The effect of secondary structures on the NLO properties of single chain oligopeptides: a comparison between β-strand and α-helix polyglycines

Abstract

The evolution of the electronic first-order longitudinal hyperpolarizability (β_zzz) and the hyperpolarizability aligned along the direction of the dipole moment (β_μ) of the α-helix and β-strand single chain H₂N–(CH₂–CO–NH)_n–CH₂–COOH (n = 1–9) oligoglycines, were investigated. For this purpose we have used Hartree–Fock, second-order Møller–Plesset perturbation theory and Coulomb-attenuating Density Functional Theory computations. For the longest chain, β_μ(β-strand) is one order of magnitude greater than β_μ(α-helix), due to the cooperative effect of the α-helices being unfavourable for the NLO properties. The β_zzz and β_μ values per unit cell of the β-strand conformation were determined, extrapolating the properties in the limit of the polymer. The calculated β_zzz values were elucidated using the two-state model involving the characteristic π–π* NV₁ electronic transition of peptides. Single chain β-strand polyglycines can be discriminated from the α-helices using second-order NLO effects.