Observation and Kinetic Characterization of Transient Schiff Base Intermediates by CEST NMR Spectroscopy

Abstract

In aqueous solution, many biochemical reaction pathways involve reaction of an aldehyde with an amine, which progresses through generally unstable, hydrated and dehydrated, Schiff base intermediates that often are unobservable by conventional NMR. There are 4 states in the relevant equilibrium: 1) gem‐diol, 2) aldehyde, 3) hemiaminal, and 4) Schiff base. For the reaction between protein amino groups and DOPAL, a highly toxic metabolite of dopamine, the 1H resonances of both the hemiaminal and the dehydrated Schiff base can be observed by CEST NMR, even when their populations fall below 0.1 %. CEST NMR reveals the quantitative exchange kinetics between reactants and Schiff base intermediates, explaining why the Schiff base NMR signals are rarely observed. The reactivity of DOPAL with Nα‐amino groups is greater than with lysine Nϵ‐amines and, in the presence of O2, both types of Schiff base DOPAL–peptide intermediates rapidly react with free DOPAL to irreversibly form dicatechol pyrrole adducts.

A structure-supporting, self-healing, and high permeating hydrogel bioink for establishment of diverse homogeneous tissue-like constructs

“…4 D). This was related to the instability of the reversible Schiff base linkages in the presence of water, which can tilt the equilibrium towards the starting materials [ 42 ]. With the continuous slow phase crosslinking between GEL and PEG-SG ( Fig.…”

“…5 C(ii)). For AHA/CMC hydrogel, due to the dehydration nature of the reversible Schiff base reaction, while the dynamic Schiff base linkages happen to be unstable in water [ 42 ], the filaments might fuse after only 3 min and result in bulk hydrogel ( Fig. 5 C(iii)).…”

“…In this work, the fast gelation and dynamic imine bonds of AHA/CMC hydrogels enabled direct printing after bioink preparation. Due to the dehydration nature of the reversible Schiff base reaction and the instability of dynamic imine bond in water, the partial healing behavior of AHA/CMC hydrogel in rheological tests was not the incomplete recovery after deformation, but the reduction of G′ caused by the hydrolysis of the imine bond [ 42 ]. It is also for this reason that the AHA/CMC constructs fused into bulk hydrogels within 3 min after printing ( Fig.…”