Second-harmonic generation of biological interfaces: probing the membrane protein bacteriorhodopsin and imaging membrane potential around GFP molecules at specific sites in neuronal cells of C. elegans
A. Lewis et al., Second-harmonic generation of biological interfaces: probing the membrane protein bacteriorhodopsin and imaging membrane potential around GFP molecules at specific sites in neuronal cells of C. elegans, CHEM PHYS, 245(1-3), 1999, pp. 133-144
Second-harmonic generation (SHG) is applied to problems of probing membrane
proteins and functionally imaging around selective sites and at single mol
ecules in biological membranes. The membrane protein bacteriorhodopsin (bR)
has been shown to have large second-harmonic (SH) intensities that are mod
ulated by protein/retinylidene chromophore interactions. The nonlinear opti
cal properties of model compounds, which simulate these protein chromophore
interactions in retinal proteins, are studied in this work by surface SHG
and by hyper-Rayleigh scattering. Our results indicate that non-conjugated
charges and hydrogen bonding effects have a large effect on the molecular h
yperpolarizability of the retinal chromophore. However, mbR, the model syst
em studies suggest that polarizable amino acids strongly affect the vertica
lly excited state of the retinylidene chromophore and appear to play the ma
jor role in the observed protein enhancement (> 50%) of the retinylidene ch
romophore molecular hyperpolarizability and associated induced dipole. Furt
hermore, the data provide insights on emulating these interactions for the
design of organic nonlinear optical materials. Our studies have also led to
the development of dyes with large SH intensities that can be embedded in
cell membranes and can functionally image membrane potential. Single molecu
les of such dyes in selected single molecular regions of a cell membrane ha
ve been detected. SHG from green fluorescent protein (GFP) selectively expr
essed in concert with a specific protein in neuronal cells in a transgenic
form of the worm C. elegans is also reported. The membrane potential around
the GFP molecules expressed in these cells has been imaged with SHG in liv
e animals. (C) 1999 Elsevier Science B.V. All rights reserved.