Determination of the acid dissociation constant of the biosurfactant monorhamnolipid in aqueous solution by potentiometric and spectroscopic methods.
The acid dissociation constant in water for a monorhamnolipid mixture extracted from Pseudomonas aeruginosa ATCC 9027 has been determined using potentiometry and two spectroscopic approaches at concentrations below and above the critical micelle concentration (cmc). Potentiometric titrations resulted in pKa values ranging from 4.28 +/- 0.16 to 5.50 +/- 0.06 depending on concentration. 1H NMR spectrochemical titrations at concentrations below the cmc revealed a pKa value of 4.39 +/- 0.06. ATR-FT-IR spectrochemical titrations on solutions well above the cmc gave a pKa value of 4.84 +/- 0.05. The value of 4.28 for the free rhamnolipid molecule for concentrations below the cmc differs markedly from that reported previously. However, the pKa of 5.50 for surface-adsorbed and solution aggregates correlates closely to that previously reported. Differences in these pKa values are rationalized in terms of the pH- and concentration-dependent aggregation behavior of rhamnolipids in aqueous solution.
Lebrón-Paler
Ariel
A
Department of Chemistry, University of Arizona, 1306 East University Boulevard, Tucson, Arizona 85721, USA.
Pemberton
Jeanne E
JE
Becker
Bridget A
BA
Otto
William H
WH
Larive
Cynthia K
CK
Maier
Raina M
RM
D000042:Absorption; D000143:Acids / Q000032:analysis*; D006017:Glycolipids / Q000032:analysis / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D011199:Potentiometry / Q000379:methods; D011550:Pseudomonas aeruginosa / Q000737:chemistry*; D012996:Solutions / Q000737:chemistry; D017550:Spectroscopy, Fourier Transform Infrared / Q000379:methods; D013501:Surface-Active Agents / Q000737:chemistry* / Q000302:isolation & purification; D014867:Water / Q000737:chemistry*
D000143:Acids; D006017:Glycolipids; D012996:Solutions; D013501:Surface-Active Agents; C418382:rhamnolipid; D014867:Water