Valence-Topological Charge-Transfer Indices, Dipole, Isoelectric Point and Fractal: From Homo/Heterocycles to Proteins

Abstract

In the present study, valence-topological charge-transfer indices are used to calculate the isoelectric point (pI). Dipoles are computed by vector semisums µvec and valence vector semisums µvec V comparing with isoelectronic series (cyclopentadiene, benzene, styrene). The valence vector semisum results intermediate between the vector semisum and experimental dipole. The steric effect stays constant and the dominating effect is electronic. The indices are applied to the dipole of homologous series of percutaneous enhancers and amino-acid pI. No collapse of Gi-Ji indices is observed in the fit of pI decreasing co-llinearity risk. A heteroatom in the π-electron systems is beneficial. Acidic and basic groups improve models of pI. Fragment fitting allows computing lysozyme pI. Globular protein models (lysozyme, myoglobin, albumin) cover a wide pI range. The purpose of the work is to extend the study to anionic lipid bilayers. The role of electrostatics is examined in cationic protein adsorption to zwitterionic phosphatidylcholine (PC) and anionic mixed PC/phosphatidylglycerol (PG) small unilamellar vesicles, via spectrofluorimetry and liquid chromatography. Parameters (partition, co-operativity) are calculated. At pI, binding reaches maximum. In Gouy-Chapman formalism, the activity coefficient rises with square charge; deviations from this model indicate asymmetric location of anionic phospholipid in the inner leaflet, in mixed vesicles for lysozyme- and myoglobin-PC/PG systems in agreement with experiments and molecular dynamics. Vesicles bind myoglobin anti-co-operatively while they attach lysozyme/albumin, co-operatively. A model is proposed for lysozyme/albumin composing two protein sub-layers with different structures and properties. The Hill coefficient reflects the subunit co-operativity of bi and tridomain proteins. Keywords: Charge distribution, dipole moment, fractal hybrid orbital analysis, isoelectric point, valence-isoelectronic series, valencetopological charge-transfer index.