Quantum chemical study of tautomerization in select pharmaceuticals

Prototropička tautomerizacija je ravnotežna reakcija pomaka protona s kiselog centra molekule na bazični, uz istovremeni pomak π-elektrona u suprotnom smjeru. Molekule koje posjeduju -CH-C(=O)-NH- fragment postoje u tri tautomerna oblika: keto, enol i iminol. Među takvim molekulama nalazi se značajan broj lijekova. Tautomeri se međusobno razlikuju po fizikalno-kemijskim osobinama, stoga je karakterizacija tautomerne smjese važna za razumijevanje farmakokinetike i farmakodinamike lijekova, kao i za postupke proizvodnje i oblikovanja. Udio svakog tautomera u ravnotežnoj smjesi ovisi o njihovim relativnim stabilnostima. Gibbsova energija molekule može se odrediti in silico, koristeći računalne programe bazirane na zakonima kvantne mehanike. U ovom radu ispitane su stabilnosti tautomera 15 molekula farmaceutika. Keto oblik najstabilniji je u svim slučajevima. Iminolni oblik većine molekula dovoljno je stabilan kako bi se smatrao značajnim dijelom ravnotežne smjese, za razliku od enola čiji se udio zbog nestabilnosti može zanemariti. Supstituenti na amidnom dušiku i α-ugljiku utječu na energije tautomera: najjači su stabilizatori elektron-akceptorske skupine, aromatski sustavi i strukture koje omogućuju uspostavu produžene konjugacije. Stabilnost se može povećati i uspostavom intramolekulske vodikove veze. Ispitivani lijekovi koriste se u terapiji velikog broja pacijenata širom svijeta. Karakterizacija tautomera pomoći će u produbljivanju razumijevanja njihovih fizikalno-kemijskih osobina te u izgradnji srodnih lijekova poboljšanih svojstava.Prototropic tautomerization is an equilibrium reaction that includes a proton shift from the acidic centre of a molecule to the basic centre, with the simultaneous shift of π-electrons in the opposite direction. Molecules possessing the -CH-C(=O)-NH- fragment exist in three tautomeric forms: keto, enol and iminol. Among such molecules is a large number of pharmaceuticals. Tautomers differ in physico-chemical properties. The characterisation of the tautomeric equilibrium is thus important for the understanding of pharmacokinetics and pharmacodynamics of drugs, as well as for their manufacturing and formulation. The percentage of each tautomer in the equilibrium depends on their relative stabilities. The Gibbs energy of a molecule can be determined in silico, using computer programs based on the laws of quantum mechanics. In this thesis the stabilities of tautomers of 15 molecules were examined. Keto form is the most stable in all cases. Iminolic form of most molecules is stable enough to be considered a significant part of the tautomeric mix, while the enolic form is very unstable, so its presence can be deemed negligible. Substituents on the amidic nitrogen and the α-carbon affect the energies of tautomers: electron-accepting groups, aromatic structures and conjugated systems possess the strongest stabilizing effect. Stability of tautomers can also be increased by the formation of an intramolecular hydrogen bond. The analyzed pharmaceuticals are widely used for treatment of patients all over the world. Characterization of their tautomers will help increase the understanding of their physico-chemical properties and aid in design of related drugs with improved features