Computational modeling anharmonic coupling in heme

and nucleic acids

NC State University

Axial ligand replacement in H93G myoglobin

Wild type myoglobin

Proximal cavity mutant of Mb

Heme

His 93

L

BackgroundIt is well-known that the iron-histidine vibrational modein heme proteins has a temperature dependent frequency.This has been interpreted in terms of anharmonic coupling of that mode with an even lower frequency heme doming mode. We have explored this bothusing experimental measurements of different axial ligandsto the heme iron in the proximal cavity mutant of myoglobin(H93G) and using density functional theory calculations.

νFe-LνFe-doming

Temperature dependence of the H93G(2-Me Im) axial mode

Differences in anharmonicity

for axial ligands

Experimental approachshows that axial ligands differ.

Horse heart is closest toH93G(4-Me Im).

H93G(2-Me Im) is the most anharmonic.

Brewer and Franzen, JPC B 2002, 106, 11641

DFT calculated intermode

anharmoniccoupling

Comparison with experiment

Frequency shift of axial-ligand mode calculated for displacement of iron doming mode.

Only H93G(1-Me Im) excluded because of Fermi resonance.

Im4-Me Im

2-Me Im

4-Br Im

2,4-diMe Im

Iron-ligand stretching and doming modes

νFe-L νFe-doming

4-Me Im

1-Me Im

143 cm-1 77 cm-1

161 cm-1 81 cm-1

Iron-ligand stretching and doming modes

νFe-L νFe-doming

Im

4-Br Im

70 cm-1

87 cm-1

143 cm-1

170 cm-1

Resonance Raman spectroscopy of iron-histidine stretching mode of

deoxy dehaloperoxidase

The frequency of the Fe-His mode is intermediate betweenthat of myoglobin (HHMb) and horse radish peroxidase (HRP).

Franzen et al., JACS 1998, 120, 4658-4661

Background for anharmoniccoupling in adenine

It is difficult to interpret vibrational spectra of adenine.The exocyclic amino group shows anomalous isotopeshifts in infared spectra. We hypothesize that this arisesfrom anharmonic coupling of the in-plane bending modes to a low-frequency out-of-plane puckering vibration. Thishas been explored using density functional theory calculations as shown in the following pages.

1.5

1.0

0.5

0.0

Wav

enum

ber (

cm-1

)

-0.10 -0.05 0.00 0.05 0.10

Mass-Weighted Nuclear Coordinate

Calculated Data Points Polynomal Fit Line

B

Anharmonicnature of amino

puckeringDisplacements obtained fromnormal mode projections.

Double well arises from projections out of plane.

Calculated data pointsPolynormial fit

1400

1200

1000

800

600

400

200

0

Wav

enum

ber (

cm-1

)

-1.0 -0.5 0.0 0.5 1.0Mass-Weighted Nuclear Coordinate

Caculated Data Points Polynomial Fit Line

Numerov - Cooley wavefunctions

Lappi, S. E.; Collier, W.; Franzen, S. J. Phys. Chem. A 2002, 106, 11446-11455

Harmonic shiftanalysis

NH2 puckering causes an upshift in NH2 bendingmodes.

Mode 34

Mode 9

Mode 20Mode 23

Mode 32

Mode 37

The exocyclic amino group vibrations form a coupled subsystem