Computational modeling anharmonic coupling in heme and nucleic acids NC State University
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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
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