Model right-handed -helix composed of seven glycine residues. (a) Illustration of molecular structure showing carbon, oxygen, and nitrogen atoms in progressively darker shades of gray. Hydrogen atoms are shown in white. (b) Division of model -helix into seven -methylacetamide monomers, with the orientations of the amide groups preserved. (c) End-on view of model -helix, showing only the carbonyl groups. Note that the CO bonds are tilted away from the long axis of the helix.
Rotation traces for the polypeptides PBLG and PELG and for the achiral surfactant DBSA adsorbed at the air∕water interface. The solid lines are fits of the data to Eq. (8). Note that the traces for the chiral interfaces PBLG and PELG are asymmetric about , and the trace for the achiral interface DBSA is symmetric.
Calculated nonlinear susceptibilities for the model -helix, shown as a function of the tilt angle of the long axis of the helix relative to the surface normal. The fundamental frequency was . The magnitude of each susceptibility is plotted in the same arbitrary units. Results are shown for an assumed square distribution of the tilt angle about the mean value, with width 10°.
(a) and (b) Calculated nonlinear susceptibilities for the model -helix, shown as a function of photon energy of fundamental radiation. The tilt angle of the long axis of the helix relative to the surface normal is 0° for , and , and 90° for . The magnitude of each susceptibility is plotted in the same arbitrary units, with shown expanded. (c) Absorption spectrum of polypeptide -helix. The solid line shows the calculated spectrum and the broken line is an experimental spectrum of poly-(-methyl--glutamate) from Ref. 33 (see the text). Note that the energy scale is in (c) only, to facilitate comparison with the nonlinear susceptibilities.
Calculated difference in hyperpolarizability tensor elements , shown as a function of photon energy of fundamental radiation for (a) the model -helix, and (b) the simplified model of seven independent -methylacetamide monomers. The real and imaginary parts and the modulus are plotted in atomic units.
Nonlinear susceptibilities of PELG and PBLG monolayer films at the air∕water interface, with . is the density of monomer constituents in the polmer films. and are effective nonlinear susceptibilities normalized to the susceptibility measured for the clean air∕water interface in this work. Two values for and shown on separate lines were derived by using alternative values of for the clean air∕water interface as shown in the table. The linear optical properties of the monolayers were assumed to be those of water.
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