PARENT SESSION
Posters P2A Type I reaction centres. Abstracts (181-218)

Computed infrared spectra of protonated and metal-bound methylimidazole. Ruili Wang^*,1, Gary Hastings^*,1, ¹ Department of Physics and Astronomy, Atlanta, GA, USA

ABSTRACT- The imidazole ring of histidine has two nitrogen atoms which can be protonated (and hydrogen bonded) or ligated. In photosystem I histidine residues HisA680 and HisB660 provide axial ligands to the chlorophyll molecules of P700. The histidine imidazole nitrogen at position 1 is ligated to the central magnesium atoms of the chlorophylls while the nitrogen at position 3 is protonated, and probably hydrogen bonded to PheA676/PheB656, respectively. It is therefore possible that the imidazole ring of HisA680/HisB660 is electronically similar to a doubly protonated imidazolium species. It is also possible that such a species contributes to (P700⁺-P700) FTIR difference spectra. To investigate these possibilities we have used hybrid density functional theory [B3LYP/6-31G(df,p)] to calculate the vibrational frequencies and intensities of the four different protonation forms of 4-methylimidazole (4-MeIm). In addition, we have calculated the vibrational frequencies and intensities for 4-MeIm bonded to a magnesium complex. The 6-31G(df,p) basis set was used for all of atoms except the metal, which was modeled using the Hay-Wadt effective core potential (LANL2DZ). A hydrogen bond to the imidazole nitrogen at position 3 was also simulated using a water molecule. Calculations were also undertaken for all of the above imidazole complexes that were fully deuterated, ¹³C and ¹⁵N labeled. The calculated spectra are compared with available experimental data.

KEY WORDS: P700, Photosystem I, Imidazole, Histidine