The cysteine radical cation: structures and … cysteine radical cation: structures and...

The cysteine radical cation: structures and fragmentation pathways

Junfang Zhao, K. W. Michael Siu and Alan C. Hopkinson*

Received 16th August 2007, Accepted 11th October 2007

First published as an Advance Article on the web 22nd October 2007

DOI: 10.1039/b712628j

A theoretical study on the structures, relative energies, isomerization reactions and fragmentation

pathways of the cysteine radical cation, [NH2CH(CH2SH)COOH]�+, is reported. Hybrid density

functional theory (B3LYP) has been used in conjunction with the 6-311++G(d,p) basis set. The

isomer at the global minimum, Captodative-1, has the structure NH2C�(CH2SH)C(OH)2

+; the

stability of this ion is attributed to the captodative effect in which the NH2 functions as a

powerful p-electron donor and C(OH)2+ as a powerful p-electron acceptor. Ion Distonic-S-1,

H3N+CH(CH2S

�)COOH, in which the radical is formally situated on the S atom, is higher in

enthalpy (DH10) than Captodative-1 by 6.1 kcal mol�1, but is lower in enthalpy than another

isomer Distonic-C-1, H3N+C�(CH2SH)COOH, by 8.2 kcal mol�1. Isomerization of the canonical

radical cation of cysteine, [H2NCH(CH2SH)COOH]�+, (Canonical-1), to Captodative-1 has an

enthalpy of activation of 25.8 kcal mol�1, while the barrier against isomerization of Canonical-1

to Distonic-S-1 is only 9.6 kcal mol�1. Two additional transient tautomers, one with the radical

located at Ca and the charge on SH2, and the other a carboxy radical with the charge on NH3,

are reported. Plausible fragmentation pathways (losses of small molecules, CO2, CH2S, H2S and

NH3, and neutral radicals COOH�, HSCH2� and NH2

�) from Canonical-1 are examined.

Introduction

In recent years, there has been considerable interest in radicals

derived from biomolecules in the gas phase; this is encouraged

partly by the fact that knowledge of the structures and

reactivities of radicals is necessary to understand the role of

transient species involved in protein radical catalysis 1 and in

oxidative damage of proteins.2,3 An additional reason for

interest in odd-electron species of biomolecules is that the

collision-induced dissociation (CID) of peptide radical ca-

tions, M�+, provides sequencing information complementary

to that of protonated peptides, [M + H]+, the even-electron

counterparts of the peptide radical cations.4 The fragmenta-

tion of radical peptides, while not as extensively studied as that

of protonated peptides, generally show more diverse chemis-

tries, as they commonly lose both even-electron and odd-

electron neutral molecules. Due to this diversity, the properties

of different amino acid-derived radicals have attracted con-

siderable attention, especially in the last few years, both from

an experimental and a theoretical perspective.5–14

In the 1900s, radical cations of amino acids and dipeptides

were typically produced by thermal desorption of the neutral

species and ionization by electron impact,15,16 or by laser

desorption followed by resonant UV two-photon ionization

at an aromatic chromophore.17 In the year 2000, Chu et al.

reported an unprecedented method for producing M�+ of

oligopeptides via the CID of electrosprayed ternary complexes

[CuII(L)(M)]�2+ in which L is a tridendate amine-diethylene-

triamine, N,N,N0,N0,N00-pentamethyldiethylenetriamine, or

2,20:60,200-terpyridine.18 Using this technique, complexes with

various combinations of M and L (including triamines, ter-

pyridines, and crown ethers) have been shown to give a wide

variety of radical cations.9,19–21

Spatial separation of the charge and radical centres plays an

important role in stabilizing radical cations of amino acids and

peptides. Ions in which the charge and spin are separated and

which do not have neutral counterparts with classical valence

bond structures, are said to be distonic. Three isomers that

have been studied22–28 both experimentally and theoretically

are:

(1) The canonical structure, H2N�+CH2COOH, formed by

electron impact on glycine, in which the majority of the spin

and charge is located on the amino group.

