A histidine residue (B), one of the functional groups in the structure of a certain enzyme, has a conjugate-acid pK_a = 7.8. What is the fraction of each form (BH and B) present at physiological pH (pH = 7.4)?

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(enzyme) :N CH₂ H B N: + H₂0 (enzyme) CH₂ :N- H + NH BH (pK₂ = 7.8) + H₂O