. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 232 REPORTS FROM THE MBL GENERAL SCIENTIFIC MEETINGS. Figure 1. Crystals of purified native GFP. (A) Dichmism (anisotropic absorbance) in visible light with crystals appearing green to light brown with the long crystal axis oriented "parallel. " and pale blue to white with the long crystal axis oriented "perpendicular, " to the polarizer. (In fact, a polarizer and analyzer were used, off-crossed hy about five degrees, to accentuate the weak visible light dichroism.) (B. C) Polarization-depen- dent anisotr
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 232 REPORTS FROM THE MBL GENERAL SCIENTIFIC MEETINGS. Figure 1. Crystals of purified native GFP. (A) Dichmism (anisotropic absorbance) in visible light with crystals appearing green to light brown with the long crystal axis oriented "parallel. " and pale blue to white with the long crystal axis oriented "perpendicular, " to the polarizer. (In fact, a polarizer and analyzer were used, off-crossed hy about five degrees, to accentuate the weak visible light dichroism.) (B. C) Polarization-depen- dent anisotropv of fluorescence excitation seen in the absence of an analvzer. The polarizer E-vector in Panel B is oriented parallel to the prominent crystal in the middle of the panel. In Panel C. it is oriented perpendicular to the length of the Mime tr\\lul. Bar = 30 They also show a weak, hut distinct, hlue-green dichroism (Fig. 1A). Illuminated with blue light of less than 450-nm wavelength, the same crystals show a very bright, green fluorescence when viewed through a 527 ± 15-nm band-pass filter. Surprisingly, the bright- ness of the fluorescence varied by a ratio of as much as 6:1 when the crystals were illuminated with polarized blue light and ob- served in the absence of an analyzer (Fig. IB. 1C). The fluores- cence was greatest when the long axis of the crystal lay parallel to the transmission direction of the polarizer E-vector. In other words, the absorption for the exciting light is six times greater with its E-vector polarized parallel to the length of the crystal axis than across. A similarly high ratio and orientation dependence was observed when the crystals were observed with non-polarized illumination but through an analyzer. In other words, the fluores- cence emitted by crystals illuminated with non-polarized light is again some six times greater for polarization parallel to the long crystal axis. Between parallel polarizer and analyzer, the orienta- tion-depe
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