(2) The distonic ion, H3N+C�HCOOH, formed by the

loss of a hydrogen atom from N-protonated glycine.27 In

this distonic ion, the charge is formally located on the NH3

group and the spin on the a-carbon. This ion has

been calculated to be lower in energy than the canonical

structure by 7.0 kcal mol�1 at the CCSD(T)/

6-31++G(d,p)//B3LYP/6-31++G(d,p) level of theory.29

(3) The structure at the global minimum is an a-radical witha hydrogen having been transferred from the a-carbon to the

carboxyl group creating ion, H2NC�HC(OH)2+. In this ion, in

addition to having the charge and the radical centres formally

separated, there is a further stabilizing feature: the radical

centre is attached to a powerful p-electron donor (NH2) and

also a p-electron-withdrawing group, C(OH)2+. This combi-

nation of electron-withdrawing (capto) and electron-donating

(dative) groups creates a planar captodative radical in which

Department of Chemistry and Centre for Research in MassSpectrometry, York University, 4700 Keele Street, Toronto, Ontario,Canada M3J 1P3. E-mail: [email protected]; Fax: 416-736-5936;Tel: 416-736-2100 ext. 77839

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there is extensive delocalization of both the spin and the

charge. This captodative effect has long been recognized as

an important factor in stabilizing radicals.30–32

In a theoretical study using an isodesmic reaction (eqn (1))

to compare the resonance stabilizing energy (RSE) of a group

X on the stability of a methyl radical, it was found that NH2 is

stabilizing by 10.7 kcal mol�1 and COOH by 5.2 kcal mol�1.33

XCH�2 þ CH4 ! CH�3 þXCH3 ð1Þ

When both groups are attached to the same centre in the

neutral glycinyl radical, H2NC�HCOOH, the radical stabiliza-

tion is 22.9 kcal mol�1, 7.2 kcal mol�1 larger than the sum of

the two individual components. This synergistic stabilization is

attributed to delocalization of the unpaired spin.

The C(OH)2+ group is much more strongly electron-with-

drawing than the neutral COOH; extension of the resonance

stabilization energy analysis to the C(OH)2+ group gives an

RSE value of 4.8 kcal mol�1, slightly smaller than that of

COOH. Separately, the sum of the individual stabilizations

from NH2 and C(OH)2+ is 15.5 kcal mol�1; however, the

combined effect of NH2 and C(OH)2+ in H2NC�HCO(OH)2

+

is 43.5 kcal mol�1, 28.0 kcal mol�1 greater than the sum of the

individual contributions.34 Comparing this with the enhance-

ment of only 7.2 kcal mol�1, attributed to the captodative

effect in H2NC�HCOOH, shows that the increase in the

electron-withdrawing power by introducing a positive charge

has a profound effect.

Cysteine, NH2CH(CH2SH)COOH, as a sulfur-containing

amino acid is of considerable interest in biochemistry and a

number of human diseases. It also is the most effective amino

acid at scavenging radicals in solution, attributable to the ease

with which it forms a radical at the sulfur atom.35 There have

been several studies36–41 examining the conformations of

cysteine and protonated cysteine in the gas phase, although

the former is comparatively less well studied than the latter.

Recently, Simon et al.42 examined computationally the struc-

tures and possible fragmentation pathways of the radical

cations of glycine, alanine, serine, and cysteine; however, for

each ion, only the canonical structure formed by direct

removal of an electron from the lowest-energy conformer of

the neutral amino acid was considered. The cysteine radical

cation has been generated by direct electron ionization (EI) of

cysteine. The major fragment ion, at m/z 76, was attributed to

HSCH2CHQNH2+; minor fragment ions at m/z 75, 74, 59

and 43 were also reported.43

In this study, we examine the plausible structures of the

cysteine radical cation, the profiles of isomerization between

these isomers, and the possible fragmentation channels of

these ions.

