Solvent effects on ultraviolet absorption and circular dichroic spectra of helical polypeptides and globular proteins. Calculations based on a lattice-filled cavity model

Solvent effects on the electronic absorption and circular dichroic spectra of helical polypeptides and globular proteins are modeled by placing the molecule in a spherical cavity in a continuous dielectric medium and filling voids in the cavity with a simple cubic lattice of solvent molecules. Effects on the amide pi-pi* spectra near 200 nm are treated. The interactions among the solute atoms, solute chromophores, and discrete solvent molecules are treated by the partially dispersive dipole interaction treatment applied previously to the isolated molecules, while the dielectric continuum is treated by a rigorous reaction field formalism. Calculations are given for helical polypeptides alpha(R)-(Ala)(10), (Pro)(10) II, beta(P)-(AbU)(6x3), beta(A)-(Abu)(6x3) and globular proteins erabutoxin, cytochrome c, and myoglobin. Predicted solvent effects are generally small, but include potentially measurable increases in intensities of CD bands in alpha(R)-(Ala)(10), (Pro)(10) II, and the three proteins, and hypochromic shifts in the absorption bands of beta-sheets.