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

Abstract

AbstractpH‐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 pKa 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 pKa ∼ 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 CN 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 CN bond is significantly longer than the exocyclic NCH3 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.

Substituent effects on the apparent pK of the reversible open-to-closed transition of sultams derived from sulforhodamine dyes

“…At the start of our work, the sulforhodamines were converted to their sulfonyl chlorides using phosphoryl chloride alone [2] but we later found that addition of a little phosphorus pentachloride gave more efficient conversion (overnight at room temperature). In either case the crude reaction mixtures, i.e.…”

“…The dipyrrolidinylsulforhodamine 4 was available from previous work [11]. Crystals of the sulforhodamines and the derived sultams had strong tendencies to retain solvent (see examples in previous data for 4 [11] and its derived N-methylsultam [2]) and we were generally unable to obtain satisfactory microanalysis data. Therefore only HRMS data are reported to verify elemental composition but the NMR spectra (apart from traces of residual solvent) contained no extraneous peaks.…”

“…The variability of the pK values for 1 described above shows the potential for manipulating the colour change of these compounds. Unlike conventional pH indicators, the colour change is not ''instantaneous'' in response to a change in pH, but takes place with pH-dependent kinetics that span nine orders of magnitude across the pH range 0e13 [2]. Near pH 7, observed rate constants for the colour change of 1 in stopped-flow experiments were w200 s À1 (at 4 C) whether approached from the coloured or colourless form [1,2].…”

“…1). More recently we have formulated a detailed kinetic scheme to explain the complex kinetics of this interconversion over the pH range 0e13 [2]. That study was conducted entirely with compound 1 (Fig.…”

Fluorescence modulation sensing of positively and negatively charged proteins on lipid bilayers

“…The ortho rhodamine B isomer exhibits much more complete quenching behavior at high pH compared to ortho-Texas Red [30,32]. Additionally, the ortho isomer of rhodamine B exhibits pHsensitive fluorescence in a more acidic pH range and is far less expensive than Texas Red.…”