E. Liepinsh et G. Otting, PROTON-EXCHANGE RATES FROM AMINO-ACID SIDE-CHAINS - IMPLICATIONS FOR IMAGE-CONTRAST, Magnetic resonance in medicine, 35(1), 1996, pp. 30-42
The proton exchange rates between water and the hydroxyl protons of th
reonine, serine, tyrosine, the amino protons of lysine, and the guanid
inium protons of arginine were measured in the pH range 0.5 to 8.5 and
for the temperatures 4 degrees C, 10 degrees C, 20 degrees C, 30 degr
ees C, and 36 degrees C. The intrinsic exchange rates of the hydroxyl
and amino protons at pH 7.0 degrees C and 36 degrees C were found to b
e in the range 700 to about 10,000 s(-1). In addition, the exchange ca
talysis by phosphate, carbonate, carboxyl-, and amino-groups was inves
tigated, The presence of these exchange catalysts at physiological con
centrations increased the proton exchange rates from hydroxyl and amin
o groups several fold, The proton exchange rates are sufficiently fast
that the total magnetization transfer between biomolecules and free b
ulk water is not rate limited by the proton exchange rate, but by the
intramolecular cross-relaxation rates between the exchangeable and non
exchangeable protons of the biomolecules, Since the cross-relaxation r
ates between surface hydration water molecules and biomolecules are us
ually vanishingly small because of too rapid exchange with the free bu
lk water, it is proposed that the contrast in MR images is a fingerpri
nt of the number of the exchangeable protons from OH and NH groups of
the tissue, as far as the contrast depends on the magnetization transf
er between biomolecules and water.