Isomerization of Aspartyl Residue in Amyloid Beta Fragments: The Kinetics by Real-Time 1H NMR Under Neutral and Basic Conditions

Isomers of aspartyl (Asp) residues have been found in various proteins in aged human tissues. Isomerization of Asp is considered as one of the potential factors to facilitate amyloidosis via the oligomerization of amyloid beta (Aβ) peptides. It is still challenging to analyze such isomerization in real time, and no information has been provided about Asp isomerization in situ. In this work, we report a real-time quantitation of the isomerization at Asp in Aβ fragment peptides, Aβ2−9 (A 2 EFRHD 7 SG 9 ) and Aβ2−16 (A 2 EFRHD 7 SGYEVHHQK 16 ) by using solution-state 1 H NMR in situ. The simultaneous NMR detection of isomerization product and reactant enables us to compare how fast Asp is converted to isoAsp in terms of the kinetics of isomerization in real time. In Aβ2−9, the rate constant, k of the increase of isoAsp7 was almost parallel with the decrease of Asp7; the rate constants were estimated to be (1.3 ± 0.1) × 10 −3 h −1 for Asp7 decrease and (1.2 ± 0.1) × 10 −3 h −1 for the increase of isoAsp7 at 70 °C. In Aβ2−16, however, k of isoAsp7 increase was much less than the Asp7 decrease. This is due to an inhibitory effect of histidine (His) on the progress of Asp isomerization; the coupling of His imidazole ring with a succinimide (Suc) intermediate makes Suc lifetime longer enough to interfere with the conversion to isoAsp.