Ring‐chain interconversion of sulforhodamine‐amine conjugates involves an unusually labile CN bond and allows measurement of sulfonamide ionization kinetics

pH‐dependent interconversion between ring and chain forms of sultams/sulfonamides derived from conjugates of sulforhodamines with amines, and the associated sulfonamide ionization, have been studied by a combination of equilibrium and kinetic methods. The colorless, ring‐closed sultam form is favored at alkaline pH with an apparent p K a of 7.37 for the color change of the methylamine conjugate of Sulforhodamine B. The ring‐closed form is also favored at very low pH (apparent p K a  ∼ 0.68) by protonation of both diethylamino substituents. The kinetics of interconversion between open and closed forms were measured at 4°C over the pH range 0–13. The observed rate constant ranges over nine orders of magnitude from 4.8 × 10 −4  s −1 at pH 1 to 2.27 × 10 6  s −1 at pH ≥ 12. Above pH 2, the data are accommodated by a mechanism that includes cleavage of the sultam CN bond in the ring opening step with a sulfonamide anion as the leaving group, and in the reverse reaction, OH − ‐dependent ionization of the sulfonamide at 4.8 (±2.0) × 10 9  M −1  s −1 . Below pH 2, H + ‐dependent protonation occurs at 1.4 (±0.4) × 10 −3  M −1  s −1 . X‐ray crystallography of a related N ‐methylsultam showed that the labile, endocyclic CN bond is significantly longer than the exocyclic NCH 3 bond (1.509 Å and 1.445 Å, respectively). Laser‐induced ring opening of the closed form has potential application as an orientation probe of biological macromolecules. Copyright © 2008 John Wiley & Sons, Ltd.