Atropisomers of asymmetric xanthene fluorescent dyes and methods of DNA sequencing and fragment analysis

An energy-transfer dye comprising:

• a donor dye capable of absorbing light at a first wavelength and emitting excitation energy in response thereto;
• an acceptor dye capable of absorbing the excitation energy emitted by the donor dye and fluorescing at a second wavelength in response;
• and
• a linker for linking the donor dye and the acceptor dye;
• wherein at least one of the donor dye and acceptor dye is an atropisomerically enriched xanthene compound having the structure:
• Z1 is OH, NH2, NHR, or NR2, wherein each R is independently hydrogen, C1-C12 alkyl, phenyl, benzyl, aryl, heterocycle, or a linking moiety;
• Z2 is O, +NHR, +NR2, wherein each R is independently hydrogen, C1-C12 alkyl, phenyl, benzyl, aryl, heterocycle, or a linking moiety;
• X is carboxylate or sulfonate;
• and said structure comprises aryl-substituted forms thereof.