Abstract:
Background Mycobacterium ulcerans is the causative agent of Buruli ulcer. The pathology of M. ulcerans disease
has been attributed to the secretion of a potent macrolide cytotoxin known as mycolactone which plays an important
role in the virulence of the disease. Mycolactone is a biomarker for the diagnosis of BU that can be detected using
the fuorescent-thin layer chromatography (f-TLC) technique. The technique relies on the chemical derivatization
of mycolactone A/B with 2-naphthylboronic acid (BA) which acts as a fuorogenic chemosensor. However, back ground interferences due to co-extracted human tissue lipids, especially with clinical samples coupled with the
subjectivity of the method call for an investigation to fnd an alternative to BA.
Methods Twenty-six commercially available arylboronic acids were initially screened as alternatives to BA using
the f-TLC experiment. UV–vis measurements were also conducted to determine the absorption maximum spectra
of mycolactone A/B and myco-boronic acid adducts followed by an investigation of the fuorescence-enhancing
ability of the boronate ester formation between mycolactone A/B and our three most promising boronic acids
(BA15, BA18, and BA21). LC–MS technique was employed to confrm the adduct formation between mycolactone
and boronic acids. Furthermore, a comparative study was conducted between BA18 and BA using 6 Polymerase Chain
Reaction (PCR) confrmed BU patient samples.
Results Three of the boronic acids (BA15, BA18, and BA21) produced fuorescent band intensities superior to BA.
Complexation studies conducted on thin layer chromatography (TLC) using 0.1 M solution of the three boronic acids
and various volumes of 10 ng/µL of synthetic mycolactone ranging from 1 µL – 9 µL corresponding to 10 ng – 90 ng
gave similar results with myco-BA18 adduct emerging with the most visibly intense fuorescence bands. UV–vis
absorption maxima (λmax) for the free mycolactone A/B was observed at 362 nm, and the values for the adducts
myco-BA15, myco-BA18, and myco-BA21 were at 272 nm, 270 nm, and 286 nm respectively. The comparable experi mental λmax of 362 nm for mycolactone A/B to the calculated Woodward-Fieser value of 367 nm for the fatty acid
side chain of mycolactone A/B demonstrate that even though 2 cyclic boronates were formed, only the boronate
of the southern side chain with the chromophore was excited by irradiation at 365 nm. Fluorescence experiments
have demonstrated that coupling BA18 to mycolactone A/B along the 1,3-diols remarkably enhanced the fuores cence intensity at 537 nm. High-Resolution Mass Spectrometer (HR-MS) was used to confrm the formation of the myco-BA15 adduct. Finally, f-TLC analysis of patient samples with BA18 gave improved BA18-adduct intensities
compared to the original BA-adduct.
Conclusion Twenty-six commercially available boronic acids were investigated as alternatives to BA, used in the f-TLC
analysis for the diagnosis of BU. Three (3) of them BA15, BA18, and BA21 gave superior fuorescence band intensity
profles. They gave profles that were easier to interpret after the myco-boronic acid adduct formation and in experi ments with clinical samples from patients with BA18 the best. BA18, therefore, has been identifed as a potential
alternative to BA and could provide a solution to the challenge of background interference of co-extracted human
tissue lipids from clinical samples currently associated with the use of BA.