EPR spectroscopy and theoretical study of gamma-irradiated asparagine and aspartic acid in solid state.

Aspartic acid (Asp) and asparagine (Asn) are vulnerable amino acids. One-electron addition or withdrawal reactions initiate many deleterious processes involving these amino acids. To study these redox processes we have irradiated by gamma-rays asparagine or aspartic acid in the solid state. The nature of the resulting free radicals was determined by electron paramagnetic resonance (EPR) and by calculations using DFT methods in various environments. Reactions initiated by electron transfer are different for both amino acids: Asn anion loses hydrogen atom whereas the cation undergoes decarboxylation. Conversely, Asp cation loses hydrogen atom from amine group, which triggers decarboxylation.

The apparent dependence of the diffusion coefficient of N-acetylaspartate upon magnetic field strength: evidence of an interaction with NMR methodology.

An inverse relationship between applied magnetic field strength and the apparent diffusion coefficient (ADC) of several important brain metabolites including N-acetyl-l-aspartate (NAA), choline and creatine, measured in vivo using proton magnetic resonance spectroscopy (MRS), has been reported. In this investigation, using phantom studies of NAA at magnetic field strengths of 3 and 7 T, these observations have been verified under controlled MRS conditions in vitro, and the ADC of NAA has been found to vary inversely with magnetic field strength, decreasing at a rate of 2.5%/T at 20 degrees C. We have also assessed whether the effect is a function of a systemic bias in methodology, or if the effect is actually on the rate of molecular diffusion. This was done using an MRS-independent method for measurement of molecular diffusion in NAA phantoms at 0, 0.025 and 7 T applied magnetic field strengths. As a result, it has been demonstrated that the observed apparent magnetic field dependence of the ADC of NAA is a consequence of the NMR measurement and is apparently not a real effect on molecular diffusion.

Specific racemization and isomerization of the aspartyl residue of alphaA-crystallin due to UV-B irradiation.

We have reported that the aspartyl (Asp)-151 residue in alphaA-crystallin in human eye lens was inverted to the D-isomer and isomerized to beta-Asp residue with age. We report here that ultraviolet (UV)-B irradiation induces the racemization and isomerization of the Asp-151 residue of alphaA-crystallin from lenses of 6-week-old rats to form D-isomer and beta-Asp residue. Simultaneous racemization and isomerization of the specific Asp residue indicate that the reaction proceeds via formation of a succinimide intermediate. This modification was not observed in UV-A irradiated and normal lenses. UV-B irradiation induced the racemization of only the Asp-151 residue and did not affect the other Asp residues in alphaA-crystallin. On the other hand, the high molecular weight fraction of the lens protein increased upon UV-B irradiation. Modification of the Asp residue would affect the three-dimensional packing array of the lens protein.