Theoretical methods

Molecular geometries were optimized and harmonic vibra-

tional frequencies calculated using the nonlocal hybrid three

parameter B3LYP method44,45 with the 6-311++G(d,p) basis

set. Spin-unrestricted calculations (UB3LYP) were used for

the open-shell systems and in our UB3LYP calculations, hS2ioperator expectation values were close to the ideal value of

0.75 (ranging from 0.751–0.766) for doublets. Optimized

structures were characterized by harmonic frequencies as local

minima (all real frequencies) and saddle points (one imaginary

frequency). In all cases, IRC calculations46 were carried out to

confirm that transition states linked the proposed reactants

and products. Net atomic charges and spin densities were

determined using the natural population analysis (NPA)

formalism.47,48 In contrast to the Mulliken population ana-

lyses, NPA gave very reasonable spins that showed only minor

polarization effects in inverted spin populations at atoms

adjacent to those with high spin densities. All calculations

were performed with the Gaussian 98 package.49

Results and discussion

In the following sections, we present the structures and relative

energies of various tautomers of the cysteine radical cation,

followed by reaction profiles for interconversion between these

tautomers. We then focus on plausible pathways for the loss of

small closed-shell molecules, CO2, NH3, CH2S and H2S, from

the canonical form of the cysteine radical cation. Fragmenta-

tion processes involving losses of neutral radicals by cleavage

of the Ca–R bonds are also examined.

1. Energies and structures of the cysteine radical cation

The potential energy surface (PES) of the cysteine radical

cation is complicated due to the large number of tautomers

that are at local minima. A preliminary scan of the surface

revealed that structures maximizing the number of hydrogen

bonds are favored. Three types of structures, the captodative,

distonic, and canonical ions, have low energies. In the cano-

nical structure, both the charge and spin are localized on the

sulfur atom, whereas for the other ions the charge is either on

NH3, SH2, or C(OH)2 and the spin on the sulfur or the

a-carbon atom. There are three distinct classes of distonic

ions: those in which the charge is localized on the NH3 group

and the spin is localized on S, we denote as Distonic-S ions;

those in which the charge is on NH3 or SH2 and the spin is on

Ca, Distonic-C ions; and those in which the charge is on the

NH3 and the spin the carboxy group, Carboxy. A fifth type of

structure formed by ionization of glycine enol is Captodative;

here the radical is formally on the a-carbon and the charge on

the protonated carboxy group. The barrier to conversion of

the carboxy radical into a rotamer of the canonical radical is

less than 1 kcal mol�1 (vide infra) and this transient ion was

only examined in the context of being an intermediate in loss

of CO2. A sixth structure, denoted Distonic-C(SH2) is an

intermediate in the loss of H2S from the Canonical ion.

Formally it has the charge on SH2 and the radical at Ca, but

this structure has a long Cb–S distance (2.691 A) and is

actually H2CQC(COOH)NH�+ solvated by H2S.

The different classes of tautomers are shown in Scheme 1.

For the four major tautomers, several conformers were opti-

mized (Fig. 1 and Table 1). The captodative ions are the

lowest-energy structures on the potential-energy hypersurface,

e.g., Captodative-1 is lower in enthalpy than Canonical-1 by

17.5 kcal mol�1. In Captodative-1, the structure at the global

minimum, all atoms except the two Ca hydrogens are coplanar

(see Fig. 1), permitting extensive p-delocalization of the charge

and spin. A natural population analysis (NPA) gave computed

spin densities on the C(OH)2, the a-C, and NH2 groups as

282 | Phys. Chem. Chem. Phys., 2008, 10, 281–288 This journal is �c the Owner Societies 2008

0.32, 0.27 and 0.29, respectively. The sulfur atom is capable of

carrying a positive charge and, in addition to the stabilization

endowed by the captodative effect, Captodative-1 is further

stabilized by a hydrogen bond from the protonated carboxy

group to the sulfur atom.

Distonic-S-1 is higher in enthalpy than Captodative-1 by

6.2 kcal mol�1, but lower than Canonical-1 by 11.3 kcal mol�1.

