Is the negative charge on RNHSO₃^-M⁺ an essential requirement for sulfamate sweetness?

Although many structure-taste studies have been carried out on sulfamate (cyclamate) sweeteners, there are still some unanswered questions - notably whether the sulfamate anion, -NHSO₃^-, is essential for sweetness in this class of compounds. The literature is contradictory on this point; therefore, 14 sulfamate esters RNHSO₃R′, which contain the sulfamate moiety but without the negative charge, i.e., -NHSO₃^-, have been synthesized and tasted under standard conditions. Almost all of the esters were found to possess strong sweetness accompanied by bitterness. Because the esters had to be heated in water to 60 °C to dissolve them, it was necessary to check for partial hydrolysis to the free sulfamic acids, RNHSO₃H, since they would be sweet and would invalidate the tasting results if formed. This was done by monitoring (gas-liquid chromatography) the formation of alcohol after heating. Negligible or very low hydrolysis to acid was found for all 14 esters. This work, in addition to answering an important structure-taste question, points the way to the potential use of suitable sulfamate esters as additives in situations where the more usual sodium sulfamate salts are unsuitable, for example, in hydrophobic media.