. When multiple, competing adsorptive interactions are operative, which is the case when cyt c adsorbs to a clean glass surface, a relatively disordered film is produced.