Investigation of the intrinsic H-bonding pattern of the guanine complex with a sizable segment (from Asn43
to Glu46) of the primary recognition site (PRS) in RNase T1 at the B3LYP/6-311G(d,p) level of theory
enables the electronic density characteristics of the H-bonding patterns of the guanine-PRS complexes to be
identified. The perfect H-bonding pattern in the guanine recognition site is achieved through the guanine
complex interactions with the large segment of the PRS. Two significant short H-bonds, O1aaaHN1 and
O2aaaHN2, have been identified. The similar short H-bond distances found in the anionic GC- base pair and
in this study suggest that the short hydrogen-bond distances may be characteristic of the multiple H-bonded
anionic nucleobases. The H-bonding energy distribution, the geometric analysis of the H-bonding pattern,
and the electron structure characteristics of the H-bonds in the guanine PRS of RNase T1 all suggest that the
O1aaaHN1 and O2aaaHN2 side-chain H-bonds dominate the binding at the guanine recognition site of RNase
T1. Also, the geometry evidence, the electron structure characteristics, and the properties of the bond critical
points of the H-bonds reveal that the side-chain H-bonding and the main-chain H-bonding are mutually
intensifying. Thus the positive cooperativity between Asn43 to Tyr45 and Glu46 is proposed.