Protonated nucleobases are not fully ionized in their chloride salt crystals and form metastable base pairs further stabilized by the surrounding anions

IUCrJ

Paulina Maria Dominiak¹  Aleksandra Pazio¹  Prashant Kumar¹  Malgorzata Katarzyna Cabaj¹ 

[1] Biological and Chemical Research Center, Department of Chemistry, University of Warsaw, ul. Żwirki i Wigury 101, Warszawa 02-089, Poland;

关键词: charge density;    multipole refinement;    quantum crystallography;    electrostatic potential;    electrostatic interaction energy;    like-charged ions;    nucleobases;    cytosine;    adenine;    guanine;    base pairs;    intermolecular interactions;    spin density;    momentum density;    hydrogen bonding;    pharmaceutical solids;    nucleic acid structures;   

DOI  :  10.1107/S2052252518006346

来源: DOAJ

【 摘 要 】

This paper presents experimental charge-density studies of cytosinium chloride, adeninium chloride hemihydrate and guaninium dichloride crystals based on ultra-high-resolution X-ray diffraction data and extensive theoretical calculations. The results confirm that the cohesive energies of the studied systems are dominated by contributions from intermolecular electrostatic interactions, as expected for ionic crystals. Electrostatic interaction energies (Ees) usually constitute 95% of the total interaction energy. The Ees energies in this study were several times larger in absolute value when compared, for example, with dimers of neutral nucleobases. However, they were not as large as some theoretical calculations have predicted. This was because the molecules appeared not to be fully ionized in the studied crystals. Apart from charge transfer from chlorine to the protonated nucleobases, small but visible charge redistribution within the nucleobase cations was observed. Some dimers of singly protonated bases in the studied crystals, namely a cytosinium–cytosinium trans sugar/sugar edge pair and an adeninium–adeninium trans Hoogsteen/Hoogsteen edge pair, exhibited attractive interactions (negative values of Ees) or unusually low repulsion despite identical molecular charges. The pairs are metastable as a result of strong hydrogen bonding between bases which overcompensates the overall cation–cation repulsion, the latter being weakened due to charge transfer and molecular charge-density polarization.

【 授权许可】

Unknown