Analysis of myoglobin adsorption to Cu(II)-IDA and Ni(II)-IDA functionalized Langmuir monolayers by grazing incidence neutron and X-ray techniques.

The adsorption of myoglobin to Langmuir monolayers of a metal-chelating lipid in crystalline phase was studied using neutron and X-ray reflectivity (NR and XR) and grazing incidence X-ray diffraction (GIXD). In this system, adsorption is due to the interaction between chelated divalent copper or nickel ions and the histidine moieties at the outer surface of the protein. The binding interaction of histidine with the Ni-IDA complex is known to be much weaker than that with Cu-IDA. Adsorption was examined under conditions of constant surface area with an initial pressure of 40 mN/m. After approximately 12 h little further change in reflectivity was detected, although the surface pressure continued to slowly increase. For chelated Cu2+ ions, the adsorbed layer structure in the final state was examined for bulk myoglobin concentrations of 0.10 and 10 microM. For the case of 10 microM, the final layer thickness was approximately 43 A. This corresponds well to the two thicker dimensions of myoglobin in the native state (44 A x 44 A x 25 A) and so is consistent with an end-on orientation for this disk-shaped protein at high packing density. However, the final average volume fraction of amino acid segments in the layer was 0.55, which is substantially greater than the value of 0.44 calculated for a completed monolayer from the crystal structure. This suggests an alternative interpretation based on denaturation. GIXD was used to follow the effect of protein binding on the crystalline packing of the lipids and to check for crystallinity within the layer of adsorbed myoglobin. Despite the strong adsorption of myoglobin, very little change was observed in the structure of the DSIDA film. There was no direct evidence in the XR or GIXD for peptide insertion into the lipid tail region. Also, no evidence for in-plane crystallinity within the adsorbed layer of myoglobin was observed. For 0.1 microM bulk myoglobin concentration, the average segment volume fraction was only 0.13 and the layer thickness was < or = 25 A. Adsorption of myoglobin to DSIDA-loaded with Ni2+ was examined at bulk concentrations of 10 and 50 microM. At 10 microM myoglobin, the adsorbed amount was comparable to that obtained for adsorption to Cu2+-loaded DSIDA monolayers at 0.1 M. But interestingly, the adsorbed layer thickness was 38 A, substantially greater than that obtained at low coverage with Cu-IDA. This indicates that either there are different preferred orientations for isolated myoglobin molecules adsorbed to Cu-IDA and Ni-IDA monolayer films or else myoglobin denatures to a different extent in the two cases. Either interpretation can be explained by the very different binding energies for individual interactions in the two cases. At 50 microM myoglobin, the thickness and segement volume fraction in the adsorbed layer for Ni-IDA were comparable to the values obtained with Cu-IDA at 10 microM myoglobin.