Protonation and hydration of biomolecules govern their structure, conformation, and function. Herein, we explore the microhydration structure in mass-selected protonated pyrimidine-water clusters (H(+)Pym-W-n,n= 1-4) by a combination of infrared photodissociation spectroscopy (IRPD) between 2450 and 3900 cm(-1)and density functional theory (DFT) calculations at the dispersion-corrected B3LYP-D3/aug-cc-pVTZ level. We further present the IR spectrum of H(+)Pym-N(2)to evaluate the effect of solvent polarity on the intrinsic molecular parameters of H(+)Pym. Our combined spectroscopic and computational approach unequivocally shows that protonation of Pym occurs at one of the two equivalent basic ring N atoms and that the ligands in H(+)Pym-L (L = N(2)or W) preferentially form linear H-bonds to the resulting acidic NH group. Successive addition of water ligands results in the formation of a H-bonded solvent network which increasingly weakens the NH group. Despite substantial activation of the N-H bond upon microhydration, no intracluster proton transfer occurs up ton= 4 because of the balance of relative proton affinities of Pym and W(n)and the involved solvation energies. Comparison to neutral Pym-W(n)clusters reveals the drastic effects of protonation on microhydration with respect to both structure and interaction strength.
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