Electrostatics of Horse Heart Cytochrome C and Montmorillonite Monolamellar Plate

ABSTRACT

Monomolecular forms of the protein Cytochrome C (CytC) and the mineral Montmorillonite (MM) were studied theoretically by pH-dependent approach of home maid program „PHEI package” with aim to find optimal conditions (pH and ionic strength) of CytC adsorption on MM. The main calculated characteristics were: net-charge titration [Z(pH)], electrostatic term and total free energy (stability) [ΔG_el(pH) and ΔG_tot(pH)], proton affinities of individual ionic groups [pK_{a, i}(pH)], Coulomb interaction of each site [E_el(pH)] and electric moments (vectors and scalars) [μ_e(pH) and μ_s(pH)]. CytC was calculated with and without reaction field effects (Born corrections) to understand the role of the water in the charge-charge interactions. Abnormal properties were obtained for many of the surface ionic sites on CytC (Lys 87, Lys 86, Lys 13, Lys 27, Lys 25, etc). In agreement with the experiments the reduced protein is more stable with -7.5 kcal/mol and both forms have isoelectric point (pI) above 10. In pH interval 5–8 all pH-dependent properties are practically non-changed. The electric moments are large and have similar orientation (parallel to the hem plane). The 3D-structure of 35x35Å MM plate was designed. The theoretical and the experimental [Z(pH)] titration curves of MM were compared at acidic and alkaline (close to pI) pH with and without counter-ions. The montmorillonite plate (pMM) undergoes strong ionic interaction between allumo-hydroxyls fixed as a monolayer under Na⁺ ions. Their intrinsic pK_a = 9 increased in 10–25 orders of magnitude and are distributed in nine “levels”. They produce a big electric moment directed normally to the plate. In the range of pH 5–10 no considerable changes in the effective charge were found.

Keywords:

• Cytochrome C
• Montmorillonite
• pH-dependent properties