In theDistonic-S ions, the proton is located on the basic amino

group and the unpaired electron is favorably located on the S

atom. Distonic-S-1 has the additional stabilizing feature of

hydrogen bonds from the NH3+ group to the S atom and to

the carbonyl oxygen. An NPA on Distonic-S-1 indicated that

the unpaired electron lies mainly on the sulfur atom (spin

density, 0.97) and the positive charge is mainly located at the

NH3 group (+0.63) and S (+0.22). Distonic-S-4 has the

proton on the less basic carboxy group and is consequently

Scheme 1

Fig. 1 B3LYP optimized structures of the cysteine radical cation. Distances are in A.


higher in enthalpy than Distonic-S-1 by 19.8 kcal mol�1; it is

included here only because it is a key intermediate in the

isomerization and fragmentation reactions of the cysteine

radical cation.

The third class of distonic structures, Distonic-C, is energe-

tically less favored than the others, probably because the

charge and spin are on adjacent atoms. Distonic-C-1 is higher

in enthalpy than Distonic-S-1 by 8.2 kcal mol�1, but lower

than Canonical-1 by 3.1 kcal mol�1.

The canonical structures have the highest energies of all the

different classes of ions. As pointed out earlier, Canonical-1 is

higher in enthalpy than the structure at global minimum,

Captodative-1, by 17.5 kcal mol�1. In Canonical-1 (see

Fig. 1), there is an intramolecular hydrogen bond between

the SH group, which acts as proton donor, and the carbonyl

oxygen, which acts as a proton acceptor. This hydrogen bond

is shorter than that calculated for the analogous neutral parent

molecule;39 this is attributed to the fact that most of the charge

in the canonical cysteine radical cation is localized on the –SH

group. As a result, the acidity of the –SH group increases and

the hydrogen bond is strengthened. An NPA on Canonical-1

gave the charge on the SH group as +0.57 and the spin density

as 0.49. Canonical-2, the only tautomer that has been

previously reported,42 has an almost identical energy to

Canonical-1 (lower by only 0.02 kcal mol�1 in enthalpy, but

higher by 0.4 kcal mol�1 in free energy at the B3LYP/

6-311++G(d,p) level). Simon et al. attributed the stability

of this conformer to a two-center–three electron hemibond

between NH2 and SH.

2. Isomerization reactions

The reaction profile for interconversion between key tauto-

mers of the cysteine radical cation is given in Fig. 2 and the

corresponding calculated energies are in Tables 1 and 2. All

energies are reported relative to Canonical-1. There are high

barriers against the direct conversion of Canonical-1 into

Distonic-C-1 (via a 1,2-H-shift and against an activation

enthalpy of 41.9 kcal mol�1) and into Captodative-1 (via a

1,3-H-shift and against 38.9 kcal mol�1); both these reactions

require large geometric distortions and major electronic redis-

tributions. In Canonical-1, both the charge and spin are

located mainly on the –SH, while in the distonic products,

the spin is formally on the Ca and the charge is on either the

NH3 or C(OH)2. An alternative, lower-energy pathway for the

conversion of Canonical-1 into Captodative-1 involves a 1,5-

proton transfer from the sulfur to the carbonyl oxygen,

forming Distonic-S-4 in the initial step; this is followed by a

1,3-H atom shift from the a-carbon to S� in the rate determin-

ing step. The enthalpy of activation via this route is 25.8 kcal

mol�1, considerably lower than the 38.9 kcal mol�1 required

for the 1,3-H-shift mechanism.

Conversion of Canonical-1 into Distonic-S-1 is essentially a

1,4-H+-shift, and has a much lower barrier (the activation

enthalpy is 9.6 kcal mol�1), consistent with the smaller geo-

metric distortions involved. Distonic-C-1 is unlikely to exist in

the gas phase as the barrier against rearrangement of this ion

to Captodative-3 via a 1,4-H+-shift is low (enthalpy of activa-

tion 5.3 kcal mol�1).

