= Solution
At ordinary physiological pH, the <arginine> side chain is predominantly <guanidinium>. Its N–H groups are donors, and its positive charge can reinforce favorable electrostatic interactions. \b[<guanine> exposes adjacent O6 and N7 acceptors in the <major groove>], allowing two <guanidinium> N–H donors to make a paired interaction without breaking Watson–Crick pairing. The diagram places this <arginine> contact on the exposed <guanine> edge, not between the paired bases.
<adenine> also exposes N7, but at the corresponding neighboring position it has the exocyclic N6 amino donor rather than a <carbonyl> acceptor. It therefore cannot make the same donor-to-two-acceptors match with <guanidinium>. A single N7 contact would be less discriminating. The classic structural recognition analysis explicitly proposed paired <arginine>–<guanine> contacts in the <major groove>.<Sequence-specific recognition of double helical nucleic acids by proteins by Nadrian C. Seeman and others>[1976, pp. 804–808, major-groove arginine–guanine recognition model]{ref} <protein> geometry, hydration and phosphate contacts also affect sequence recognition; this local motif explains a preference rather than guaranteeing that every <arginine>-containing <protein> recognizes every G–C site.
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