In a solution at pH 8.5, which of the forms of the histidine side chain (depicted below) will be present in a lower concentration?
a) H₃N⁺-CH(CH₂) ₃
b) H₂N-CH(CH₂) ₃
c) H₃N⁺-CH(CH₂) ₂
d) H₂N-CH(CH₂) ₂