Buffer

Añadido: 2026-05-25

[00:00:07]Hello students, it's me again, your lecturer funman.

[00:00:11]You know, today is Monday and uh last weekend I got a cold here, so my voice is a bit different. I'm under the weather, but I can still speak. If I lost my voice, I'll still continue to record video for you, but that might be more ASMR.

[00:00:25]Now I'm very grateful to be at UCD and hope you are as well to be in this community of learning. So let's continue our class here chemistry for biology.

[00:00:35]Now this is as in basis part two here right first of all remind yourself if you can with fun man you can have fun man. Okay for some of you you have started your lab this week some you're going to do this week. Remember the last couple of sessions where we talked about acid bases indicators. This is something you see in the lab there. Okay. Test tube test your bra and the droplets of the chemicals.

[00:01:03]Do enjoy learning. And we're right here.

[00:01:06]Almost halfway done.

[00:01:09]Okay. And they are the colors of some of the indicators. Right. I will let you guess what is it and tell me afterwards here after doing your lab.

[00:01:19]Some of the indicators you use nitrine yellow, bromemo phenol blue, etc. Okay, I took this in lab about a month before you came in to make sure the experiments are okay. You know, me and my teaching assistant film some short videos which we would show you during the class, right? It's very real and bonafide. We make mistake. We'll show you so that you don't make the same mistake as us as him. Okay, not me.

[00:01:48]Now at the end of today's class right you should be able to articulate explain the meaning of the term buffers buffering capacity and blood buffers.

[00:01:58]How are buffers applicable useful to humans they are in our blood in our body. What are the effects of pH on the structure electrical charge of proteins our blood's array because of hemoglobin and the iron tree. Okay, as human the blood of snake is not the same. Okay, other animals and creatures might have a copper in the blood. So they have a different color. But as you have watched my other video on how to calibrate the pH meter, you have seen these three normal buffer to calibrate it.

[00:02:43]Right? So in the classical way we use different chemicals to calibrate the buffer solutions.

[00:02:51]The first thing is right you need to know what is a buffer. It's a solution that you add that can resist a small change in pH when an amount of acid or base is added to it. So it has like a shield right? So that when you drop some acid or base accidentally the pH of the solution doesn't change that much. It's like a buffer. you know you have buffer time you have buffer money buffer whatever and the reason is because you have the acid base conjugate pair so that when you have either side the H+ or hydroxy come in some of them will grab it and change to the other property it doesn't change the pH that much and the buffering capacity we denote as B is expressed as the number of moles of a strong acid or base called delta D over the delta pH. Delta in science means change, right? This is capital delta like D. Change in the D over change in the pH there. Okay. And I always refer to the same diagram here. If you add a bit, it will change you know very slightly. Okay.

[00:04:02]So when you have a pH close to pKa in a buffer you have maximum buffering capacity because these two they are acid base conjugate pair they have one to one ratio it's like saying that your defense and attack are top and maximum okay so that you know whoever comes in you can defend you can also attack when this ratios one to one there okay so when H+ is added to it right the A minus grabs it and form a H. When there's hydroxide present, the H+ that's dissociated will collide with the O minus and form A minus. So you see in the end, you're still having a minus H.

[00:04:45]Ah, you don't really change the system.

[00:04:48]But of course, when you spam too much of the acid and base here, no amount of buffer can change it, right? Buffer is just like a little cushion, a thin cushion there. It's not a big pillow.

[00:04:58]Okay, remember it's a thin cushion. the cushion.

[00:05:02]So in the blood here is regulated between pH 7.35 to 7.45 and you saw in the last class is the carbonic acid that's present in the blood buffer system.

[00:05:18]Okay, that's the B value. Okay, and the percentage of the total amount that's present.

[00:05:26]So if your pH drops to acidic site really you got death you know you won't survive this is the normal pH range is a bit acidic you get acidtosis you feel unwell to also not good basically extreme pH is so bad okay so sometimes when you feel unwell it could probably be due to your pH level which are influenced by other factors too but I shan go there I shan digress when I saw this page scale I it kind of remind me of the UPAC logo, international year of pure applied chemistry. I was just going through my diary and oh that was so many years ago still younger man in Paris visor there. Still very proud because you meet with Nobel Prize winners who inspire you. Yeah, it was good times before co.

