1

UV PHOTOFRAGMENTATION SPECTROSCOPY OF MODEL LIGNIN-ALKALI ION COMPLEXES: EXTENDING LIGNOMICS

INTO THE SPECTROSCOPIC REGIME

JACOB C. DEAN, NICOLE L. BURKE, JOHN R. HOPKINS, JAMES G. REDWINE,a BIDYUT BISWAS, P. V. RAMACHANDRAN, SCOTT A. MCLUCKEY and TIMOTHY S. ZWIER

Department of Chemistry, Purdue University, West Lafayette, IN 47906-2084 U.S.A.;

a Present address: Kalsec, Kalamazoo, MI 49006 U.S.A. 

Inte

nsity

(arb

. uni

ts)

3560035500354003530035200351003500034900Wavenumber (cm

-1)

*

Lignin: Cell Wall Foundation

• Aromatic biopolymer concentrated in cell wall.

• “Lignomics” accomplished with tandem MS. Ionization accomplished with negative ESI or alkali metal complexation.

• Each monolignol is a natural chromophore, providing potential for UV spectroscopy.

p-Hydroxyphenyl

Guaiacyl

Syringyl

β-O-4

β- β

HO

OH

HO

OH

H3CO

HO

OH

H3CO

H3CO

HO

H3CO

OH

OH

O

H3CO

H3CO

HO

O

O

OH

OCH3

O

HO

OCH3

OCH3

OH

Vanholme, R, et. al. Plant Physiol. 2010, 153, 895-905

β-5

HO

OCH3

OH OH

O

H3CO

OH

OH

O

OCH3

O

O

OCH3

O

HO

OH

O

HO

OCH3

HO

Monolignols Dilignols Oligolignols

Dean,J, et al. Chem. Sci., 2014, 5, 1940-1955 Dean,J, et al. J. Chem. Phys, 2013, 139 , 144313.

Instrument for Cold Ion Spectroscopy

• Ions are generated via nano-electrospray ionization (nESI)

• q2 and q3 are set up as linear ion traps• The cold trap is a 22-pole ion trap held at ~5K

UV photofragment spectroscopy of cryocooled ions

(M+Li)+*

(M+Li)+S0

S1

Dissociation threshold

Scan UV

laser

UnimolecularDissociation

(Frag2+Li)+

+YInternal

Conversion

(M+Li)+‡

(Frag1+Li)++ X

ExcitedState

Fragmentation

Conformer AConformer B

AB

Protonated tyrosine

AB

IR-induced Fragment Ion Gain Spectroscopy(IRFIG)

UnimolecularDissociation

(M+Li)+

(M+Li)+*

(Frag2+Li)++Y

S0

S1

Internal Conversion

(M+Li)+‡Dissociation threshold

Fix UV dissociation laser to thered of cold transitions

Scan IR Laser

(Frag1+Li)++ X

Dt=100 ns

ExcitedState

Fragmentation

UV wavelength fixed

35900357003550035300

IR produces “warm”ion gain signal

(erythro) β-O-4+Li+ UV Spectroscopy In

ten

sit

y (

arb

. un

its

)

36500360003550035000Wavenumber (cm

-1)

*In

ten

sit

y (

arb

. un

its

)

3560035500354003530035200351003500034900Wavenumber (cm

-1)

*

OH

OCH3

HO

HO

O

OCH3

Li

• Three conformer “terraces” at 34985, 35083, and 35161 cm-1 present.

(erythro) β-O-4+Li+ IR

375037003650360035503500Wavenumber (cm

-1)

• IRFIG spectrum shows only three bands, indicating single dominant linkage conformation for all three conformers.

• Cation solvated by four oxygen atoms, making binding at this linkage most likely in an oligomer with multiple linkage types.

1

2

3

1 2 3

β-O-4 Na+ UV

OH

OCH3

HO

HO

O

OCH3Na+

• Two or three conformers present, as with lithiated species, albeit at lower frequencies with more congestion.

