The stability of the NHS esters differs between the individual dyes.
For non-dissolved dyes which are kept at dry conditions at 4 °C the shelf life is typically 6 - 12 month. The shelf life increases significantly if they are stored at -20 °C or better -80 °C.
For dissolved NHS esters the shelf life depends strongly on the solvent used for dissolving or diluting the dye. Best use water free solvents.
In water containing solvents NHS esters are very labile.
Therefore we recommend dissolving NHS esters in water free Dimethylformamide. Alternatively water free and very pure Dimethylsulfoxide may be used.
N-Hydroxysuccinimidyl (NHS) esters of the carboxylic acids (RCOOH) look like RCOONS (where NS is N-succinimidyl residue). They readily react with primary amines (R'NH2) forming hydrolytically stable amides RCONHR' and N-hydroxysuccinimide.
N-Hydroxysuccinimidyl carbonates of the alcohols (RCH2OH) look like RCH2OCONS, where NS is N-succiimidyl residue (as above). They readily react with amines R'NH2 forming hydrolytically stable urethanes RCH2OCONHR' and N-hydroxysuccinimide.
N-hydroxysuccinimidyl carbonates are expected to react with amines a bit slower, but their stability is typically higher that that of N-hydroxysuccinimidyl esters.