A peptide toolkit

A string of one-letter codes gives a short peptide. You will count its peptide bonds, find its mass, work out its net charge at neutral pH, and count its amino acids by class.

The idea. A chain of $n$ amino acids has $n - 1$ peptide bonds. Its mass is the sum of the residue masses plus $18.0\ \text{Da}$ for the water that stays with the chain. At neutral pH, an acidic residue (D or E) carries one negative charge and a basic residue (K or R) one positive charge. The two ends cancel.

Your tasks.

1. Set bonds to the number of peptide bonds.
2. Set mass to the mass of the peptide in daltons.
3. Set charge to the net charge at neutral pH.
4. Set classes to a dictionary with the counts under "acidic", "basic" and "other".