2007/Sep/10

กรดอะมิโนที่มี a-amino group 1 หมู่, มี a-carboxyl group 1 หมู่ และมีหมู่ R ที่ไม่แตกตัว จะมี titration curve ที่คล้ายคลึงกับ titration curve ของ glycine กรดอะมิโนเหล่านั้นจะมี pKa ที่ใกล้เคียงกันแต่ไม่เท่ากัน ค่า pKa ของหมู่ COOH อยู่ในช่วง 1.8-2.4 และ pKa ของหมู่ ---NH3+ อยู่ในช่วง 8.8-11.0

กรดอะมิโนที่มีหมู่ R ที่แตกตัวได้จะมี titration curve ที่ซับซ้อนกว่า โดยจะมีสามขั้นตอนเพราะมีสามขั้นตอนการแตกตัวที่เป็นไปได้ (ดังนั้นจะมีสามค่า pKa) ขั้นตอนของการไตเตรตที่เพิ่มมานี้จะกลมกลืนไปกับสองขั้นตอนเดิม titration curve ของกรดอะมิโนประเภทนี้มีลักษณะดังแสดงในรูป

Titration curve ของ histidine

Isoelectric point จะสะท้อนธรรมชาติของหมู่ R ที่สามารถแตกตัวได้ของกรดอะมิโนชนิดนั้นๆ ยกตัวอย่างเช่น glutamate มี pI ที่ 3.22 (ซึ่งต่ำกว่า glycine, pI = 5.97) เป็นเพราะว่ามีหมู่คาร์บอกซิลอยู่ 2 หมู่ ที่ pH = pKa เฉลี่ย (3.22) จะให้ประจุสุทธิ = -1 ซึ่งสมดุลกันพอดีกับประจุ +1 ของหมู่อะมิโน ในลักษณะเดียวกัน histidine (ซึ่งมี 2 หมู่ที่ให้ประจุบวกเมื่อถูก protonate) มี pI = 7.59 (ค่า pKa เฉลี่ยของหมู่อะมิโนและหมู่ imidazole) ซึ่ง pI ค่านี้สูงกว่า pI ของ glycine มาก

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