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Evolution of electric dipole (hyper)polarizabilities of β-strand polyglycine single chains: an ab initio and DFT theoretical study.

Ab initio and density functional theory calculations have been carried out on dipole moments (μ), polarizabilities (α), and second-order hyperpolarizabilities (γ) of single chain β-strand oligoglycines in the gas phase. Basis set and electron correlation effects have been investigated. The long-range corrected CAM-B3LYP and ωB97X-D functionals furnish satisfactory results in agreement with correlated ab initio computations. The dependence of μ, α, and γ values per unit cell on chain length has been explored, extrapolating the properties in the limit of the polymer. The increase of the response electronic properties with the number of glycine units has been rationalized through the two-state model involving the typical π-π* NV1 electronic transition of peptides. The effect of the secondary structure on the electric properties has been discussed. At variance from the dipole moments, the γ values increase on going from the α-helices to the β-strands, whereas the α values are little influenced by the conformation of the oligoglycines.