β-O-4 Na+ IR

• As with lithiated species, single linkage binding conformation similar to B-O-4 Li+ in cold trap.

• Conformation essentially identical to lithiated species.

Pinoresinol + Li+ UV and IR ( -b b linkage) In

ten

sit

y (

arb

. un

its

)

36500360003550035000Wavenumber (cm

-1)

* • First UV transition at 34959 wavenumbers, shifted down by 350 wavenumbers compared with neutral due to metal cation complexation.

• Second larger transition at 35199 wavenumbers

HO

O

O

OH

H3CO

OCH3

Li

DFT/M05-2X/6-31+G(d) theory for calculations

Pinoresinol +Li+ UVPD

500400300200100m/z

305214

159

UV Photofragment MS

• 305 Da peak due to loss of methoxy and OH groups. • 214 and 159 Da peaks two separate fragments from same pathway.• Relative abundances show higher binding energy of Li cation for the 214

fragment.

HO

O

O

OH

H3CO

OCH3

Li

Parent (365 Da)

Pinoresinol + Na+ SpectroscopyIn

ten

sit

y (

arb

. u

nit

s)

3560035400352003500034800Wavenumber (cm

-1)

*

HO

O

O

OH

H3CO

OCH3

Na

Inte

ns

ity

(a

rb.

un

its

)

37003650360035503500Wavenumber (cm

-1)

• First sharp transition at 35258 cm-1.

• Absorption spectrum spans 300 cm-1 to the red indicating possible unresolved bands or excited state processes.

• Two distinct bands in IRFIG spectrum, compared with only one for lithiated species.

• Could be due to different conformers or isomers.

Pinoresinol+Na+ UVPD MS

HO

O

O

OH

H3CO

OCH3

Na

HO

O

O

OH

H3CO

OCH3

Molecular Weight: 358.39

S1 Charge Transfer Na

OH

OCH3

Na • Excited state surfaces steering fragmentation pathways

• Even slightly different metal ions results in different fragmentation500400300200100

m/z

147 [G+Na+]

359 [M+]

202 [M+Na+

2]

Structural Perturbation by Alkali Metal Complexation

Li+

Li+

• Loss of all linkage H-bonds

• Complete charge solvation by linkage

• β-O-4 ideal binding site in oligomers

β-O-4•Li+ β-O-4•Na+ Pino.•Li+ Pino.•Na+

Binding E. (eV)

3.69 2.68 2.66 1.88

RO-M (Å) 1.97 2.30 1.89 2.26

β-O-4 Li+ CID vs. UVPD

OH

OCH3

O

OCH3Li+

m/z = 279

• 279 Da peak most prominent fragment in both CID and UVPD of lithiated β-O-4 dilignol.

• UVPD yields 294 and 312 Da fragments not seen in CID.

UVPD

CID taken with AB SCIEX QTRAP 4000

Parent (327 Da)

β-O-4 Na+ CID vs. UVPD

OHNa+

m/z =143

• UVPD of β-O-4 Na+ results in substantially different fragments vs. lithiated species.

• UVPD fragments different

from CID fragments: 143, 156, and 172 Da peaks not seen in CID spectrum.

CID

UVPD

HO

OCH3

Na+

343 [M+Na+ ]

m/z 173

Conclusions

Ion spectroscopy adds another dimension to lignomics.

Dilignol transitions resolvable with ion spectroscopy.

Photofragmentation mass spectrometric signatures differ compared to CID.

UVPD of lithiated and sodiated β-O-4 dilignol markedly different. In both cases UVPD provides complementary fragmentation to CID.

Different features found with different linkages.

Future work includes studies of negative ion dilignols.

Acknowledgements

NSF CRIF CHE-0820766 NSF CHE-1213289 DOE FG02-00ER15105 All group members of the McLuckey and Zwier groups.

McLuckey Group Zwier Group