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Biochemistry Practical - Experimental Layout Background 2-aminoquinoline (2AQ) can bind to a Tec SH3 domain in humans. It does this by the formation of a salt bridge using a resonance delocalised negative charge on aspartic acid present on an RT loop in close proximity to tryptophan 215 and a resonance delocalised positive charge on 2AQ. Binds with greater affinity at lower pH, supporting salt bridge assumption. Needs to form a good pi-pi stack between Trp and 2AQ. Both aromatic rings have delocalised pi electron clouds, one (Trp) which is slightly negative and the other (2AQ) which is slightly positive. Hence polarity exists. The protein sequence bound by SH3 domains is proline rich, often of sequence PxxP. Three binding pockets on flat hydrophobic ligand-binding surface, two occupied by hydrophobic-proline dipeptides. There are 511 SH3 domains across human proteins. Some have 5-6 such domains. They serve as scaffolds