Differentiating βAsp Residue by PSD in a Curved Field Reflectron (1)

Introduction

Aspartic acid in proteins is known to form a 5-membered ring when it undergoes isomerization (isoaspartate; isoAsp or βAsp) due to ultraviolet irradiation or aging, etc. As this isomerization means the formation of a bond between the C=O group of an aspartic acid side chain and the NH group of a neighboring residue, it is thought that this imparts instability to the main chain of the protein, eventually leading to modification of the protein structure and cohesion between proteins. In fact, it has been reported that α crystallin including isoaspartate exists in the crystalline lens of cataract patients. The detection and quantitation of isoaspartate is mainly conducted by protein sequencing and HPLC, but due to the difficulty in separating the isomers and the very small amounts present, analysis is generally difficult. MALDI-TOFMS is an effective method of analyzing trace level analytes, but because the masses of isomer residues are identical, isoaspartate detection analysis is not possible using simple molecular weight measurement. Here, we report the results of detection of the characteristic fragment ion of βAsp and its differentiation from ordinary Asp using TOF post source decay (PSD) analysis with Shimadzu’s original Curved Field Reflectron technology.

September 16, 2011 GMT