Semiempirical PM5 Molecular Orbital Study on Chlorophylls and Bacteriochlorophylls:Comparison of Semiempirical,Ab Initio,and Density Functional Results

摘要

The semiempirical PM5 method has been used to calculate fully optimized structures of magnesiumbacteriochlorin,Magnesium-chlorin,magnesium-porphim,mesochlorophyll a,chlorophylls a,b,c_1,c_2,c_3,and d,and bacteriochlorophylls a,b,cd,e,f,g,and h with all homologous structures.Hartree-Fock/6-31G~* ab initio and density functional B3L YP/6-31G~* methods were used to optimize structures of methyl chlorophyllide alpha,chlorophyll c_1,and methyl bacteriochlorophyllides a and c for comparison.Sepectroscopic transition energies of the choromophores and their 1:1 or 1:2 solvent complexes weer calculated wtih the Zindo/S CIS method.The self-consistent reaction field model was used to estimate solvent shifts.The PM5 calculations predict planar structure of the porphyrin ring and central position of the four coordinated magnesium atoms in all pigments studied,in accord with the experimentalab initio,and density functional results,a significant improvement as compared to the older semiempirical PM3 approach.Only small differences in PM5 and B3LYP/6-31G~* or Hartree-Fock/6-31G~* minimum energy geometries of hte reference molecules were observed.Calculations show that in 1:1 solvent complexes,where the magnesium atom is five coordinated,the magnesium atom is shifted out of the plant of the porphyrin ring towards the solvent molecule,while the hexa coordinated 1:2 complexes are again planar.The PM5 method gives tomic charges that are comparable with those obtained from the Hartee-Fock/6-31G~* and B3LYP/6-31G~* calculations.The Single point ZINDOS/S CIS calculations with PM5 minimum energy structure gave excellent correlations between calculated and experimental transition energies of te chlorophylls and bacteriochlorophylls studied.Such Correlations may be used for prediction of transitin energies of the chromophores in protein binding sites.Calculations also predict existence of dark electronic states below the main soret absorption band in all chromophores studied.The results suggest that the semiempirical PM5 method is a fairly reliable and computationally efficient method in prediction molecular parameters of porphyrinlike molecules.