Figure 3.

Fluorescence excitation spectra at 25 °C (native) of the 2Ap- and PC-labeled constructs: (A) excitation at 305 nm, the 2Ap excitation maximum; (B) excitation at 340 nm, the PC excitation maximum. Shown are the steady-state emission spectra of constructs with an eight-CAG repeat loop in a fixed position (red), a six-CAG repeat loop in three possible loop positions (purple), a four-CAG repeat loop in five possible loop positions (blue), and a two-CAG repeat loop in seven possible loop positions (green). Also shown are the emission spectra of our construct when the repeat is fully base-paired (light-brown) and fully single-stranded (black). It should be noted that the PC fluorescence signal (460 nm peak) in (A) may be due in part to energy transfer from either 2Ap or the other bases, as PC excitation at this wavelength is at a minimum.