Detection of synthetic protein isomers and conformers by electrospray mass spectrometry

Electrospray mass spectrometry (ESMS) has been used to investigate the structural properties of a protein prepared by total chemical synthesis. Construction of an analog of the tenth type III module from fibronectin (10F3) by chemical ligation of the unprotected synthetic peptides 10F3(1-40) alpha COSH and BrAc (42-94) 10F3 was found to give two major products, both of which possessed a mass corresponding to the expected product, [(COS)40-41]10F3. Comparisons of the ESMS charge distributions obtained for these two synthetic products with that obtained for recombinant 10F3 suggested that one of the synthetic 10F3 analogs was correctly folded and the other was somehow misfolded. This was further confirmed by 1D and 2D NMR analysis. Exposure of the misfolded synthetic [(COS)40-41]10F3 to high pH and elevated temperature followed by analysis using liquid chromatography-mass spectrometry revealed a beta-ester linkage between residues Asn42 and Ser43, produced by an N-->O acyl shift rearrangement at Ser43, as the origin of the misfolding. ESMS was also used to measure the H-D exchange rates of labile protons within the synthetic and recombinant 10F3s. This application, which allows the number of slow exchanging backbone amides within a protein to be calculated, revealed clear differences in the H-bonding networks of the folded and unfolded synthetic protein modules. Replacement of Ser43 by an alanine was found to circumvent the N-->O acyl shift, and the resulting synthetic protein analogue, [Ala43, (COS)40-41]10F3, possessed identical structural properties to recombinant 10F3.