3. Fragmentation reactions

In principle, the canonical cysteine radical cation can cleave at

Ca to lose four different non-Ca-containing radicals (COOH�,

H�, NH2� or HSCH2

�), or cations (COOH+, H+, NH2+ or

HSCH2+). Simon et al.42 calculated the energetics of these

reactions and found that losses of the radicals are energetically

much more favorable. The loss of COOH� was found to have

the lowest endothermicity (18.1 kcal mol�1). Here, we have

calculated the losses of cationic and neutral radical fragments

at B3LYP/6-311++G(d,p) i.e., using a slightly larger basis set

than that used previously. In addition, we have calculated the

Table 1 Electronic (Et) and ZPE-corrected (EZPVE) energies of selected tautomers of the cysteine radical cation. Enthalpies and free energies arerelative to Canonical-1

Species Et/Eh EZPVE/Eh DH1

0/kcal mol�1 G1

298/Eh DG1

298/kcal mol�1

Canonical

1 �721.7567534 �721.650210 0.0 �721.684304 0.02 �721.7583389 �721.650237 0.0 �721.683709 0.43 �721.7555362 �721.647331 1.8 �721.679960 3.8

Distonic-C

1 �721.7627515 �721.655144 �3.1 �721.689576 �2.62 �721.7610566 �721.653238 �1.9 �721.687723 �1.33 �721.7466705 �721.639802 6.5 �721.674469 6.24 �721.7465963 �721.639327 6.8 �721.673193 7.0

Distonic-S

1 �721.7802974 �721.668178 �11.3 �721.701575 �10.82 �721.7701976 �721.658260 �5.1 �721.692212 �5.03 �721.7642757 �721.652636 �1.5 �721.685811 �0.94 �721.7456194 �721.636678 8.5 �721.669678 9.2

Captodative

1 �721.7856087 �721.678154 �17.5 �721.711297 �16.92 �721.779994 �721.673013 �14.3 �721.706891 �14.23 �721.7725992 �721.665641 �9.7 �721.699429 �9.54 �721.7714246 �721.664073 �8.7 �721.697201 �8.1


energetics of probable fragmentation pathways involving the

losses of small molecules, CO2, H2S, CH2S, and NH3, some of

which require multi-step reactions.

Fragmentation reactions of the Ca–R bonds. Table 3 lists the

enthalpies and free energies for cleavage of the Ca–R bonds,

all with respect to Canonical-1. There is good agreement with

the literature values.42 As found previously, the fragmentation

energies for the losses of neutral radicals, COOH�, HSCH2�,

H�, and NH2� are significantly lower than those for the losses

of cations, H+, NH2+, and COOH+; i.e., the more favorable

processes are those that leave the charge on the fragment with

the lower ionization energy. Among the four possible reactions

that lose R�, the most favorable (lowest in energy) is the loss of

COOH�, giving H2N+QCHCH2SH (m/z 76, the most abun-

dant ion in the EI spectrum of the cysteine radical cation43).

The second most favorable is the loss of HSCH2� to give

H2N+QCHCOOH (m/z 74).

Losses of small molecules CO2, H2S, CH2S and NH3. The

loss of carbon dioxide is a common phenomenon in

the fragmentation of radical cationic peptides, whereas

it is rare from protonated peptides. Fig. 3 shows the reaction

profile for a proposed mechanism leading to the loss of

CO2 from Canonical-1. The initial step along the reaction

profile involves rotation about the Ca–C bond to form

Canonical-3 (1.8 kcal mol�1 higher in enthalpy than

Canonical-1), via a barrier of 14.0 kcal mol�1. In the

second step, a proton is transferred from the hydroxyl

group to the amino group, creating a distonic ion in

which the charge is on the NH3 and the radical on the

carboxy group; the barrier to this 1,4-H+ shift is

8.5 kcal mol�1. Stretching the Ca–C bond in this ion is

Fig. 2 Energy profiles for the isomerization processes of the cysteine radical cation. Upper numbers are enthalpies at 0 K and lower italicized

numbers are free energies at 298 K. All numbers (in kcal mol�1) are relative to the Canonical-1 ion.

