Second-harmonic generation optical activity of a polypeptide alpha-helix at the air/water interface

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DOIResolve DOI: http://doi.org/10.1063/1.1862613
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TypeArticle
Journal titleThe Journal Of Chemical Physics
Volume122
Issue11
Pages1147078; # of pages: 114700
Subjectchirality; interface phenomena; molecular configurations; nonlinear optical susceptibility; optical harmonic generation; optical rotation; polarisability; polymers
AbstractQuantitative measurements of second-harmonic generation optical activity (SHG-OA) have been performed for αα-helical polypeptides poly-(γγ-benzyl-LL-glutamate) and poly-(γγ-ethyl-LL-glutamate) adsorbed at the air∕water interface, with the fundamental frequency ℏω=2.96eVℏω=2.96eV (λ=417nm)(λ=417nm). The chiral component of the nonlinear susceptibilityχ(2)XYZχXYZ(2) is small for both polymers, being comparable in magnitude with the susceptibility χ(2)XXZχXXZ(2) of the clean air∕water interface. The microscopic origin of the nonlinear response has been investigated by using semiempirical ZINDO∕S calculations in conjunction with standard time-dependent perturbation theory to evaluate the molecular hyperpolarizability tensor of a model αα-helix composed of glycine residues. Calculated nonlinear susceptibilities (per monomer unit) are in good agreement with experimental measurements for both the chiral and achiral response. The computational results indicate that charge transfer transitions of the αα-helix have a large influence on the achiral components of the hyperpolarizability tensor, and produce characteristic features in the response under suitable experimental conditions. The dominant origin of SHG-OA for the model αα-helix is a structural effect due to the tilt of the plane of each amide group of the helix relative to the helical axis. SHG-OA is associated with the orientational distribution of isolated, achiral chromophores, and is present in the absence of electronic coupling between the amide subunits of the polypeptide αα-helix.
Publication date
LanguageEnglish
AffiliationNational Research Council Canada; NRC Steacie Institute for Molecular Sciences
Peer reviewedYes
NPARC number12328140
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Record identifierdf0a8259-4bd5-44b8-8226-d7626967c07c
Record created2009-09-10
Record modified2017-03-23
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