In vacuo esterification of carboxyl groups in lyophilized proteins

Methylating peptides to prevent adduct ion formation also directs cleavage in collision-induced dissociation mass spectrometry

We investigated the effect of N-terminal amino group and carboxyl group methylation on peptide analysis by electrospray mass spectrometry (ESI-MS) and tandem mass spectrometry (ESI-MS/MS). Permethylation of the N-terminal amino group and the carboxyl groups can reduce metal ion adducts but does not enhance sensitivity in electrospray as previously observed for matrix-assisted laser desorption/ionization (MALDI) mass spectrometry. N-terminal trimethylated peptides exhibit collision-induced dissociation (CID) tandem mass spectra that differ from their unmodified analogs; the results support the mobile proton hypothesis of peptide fragmentation. A permanent positive charge at the N-terminus leads to competition between permanent-charge directed processes and loss of the N-terminal trimethyl amino group. Carboxyl methylation has no effect on fragmentation behavior other than to shift the mass of fragments containing methylated carboxyl groups. Comparison of regular and tandem mass spectra of different methylated peptides allowed probing the location of incomplete methylation, the proton displaced by alkali metal ions and the purity of a mass-selected methylated peptide ion.

Covalent cross-linking of proteins without chemical reagents

A facile method for the formation of zero-length covalent cross-links between protein molecules in the lyophilized state without the use of chemical reagents has been developed. The cross-linking process is performed by simply sealing lyophilized protein under vacuum in a glass vessel and heating at 85 degrees C for 24 h. Under these conditions, approximately one-third of the total protein present becomes cross-linked, and dimer is the major product. Chemical and mass spectroscopic evidence obtained shows that zero-length cross-links are formed as a result of the condensation of interacting ammonium and carboxylate groups to form amide bonds between adjacent molecules. For the protein examined in the most detail, RNase A, the cross-linked dimer has only one amide cross-link and retains the enzymatic activity of the monomer. The in vacuo cross-linking procedure appears to be general in its applicability because five different proteins tested gave substantial cross-linking, and co-lyophilization of lysozyme and RNase A also gave a heterogeneous covalently cross-linked dimer.

Improved peptide detection with matrix-assisted laser desorption/ionization mass spectrometry by trimethylation of amino groups

Trimethyl alpha-amino derivatives of peptides (penta to deca) with a permanent positive charge on their alpha-amino groups were prepared by in vacuo reaction with iodomethane and subjected to matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). Compared to the unmodified peptide, the signal intensity of the trimethyl alpha-amino derivative in MALDI-MS is increased by at least an order of magnitude. Similarly, an octapeptide with a trimethylated epsilon-amino group derived from the solitary lysine residue of the B-chain of insulin also shows the same relative increase in signal intensity. Another advantage of the in vacuo methylation procedure is that trimethylation of a peptide amino group can be carried out readily with a combination of isotopes (13)CH(3)I and (12)CH(3)I or CD(3)I and CH(3)I, yielding a doublet signal either 3 or 9 units apart, respectively. The presence or absence of such a doublet signal can be used as a criterion to discriminate between peptide and non-peptide signals in the mass spectrum.