Table 2 Electronic (Et) and ZPE-corrected (EZPVE) energies of selected transition states of the cysteine radical cation. Enthalpies and free energiesare relative to Canonical-1

Transition state Et/Eh EZPVE/Eh DH1

0/kcal mol�1 G1

298/Eh DG1

298/kcal mol�1

TS1 �721.6845864 �721.583376 41.9 �721.617618 41.8TS2 �721.6891124 �721.588297 38.9 �721.622061 39.1TS3 �721.7403966 �721.634989 9.6 �721.668586 9.9TS4 �721.7419607 �721.638063 7.6 �721.670847 8.4TS5 �721.7130369 �721.609048 25.8 �721.641147 27.1TS6 �721.7503426 �721.646646 2.2 �721.680220 2.6TS7 �721.7324116 �721.627923 14.0 �721.661684 14.2TS8 �721.7417832 �721.636604 8.5 �721.668759 9.8TS9 �721.7253138 �721.620353 18.7 �721.654527 18.7TS10 �533.1002471 �533.013271 — �533.041670 —TS11 �721.7255413 �721.620260 18.8 �721.653188 19.5TS12 �721.7219267 �721.620592 18.6 �721.653830 19.1TS13 �721.7285978 �721.622797 17.2 �721.658234 16.4TS14 �721.6794881 �721.576246 47.5 �721.610654 46.2


the rate-determining step via transition state TS9. The

immediate product from the intrinsic reaction coordinate

(IRC) calculation is the ion–molecule complex

H3N+CH�CH2SH� � �CO2. However, the observable experi-

mental product would be the ethenamine radical cation,

[H2NCHCH2]�+, (m/z 43, a minor product in EI spectrum)

after losses of both CO2 and H2S; the collision energy

deposited onto the cysteine radical cation to overcome TS9

would also drive the reaction past TS10, a barrier of only

1.1 kcal mol�1. The loss of CO2 from Canonical-1 is exother-

mic by 8.4 kcal mol�1, while the combined loss of CO2 and

H2S is exothermic by 15.4 kcal mol�1.

In addition, there are two transition states of note, TS11

and TS12 (see Fig. 4 and Table 2), that have energies almost

identical to TS9. The enthalpy of activation for the loss of

CH2S is 18.8 kcal mol�1 and for the loss of H2S 18.6 kcal

mol�1, very comparable to the activation enthalpy for the

combined loss of CO2 and H2S at 18.7 kcal mol�1. Conse-

quently, all three fragmentation reactions, involving the loss of

(CO2 + H2S), H2S, and CH2S from Canonical-1 should be

competitive at low collision energies. The reaction leading to

the captodative a-radical cation of glycine (m/z 75) by losing

the CH2S molecule is endothermic by 10.3 kcal mol�1; by

comparison, the reaction leading to the

[NH2C(CH2)COOH]�+ radical cation (m/z 87) by losing

H2S is exothermic by 1.0 kcal mol�1.

The loss of ammonia is the major dissociation pathway

for protonated cysteine at low collision energies.40 Here

we have studied possible fragmentation mechanisms for am-

monia loss from the cysteine radical cation, starting from the

two distonic structures, Distonic-C-1 and Distonic-S-2 (Fig. 5).

The barrier against cleavage of the Ca–N bond in Distonic-C-1

(47.5 kcal mol�1) is much higher than that in Distonic-S-2

(17.2 kcal mol�1). Conversion of Canonical-1 into Distonic-S-2

involves a series of rotations about C–C bonds plus the proton

transfer reaction shown in Fig. 2; all these steps have low

barriers. By comparison, conversion of Canonical-1 into

Distonic-C-1 has a high barrier (41.9 kcal mol�1, Fig. 2) and

the subsequent loss of NH3 requires even higher energy

(endothermicity = 48.6 kcal mol�1). The loss of NH3 from

Canonical-1 then will proceed via Distonic-S ions and the

overall endothermicity of 25.9 kcal mol�1 is sufficiently high

to make it uncompetitive with pathways leading to losses of

H2S, H2CS, and (CO2 + H2S).

