QUANTUM-CHEMICAL CALCULATIONS ON THE INTRAMOLECULAR HYDROGEN-BOND OF CIS-UROCANIC ACID

Citation
A. Lahti et al., QUANTUM-CHEMICAL CALCULATIONS ON THE INTRAMOLECULAR HYDROGEN-BOND OF CIS-UROCANIC ACID, Journal of molecular structure. Theochem, 452, 1998, pp. 185-202
Citations number
53
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Chemistry Physical
ISSN journal
0166-1280
Volume
452
Year of publication
1998
Pages
185 - 202
Database
ISI
SICI code
0166-1280(1998)452:<185:QCOTIH>2.0.ZU;2-L
Abstract
The intramolecular hydrogen bond (IMHB) of the neutral, cationic, and anionic cis-urocanic acid (cis-UCA, (Z)-3-(I'H-imidazol-4'(5')-yl)prop enoic acid) is studied using ab initio calculations up to the MP2/6-31 +G* level of theory. The IMHB energies are estimated by first showing a near to perfect linear correlation between the hydrogen bond energi es of modified cis-urocanic acid structures and the Mulliken bond orde rs of their hydrogen bonds - the electron densities at the critical po ints of the hydrogen bonds have a weaker correlation with the hydrogen bond energies than the bond orders - and then using this correlation in order to evaluate the IMHB energies of the cis-UCA structures thems elves. This method gives high ( > 50 kJ mol(-1)) IMHB energies for the neutral and cationic cis-WCA structures and very high ( > 100 kJ mol( -1)) IMHB energies for the anionic cis-UCA structures in the gas phase . The effect of solvation on the IMHB in cis-UCA is studied using both MP2/6-31G water-UCA supermolecule optimizations and HF/6-31G* SCI-PC M (self-consistent isodensity polarized continuum model) reaction fiel d calculations. Solvation changes the IMHB energies by up to 30 kT mol (-1). The IMHB of the imidazole protonated structures is weakened, whi le that of the carboxylate group protonated structures is strengthened in aqueous solution. Solvation does not change the optimal conformati on of cis-UCA from the IMHB-ed closed conformation to an open conforma tion. The supermolecule calculations and the SCI-PCM calculations give consistent changes of the IMHB strengths in aqueous solution. Inclusi on of only 1-3 water molecules nearest to the IMHB seems to be enough to demonstrate most of the solvent effect on the IMHB. The semiempiric al AM 1 method is shown to be a valuable tool in finding good initial positions for the water molecules in ab initio calculations. The adequ acy of cis-UCA as a model compound for the hydrogen bonded His-Asp com ponent of the catalytic triad in serine proteases is investigated. The IMHB in the anionic cis-UCA is a low-barrier hydrogen bond (LBHB) in the gas phase, while the IMHB in the neutral cis-UCA is not. In aqueou s solution, modeled by HF/6-31G SCI-PCM calculations, the IMHB in bot h the neutral and the anionic cis-UCA seems to be an LBHB. The anionic cis-UCA is suggested as an experimental model for the initial state, and the neutral cis-UCA as that for the final state of the proton tran sfer process in the His-Asp system of the catalytic triad. (C) 1998 El sevier Science B.V. All rights reserved.