6-SFX (full name: 6-(Fluorescein-6-carboxamido)hexanoic acid, succinimidyl ester, single isomer) has a 6-atom aminohexanoyl spacer group that separates the fluorescein dye and the amine-reactive group.
&lamba;Ex/&lamba;Em (pH 9) = 494/521 nm
Yellow solid soluble in DMF, DMSO or pH>6.5 buffers