CALCULATION OF RELATIVE HYDRATION FREE-ENERGY DIFFERENCES FOR HETEROAROMATIC-COMPOUNDS - USE IN THE DESIGN OF ADENOSINE-DEAMINASE AND CYTIDINE DEAMINASE INHIBITORS

Heteroaromatic compounds frequently undergo reversible covalent hydrat ion in aqueous solution with the extent of hydration dependent on the heterocycle and its substituents. Using a combined quantum mechanical and thermodynamic cycle perturbation (TCP) approach, relative hydratio n free energy differences (Delta Delta G(hyd)) were calculated for a v ariety of pteridine, quinazoline, pyrimidine, and purine analogues. Go od agreement with experimental data was obtained for heteroaromatic co mpounds exhibiting a wide range of hydration equilibrium constants (10 (-6)-10(3)). Differences in hydration were attributed to a multitude o f molecular factors including both electronic and steric effects. Diff erences in the resonance energy lost during hydration of the heteroaro matic ring accounted for the 10(7)-fold greater hydration of pteridine relative to 9-methylpurine (Delta Delta G(hyd) (exp) approximate to - 8.8 kcal/mol; Delta Delta G(hyd) (calc) = -9.3 kcal/mol). An analysis of purine riboside and its 8-aza analogue showed that the 400-fold gre ater adenosine deaminase (ADA) inhibitor potency exhibited by the 8-az a analogue is accurately calculated by summing the hydration free ener gy difference with the relative binding foe energy difference for the corresponding hydrated species. The greater inhibitor potency was attr ibuted to increased hydration since hydration of 8-aza-9-methylpurine was strongly favored over 9-methylpurine (Delta Delta G(hyd) = -7.1 kc al/mol), whereas the relative binding free energy calculated using the TCP method and the murine ADA structure favored the purine riboside h ydrate (Delta Delta G(bind) = 3.1 +/- 0.7 kcal/mol). Increased desolva tion costs for the 8-aza analogue and an unfavorable electrostatic int eraction between the 8-nitrogen and Asp296 accounted for the loss in b inding affinity. The combined results gave an apparent inhibition cons tant for the 8-aza analogue similar to the experimental value and demo nstrated the potential importance of hydration free energy calculation s in drug design.