Cooperative effect of CH•••O bonds in models for biological systems

In order to calculate CH center dot center dot center dot O interactions appearing in biologically important systems, a preliminary study is performed for smaller models. MO bond indices I-XHY are calculated for dimers involving HCN, acetic acid, acetamide, glyoxal and formamide, as well as a ribose33-U35 pair appearing in tRNA(Phe) and 3MU-U, also related to RNA. The distance dependence obeys the same pattern for I-OHO, I-NHO and I-CHO. It is shown that the negative I-XHY sign is related to the lengthening of the XH distance; the fluctuation of net charges for the involved atoms is related to the bond index. All results point at a definite cooperative effect.