PROTON-EXCHANGE RATES FROM AMINO-ACID SIDE-CHAINS - IMPLICATIONS FOR IMAGE-CONTRAST

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.