194
M. Dantus and C.L. Kalcic
forms. While the delocalized pi systems stabilize the resulting radical, we do observe significant radical-directed cleavage of the C α –C β bonds in all three amino
acids, giving rise to a singly charged product ion corresponding to the mass of the
amino acid’s side chain. The other major product in these fs-LID spectra is a doubly
charged product ion that has lost neutral CO 2 from the carboxylic acid end of the
molecule. Based on our observations, the aromatic amino acids are the most likely
sites for radical formation when a peptide is subjected to fs-LID.
8.4.3 Acidic/Basic Amino Acids
Aspartic acid and glutamic acid show no ionization or dissociation into product
ions via fs-LID, regardless of derivatization, which is consistent with their high
(∼ 10 eV) ionization energy. Histidine also gives rise to no fs-LID product ions,
which is surprising given the low (∼ 8 eV) estimated ionization energy of the neutral form. Most likely, protonation of the histidine side chain is interfering with the
conjugated pi system of electrons, making them less polarizable and therefore less
susceptible to strong field ionization.
Lysine and arginine show limited degrees of photoionization by fs-LID only after derivatization as an N-acetyl methyl ester and N-benzoyl derivative, respectively.
Since the side chains of these residues are basic, they are probable sites of protonation, leaving few lone pair electrons susceptible to photoionization. Overall, the
acidic and basic amino acids are unlikely origins for radical formation.
8.4.4 Polar Amino Acids
Glycine does not photoionize in any form, which is not surprising given that the hydrogen atom side chain does not enhance the polarizability of the amino acid backbone. More surprisingly, glutamine and its derivatives showed no fs-LID product
ions, while asparagine gave rise to a small signal as an N-benzoyl derivative, as did
serine and threonine. While the interaction of the benzoyl group with the backbone
of each amino acid and the resulting stereochemistry are unique, the aromatic group
does increase the polarizability of some of these previously inactive polar amino
acids to the point that fs-LID signal can be observed. The bulky benzoyl group did
not improve the amenability of cystine or cysteine to fs-LID, but a simple lengthening of the backbone in the N-acetyl methyl ester forms was sufficient to observe
limited photoionization.
8.4.5 Non-polar Amino Acids
The susceptibility to fs-LID of the non-polar amino acids was found to increase
with size and therefore polarizability. Alanine and valine were completely inactive,
M. Dantus and C.L. Kalcic
forms. While the delocalized pi systems stabilize the resulting radical, we do observe significant radical-directed cleavage of the C α –C β bonds in all three amino
acids, giving rise to a singly charged product ion corresponding to the mass of the
amino acid’s side chain. The other major product in these fs-LID spectra is a doubly
charged product ion that has lost neutral CO 2 from the carboxylic acid end of the
molecule. Based on our observations, the aromatic amino acids are the most likely
sites for radical formation when a peptide is subjected to fs-LID.
8.4.3 Acidic/Basic Amino Acids
Aspartic acid and glutamic acid show no ionization or dissociation into product
ions via fs-LID, regardless of derivatization, which is consistent with their high
(∼ 10 eV) ionization energy. Histidine also gives rise to no fs-LID product ions,
which is surprising given the low (∼ 8 eV) estimated ionization energy of the neutral form. Most likely, protonation of the histidine side chain is interfering with the
conjugated pi system of electrons, making them less polarizable and therefore less
susceptible to strong field ionization.
Lysine and arginine show limited degrees of photoionization by fs-LID only after derivatization as an N-acetyl methyl ester and N-benzoyl derivative, respectively.
Since the side chains of these residues are basic, they are probable sites of protonation, leaving few lone pair electrons susceptible to photoionization. Overall, the
acidic and basic amino acids are unlikely origins for radical formation.
8.4.4 Polar Amino Acids
Glycine does not photoionize in any form, which is not surprising given that the hydrogen atom side chain does not enhance the polarizability of the amino acid backbone. More surprisingly, glutamine and its derivatives showed no fs-LID product
ions, while asparagine gave rise to a small signal as an N-benzoyl derivative, as did
serine and threonine. While the interaction of the benzoyl group with the backbone
of each amino acid and the resulting stereochemistry are unique, the aromatic group
does increase the polarizability of some of these previously inactive polar amino
acids to the point that fs-LID signal can be observed. The bulky benzoyl group did
not improve the amenability of cystine or cysteine to fs-LID, but a simple lengthening of the backbone in the N-acetyl methyl ester forms was sufficient to observe
limited photoionization.
8.4.5 Non-polar Amino Acids
The susceptibility to fs-LID of the non-polar amino acids was found to increase
with size and therefore polarizability. Alanine and valine were completely inactive,