J. Phys. Chem. B 2005, 109, 20009-20026
20009
Electronic and Vibrational Second-Order Nonlinear Optical Properties of Protein Secondary Structural Motifs John M. Perry, Andrew J. Moad, Nathan J. Begue, Ronald D. Wampler, and Garth J. Simpson* Department of Chemistry, Purdue UniVersity, 560 OVal DriVe, West Lafayette, Indiana 47907 ReceiVed: February 8, 2005; In Final Form: August 12, 2005
A perturbation theory approach was developed for predicting the vibrational and electronic second-order nonlinear optical (NLO) polarizabilities of materials and macromolecules comprised of many coupled chromophores, with an emphasis on common protein secondary structural motifs. The polarization-dependent NLO properties of electronic and vibrational transitions in assemblies of amide chromophores comprising the polypeptide backbones of proteins were found to be accurately recovered in quantum chemical calculations by treating the coupling between adjacent oscillators perturbatively. A novel diagrammatic approach was developed to provide an intuitive visual means of interpreting the results of the perturbation theory calculations. Using this approach, the chiral and achiral polarization-dependent electronic SHG, isotropic SFG, and vibrational SFG nonlinear optical activities of protein structures were predicted and interpreted within the context of simple orientational models.
Introduction Second harmonic generation (SHG) and sum-frequency generation (SFG) have emerged as selective and sensitive probes of local structure in biological systems.1-9 The unique symmetry properties and experimental simplicity of frequency doubling are finding increasing applications in SHG microscopy investigations.1-5,10 SHG is symmetry forbidden in randomly oriented media, but allowed in systems that locally lack inversion symmetry. Consequently, SHG microscopy provides image contrast related directly to structure, complementing other optical characterization methods such as one- and two-photon induced fluorescence. In recent years, relatively large SHG intensities have been observed for several native protein structures, including collagen networks and connective tissue.2,5,6 These protein structures routinely exhibit intricate polarization and angle dependences, suggesting the potential for utilizing polarization analysis for interpreting microscopic protein structure.2,5,7-9 Nonlinear optical (NLO) polarization analysis in biological systems is particularly intriguing given the high sensitivity of second-order NLO measurements to chirality at interfaces.11-21 The applications of vibrational SFG in biomolecular studies are also rapidly increasing.22-41 Recent observations of significant chiral-specific SFG activity in isotropic solutions have the potential to lead to a host of novel spectroscopic methods, since the chiral response detected by SFG arises from distinctly different interactions than those driving absorbance circular dichroism (CD).33-40,42 The rich structural information contained within the polarization dependence of NLO phenomena will most certainly be increasingly applied to structural studies of proteins and biomolecules. A significant hurdle in further advancing the analysis of SHG and SFG polarization measurements of biomolecular assemblies is the absence of a significant body of literature on predicting the NLO properties of proteins. The development of practical bottom-up approaches for understanding the relationships connecting the macroscopic polariza* To whom correspondence should be addressed. E-mail: gsimpson@ purdue.edu.
tion measurements to the microscopic protein structures will help provide a framework for interpreting the rich information contained within second-order NLO measurements of protein assemblies. Given the established success of Moffitt’s exciton coupling theory for interpreting absorbance and circular dichroism spectra of protein structures and other polymers,43-48 it seems natural to adopt a similar approach for describing the NLO properties of proteins. The electronic absorbance spectra of polyamides can be understood using perturbation theory, in which the interactions between adjacent chromophores are introduced as corrections to the system of uncoupled chromophores.49
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