Ring-chain interconversion of sulforhodamine-amine conjugates involves an unusually labile CN bond and allows measurement of sulfonamide ionization kinetics

Graphical Abstract

The kinetics of this process have been studied over the pH range 0–13. The rate of color change varies by nine orders of magnitude over this range and ring-opening involves unimolecular CN bond fission. Deprotonation of the sulfonamide by OH⁻ is diffusion-controlled.

Abstract

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 pK_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 pK_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 × 10⁶ 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) × 10⁹ 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 NCH₃ 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.