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Fig. 5 Experimental (a, b) and calculated in vacuo (c, d) preresonance Raman spectra of cGFP
and tGFP (a, c) and cCFP, tCFP (b, d); the spectrum of the trans form is shown inverted around the
x-axis. Calculations are based on the B3LYP functional with Gaussian bases (GB), considering an
excitation at 514.5 nm. As known, GB-B3LYP approach tends to overestimate the mode energies
[ 42 ]; this is reflected in the slightly different scale between theoretical and experimental results.
Gray ellipses highlight corresponding fingerprint modes for the cis-trans transition. Small circles
emphasize stretching modes mostly localized on double bonds in the chromophore region,
highlighted in the insets of panels (c) and (d) with corresponding colors: black ,C
O; green ,
(e, f) Schematic representations of atomic displacements in the modes responsible for the peaks
inside the solid gray ellipse in panel (c) for cGFP, and tGFP, respectively. Panels (a) and (b)
reprinted with permission from [4]. Copyright 2009 American Chemical Society
C on the bridge; orange ,C
C on the phenolic (panel c) and indolic (panel d) rings; blue ,C
deformation of both rings and of the bridge (see Fig. 5e ). Very interestingly, this
mode appears blue shifted of about 30 cm 1 after irradiation of the system in the
UV. An identical shift is observed in the calculation on the trans configuration of
the chromophore, clearly indicating this mode as the fingerprint one for the
cis-trans transition, as will be discussed in more details in Sect. 4.3 .
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