The gas phase basicities (GPB) of purines and their hydroxyl radical modified analogs were characterized by different methods and diverse basis sets. The macroscopic and microscopic protolytic properties of six derivatives were analyzed in details. Most of studied model purine analogs, namely 8-oxo-adenosine (AB), 8-oxo-guanosine (GA), xanthosine (GB) and fapy-guanosine (GC) have reduced basicities from 2.0 kcal/mol (AB) to 6.4 kcal/mol (GA) compared to non-modified model purine nucleosides. The fapy-adenosine (AC) and 2-OH-adenosine (AA) are characterized by higher basic character compared to non-modified adenosine. Besides, it is presented the detailed analysis of GPB in accuracies estimated by means of HF, MP2, B3LYP, G3MP2 and G3MP2B3 methods. The B3LYP/aug-cc-pvdz approach seems to be the most accurate among studied methods and precise enough for estimation of GPB. However, the microscopic protonation features are much more sensitive to applied method since the difference in energies between some tautomers are often less than 1 kcal/mol with method dependent succession. The correct sequence of neutral and cationic forms may be however obtained using one of the model composite chemistry approaches, e.g. G3MP2B3. In the cases where B3LYP/aug-cc-pvdz and G3MP2B3 methods lead to contradictory predictions of order of neutral or protonated tautomers the latter is suggested to be used in the interpretation of microscopic protonation properties. Nevertheless, if only macroscopic property is necessary the B3LYP/aug-cc-pvdz level is sufficient since it provides GPB values with 1.0 kcal/mol accuracy.
JavaScript jest wyłączony w Twojej przeglądarce internetowej. Włącz go, a następnie odśwież stronę, aby móc w pełni z niej korzystać.