$[$Exploratory$]$ Given that there are $20$ amino acids in humans, how many distinct pentapeptides are possible?

Characterize the interactions between amino acids shown in the diagram below.

a$.$ vo trese mteractions comsist or mtermuecurar iorces? $"$ so$,$ what kind$($s$)?$

b$.$ Describe a specific $pH$ change that could disrupt one of the interactions below $($and briefly describe the disruption$).$