Environmental Science and Technology 2013, Vol 47: 4080−4087
Kinetic studies showed that [Asp3, Dhb7]MC-RR reacted with
mercaptoethanol hundreds of times more slowly than MC-RR and a range of
other [Mdha7]-containing microcystin congeners. The difference in reaction
rate was sufficiently large that derivatization of microcystin-containing samples
with mercaptoethanol, followed by LC-MS analysis, clearly discriminated
between microcystins containing the isobaric [Dhb7]- and [Mdha7]-groups.
Application of this approach, using LC-MS with both-ion trap and triplequadrupole
mass spectrometers, to water samples and Planktothrix cultures from Lake Steinsfjorden, Norway, demonstrated the
presence of [Asp3, Dhb7]MC-RR (5), [Asp3]MC-RY (14), and [Asp3]MC-LY (16), as well as analogues tentatively identified as
[Asp3]MC-RR (4), [Asp3, DMAdda5, Dhb7]MC-LR (6), [Asp3, Dhb7]MC-HtyR (8), [Asp3]MC-HtyR (9), [Asp3, Dhb7]MC-LR
(10), [Asp3]MC-LR (11), [Asp3, Dhb7]MC-RY (15), and [Asp3, Dhb7]MC-LY (17), together with low levels of several other
analogues. This is the first use of this thiol-based LC-MS approach to identify Dhb-containing microcystins, and allowed
identification of LC-MS peaks in a mixture of [Mdha7]- and [Dhb7]-congeners of [Asp3]MC-RR (4, 5), -RY (14, 15), and -LY
(16, 17) in the samples from L. Steinsfjorden. This is also the first report of MC-RY-congeners outside of Africa, or in
Planktothrix spp. Analysis of European crayfish (Astacus astacus) taken from L. Steinsfjorden revealed the presence of only trace
levels of microcystins in the edible parts.