This product is soluble in water and aqueous buffers. It is recommended to prepare solutions fresh as solutions will degrade quickly. At pH 7.0 – 7.5, NHS esters react optimally with free amines. However, NHS esters can react with free amines with a pH as low as 6.0. As the pH increases, the hydrolysis rate of the NHS ester increases. Indeed, at pH 7 and 0°C, the NHS ester half-life is 4 to 5 hours in aqueous buffer, while at pH 8 and 25°C, the half-life of an NHS ester is 1 hour.
Alternative solvents have been cited in the literature. Sources state that a stock solution of 10 mm Mal–dPEG24–NHS ester can be prepared in chloroform and subsequently diluted in dimethylformamide. See the link below to review this article:
https://onlinelibrary.wiley.com/doi/full/10.1002/admt.202200490