Please please please help me out if you can (I hope I am allowed to post this here, if I am not I am sorry!) I am so confused about the following 2 questions. Also, can someone explain to me in general what we need to know about pI's and pKa's for the MCAT. THAT WOULD BE BEYOND AWESOME!! BTW, it gives structure of ADH with this info: Vasopressin. The pKa of the N-terminus is 8.0, the phenolic hydroxyl is 10.0, and the guanidino group is 12.5. (See post below for structure given) 139. What is the isoelectric point (pI) of ADH? A. 9.00 B. 10.25 C. 11.25 D. 15.25 - C is the best answer. Vasopressin has 3 dissociable hydrogens. One is on the N-terminus (pKa ≈ 8.0), one is on the phenolic hydroxyl (pKa ≈ 10.0), and the last is on the guanidino group of arginine (pKa ≈ 12.5). The isoelectric point (pI) is that pH at which the molecule (vasopressin) has no net charge and is in the zwitterion form. As shown by the structure of vasopressin in Figure 1, there is an overall +2 charge. What pKa must be exceeded before we get to a vasopressin molecule with a +1 overall charge? The first hydrogen to come off, if we titrate the molecule with a base is the one with the lowest pKa. The hydrogen that comes off first is the N-terminus hydrogen. Once this happens, there is no longer a positive charge on that nitrogen. The vasopressin molecule now has an overall charge of +1. The next hydrogen to come off belongs to the phenolic hydroxyl. Removal of that hydrogen places a -1 charge on the phenolic oxygen atom. At this point, vasopressin now has a -1 charge on the phenolic oxygen and a +1 charge on the guanidino group. The overall charge is zero, and we have found the zwitterion. The pI is simply the average of the two pKa values on either side of the zwitterionic form. --> WHY!?! 141. What is the net charge on vasopressin, if it is being eliminated in urine that has a maximum alkaline pH of 8.0? Answer: 1.5. WHY!?