Table 3 Energies of fragmentation products of the cysteine radical cation. Thermodynamic quantities are relative to Canonical-1

Species Et/Eh EZPVE/Eh DH1

0/kcal mol�1 G1

298/Eh DG1

298/kcal mol�1

R+ + COOH �721.7262599 �721.623261 16.9 (18.1)a �721.676085 5.2R+ + H2CSH �721.7105735 �721.610291 23.3 (24.1)a �721.664561 11.0R + H2CSH

+ �721.6974334 �721.596109 33.9 (33.8)a �721.648406 22.5R+ + H� �721.6946974 �721.597687 33.0 (32.3)a �721.64103 27.2R+ + NH2

� �721.6779549 �721.579272 44.5 (44.1)a �721.628017 35.3R� + COOH+ �721.6423353 �721.542865 67.4 (72.3)a �721.595624 55.6R� + NH2

+ �721.4795327 �721.384857 166.0 (168.5)a �721.435209 156.3R� + H+ �721.4419589 �721.347251 190.0 (191.6)a �721.391412 183.8

a At B3LYP/6-31++G(d,p), ref. 42.

Fig. 3 Energy profile for the loss of (CO2 + H2S) from Canonical-1. Upper numbers are enthalpies at 0 K and lower italicized numbers are free

energies at 298 K. All numbers (in kcal mol�1) are relative to Canonical-1.


4. Fragmentation versus isomerization reactions

The barriers against isomerization from Canonical-1 to

Distonic-C-1 and to Captodative-1 are much larger than

those leading to the loss of COOH� and other neutral

closed-shell molecules. However, isomerizations of Canonical

isomers to Distonic-S isomers, by 1,4-proton transfers,

have low barriers (9.6 kcal mol�1 in terms of enthalpy in the

example used in Fig. 2) making it competitive with

the fragmentation reactions. When we compare relative Gibbs

free energies (DG298), which takes into account of zero-point

energy correction, thermal correction, and entropy, the

fragmentation leading to the loss COOH� is more favorable

than the isomerization to Distonic-S ions. As expected,

everything being equal, the entropic effects favor fragmenta-

tion over isomerization, as the former creates more than one

product. The free energy change (also the activation free

energy) for the loss of COOH� is only 5.2 kcal mol�1, much

smaller than the activation free energy values for the dissocia-

tion reactions as shown in Fig. 3–5. As a result, the loss of

Fig. 5 Energy profiles for the loss of NH3 from Distonic-C and Distonic-S conformers. Upper numbers are enthalpies at 0 K and lower italicized

numbers are free energies at 298 K. All numbers (in kcal mol�1) are relative to Canonical-1.

Fig. 4 Energy profiles for the loss of H2S and H2CS from Canonical-1. Upper numbers are enthalpies at 0 K and lower italicized numbers are free

energies at 298 K. All numbers (in kcal mol�1) are relative to Canonical-1.


COOH� is the most favorable gas-phase reaction of the

cysteine radical cation.

Conclusions

Tautomers of the cysteine radical cation have been examined

in detail. The ion at the global minimum has a captodative

structure, with the unpaired electron on the a-carbon and the

charge on a protonated carboxy group. This ion is 17.5 kcal

mol�1 lower in enthalpy than the lowest-energy canonical

structure, but there is a large barrier (25.8 kcal mol�1 above

the canonical structure) against conversion into the capto-

dative ion—a similar situation to that found previously for the

glycine radical cation. Of the other two distonic ions, the one

with the radical centre at the sulfur atom and the charge at the

protonated amino group has the lower energy. In addition,

this ion can be formed with a relatively low barrier from the

canonical ion.

Despite several attempts at experimentally producing the

cysteine radical cation via techniques used successfully for

other amino acid cations, we have not observed this ion. This

absence could be a consequence of the low free energy (5.2 kcal

mol�1) required for dissociation via the loss of COOH�. The

electron ionization mass spectrum of the cysteine radical

cation43 shows the loss of COOH� with the formation of

H2N+QCHCH2SH (m/z 76) as the dominant fragmentation

pathway. A number of reaction pathways involving the loss of

radicals and neutral molecules also have low activation energies.

Acknowledgements

This work was supported by the Natural Sciences and

Engineering Research Council (NSERC) of Canada.

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The cysteine radical cation: structures and … cysteine radical cation: structures and...

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