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A
13
(a)
15.0
pK
2
= 4.9
14.6
14.2
pK
1
= 2.8
13.8
13.4
1234
pH
56
(b)
6
5
4
3
2
0 0 0 0 0 0
Time (min)
(c)
Figure16.9
(a) 300MHz EPR spectrum of a 2ml aqueous solution of 2mM API probe. Spectrometer settings
wereasfollows:microwavepower,5mW;modulationamplitude,1G.(b)Thedependenceofhyperfinesplitting
(a
N
) on pH calculated from corresponding 300MHz EPR spectra of the API nitroxide measured as a half a
distancebetweenlowandhighfieldcomponents,A
13
/2(Figure16.9a).Thesolidlinewascalculatedaccording
tothestandardtitrationequationforthecompoundwithtwoionizablegroups.
18
NoteconvenientpHsensitivity
range from pH 1.8 to pH 6, ideally optimized for monitoring of stomach acidity. (c) Time dependencies of pH
changesinthestomachmeasuredbyLODEPRspectrometeroperatedat304MHzEPRexcitationfrequency,after
giving3ml ofgavagecontaining5mM APIalone(
).Afterdosing theratswere
placed in the LODEPR coil assembly and a seriesof spectrawere obtained.The pH valuesfrom each spectrum
weredeterminedusingcorrespondingcalibrationcurvefora
N
.
•
)or with50mM bicarbonate(
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