Fluorescent dyes, fluorescent dye kits, and methods of preparing labeled molecules

A method for preparing a labeled protein comprising:

• (a) providing i) an amine-reactive detectable dye or an amine-reactive detectable label which is a biotin or a digoxin and ii) a first amine which is a weak primary aliphatic amine;
• (b) combining the amine-reactive detectable label or the amine-reactive detectable dye and the first amine, in a buffer, with a sample solution comprising a target protein, the target protein comprising a second amine, to form a combined solution;
• and
• (c) allowing the amine reactive detectable dye or the amine-reactive detectable label to react with the first amine and the second amine in said combined solution;
• wherein said amine-reactive detectable label or amine-reactive detectable label comprises an activated ester and wherein the amine-reactive detectable label or amine-reactive detectable dye in said combined solution reacts more strongly with the second amine than the first amine, thereby producing the target protein labeled with said target protein labeled with said detectable dye or said detectable label.