How the second-order nonlinear optical response of the collagen triple helix appears: A theoretical investigation

The origin and the nature of the first hyperpolarizability of collagen has been unraveled by performing first-principles calculations on the PPG10 compound, a molecular model for the collagen triple helix structure, and on its building blocks. The first hyperpolarizability of the triple helix originates from the amide groups. Owing to the rigidity of the structure and of the proline units, the β-tensor components parallel to the helical axis add to each other, whereas the perpendicular components cancel each other, which result in a dipolar first hyperpolarizability and a depolarization ratio close to 9. The calculations have also shown that the resulting β values cannot be viewed as the simple sum of the amide group contributions. Indeed, for a given chain, the β per amino acid increases with the size of the chain, whereas the β of the triple helix is smaller than three times the β of a single chain. The calculations, performed at different levels of approximation, demonstrated also the reliability of the ONIOM scheme when combining high and low layers described with different basis sets and the weak impact of electron correlation.