Time-domain chirally-sensitive three-pulse coherent probes of vibrational excitons in proteins

TL;DR

This paper develops a theoretical framework to analyze the third-order optical response of vibrational excitons in proteins, revealing chirally-sensitive signals that distinguish protein secondary structures.

Contribution

It introduces a Green's function approach to calculate nonlinear exciton responses, predicting new chiral response components in isotropic protein structures.

Findings

Chirally-sensitive signals differentiate alpha helices and beta sheets.

The nonlocal response reduces computational complexity.

Application to peptide vibrations demonstrates practical relevance.

Abstract

The third order optical response of bosonic excitons is calculated using the Green's function solution of the Nonlinear Exciton Equations (NEE) which establish a quasiparticle-scattering mechanism for optical nonlinearities. Both time ordered and non ordered forms of the response function which represent time and frequency domain techniques, respectively, are derived. New components of the response tensor are predicted for isotropic ensembles of periodic chiral structures to first order in the optical wavevector. The nonlocal nonlinear response function is calculated in momentum space, where the finite exciton-exciton interaction length greatly reduces the computational effort. Applications are made to coupled anharmonic vibrations in the amide I infrared band of peptides. Chirally-sensitive and non sensitive signals for alpha helices and antiparallel beta sheets are compared.