[00:06:23]So you look at the background of this photo, you have lactic acid, you have fatigular muscles when you do spots, you have the sodium hydrogen phosphate, sodium hydrogen carbonate, the buffer solutions there in the lab.

[00:06:39]It's taken from your lab manual here.

[00:06:43]Hope that captures your memory.

[00:06:46]Now in biology, you learn about amino acids that will form proteins, right? So in amin acid like the word suggest you have got amin group and acid group here they can either be deeproinated for the acid or proteinated for the base right in the side chain. So here's a typical example of a general formula of a a amino acid.

[00:07:10]You have a lone pair on the nitrogen that's why I pick up the H+ and form this okay amine CO HD acid caroxile group example is a serene there you have sistain containing sulfur this a bigger atom is yellow because it's in the third period and so this R part that's not part of this normal chain is the side chain of the amino acid now if you refer to here on the left side is the low pH on the right side has high pH there. How would the change in the pH influence the charge? When you have a very low pH that means you have a lot of H+ surrounding you know the a lot of H+. So of course you spam H+ anything that's basic will get protonated and the CO will be protonated also. Basically when your surrounding is very low pH a lot of H+ the environment like to be positive. So there cannot exist any negative charge. That's how I rationalize things. But as you slowly increase the pH what happens is you have this here right plus one minus one here as ion okay so we start to deproinate the acidic proton first okay slowly and then on the other hand when you have very high pH the environment becomes super alkaline you have plenty of O minus which means that the H+ or plus charge cannot exist because if I have H+ you gets snuck by the hydroxide and forms water. So just translate high pH means excessive hydroxide. Hydroxide is negative charge.

[00:08:50]Environment is negative. Negative charge predominates.

[00:08:54]Acidic site H+ around you like to form positive charge. Everything that can be protonated gets protonated in between is half half.

[00:09:06]Okay.

[00:09:10]So the N group and the side chain can also be protonated right like you see here in the C terminus the caroxyic terminus here CO minus is the conjugate base of the caroxic acid not conjugate base it is a base a base can be protonated the N terminus in this case is N plus there's a tendency for H+ to come out as H+ ion so this is acidic right weak acid of course here give you two pKa value okay of the lysin and the arginine side chain. How do I know the side chain? Because here it does not connect to that common backbone there.

[00:09:53]Same thing here for ospotic acid glutamic acid as the term suggests they're acidic. So they're able to give out H+. See the value here much lower than here because they are strong acid.

[00:10:05]they are called acid or the in because it's the amin you see the relationship there hope we can find this uh the strand now some of the pKa of this aa side chain right I will just show you please don't memorize okay don't memorize but understand it form a pattern form a system on your own the pKa of the side chain etc and the pKa in the protein is a different how so has a sulfur again can form dulfide bridges in the tertiary structure of your protein aspartic acid e of H+ we went through this before histadin is the five member cyclic structure next we have lysine you can tell from the name here it's a more amin like pka hiden Seven. Okay.

[00:11:08]What you need to know though is just to show me before and after. If I give you arginine, I show you the structure, you're able to draw for me the other side where the protonated.

[00:11:18]Okay, that's all.

[00:11:22]Now, how does the pH on the electrical charge get the effect? Right? As I said just now we have extreme pH hydroxide predominous surrounding is negative the whole AA or protein become more negative H+ cannot exist. So let's say I give you this structure of the beta lacto globaline the amino acid sequence as such that you have lysine etc all the way right a primary structure the polyeptide here okay with 162 aa okay I stands for iso electric point which means remember just now we have the extreme pH there in the middle when you have no charge right it's called iso electric point when you have a balance of the plus and minus charge and that exists at a certain pH and this pH is different for different amino acid. So every amino acid has a different pi iso electric point that refers to the pH at which this amino acid has no net charge.

[00:12:30]Such a mouthful. Okay, over here no net charge. Got it?

[00:12:39]Okay. So you see that this is a monomer and then because you have charged particles you aggregate together to drize and then you form octoma eight per molecule and then diamond again. You might see how come over here it is an octto when is electric because you have like head and tail and tail in out in out. So it can kind of pack together. We have too much of a positive charge here.

[00:13:04]You know you kind repulsion, right? You you cannot connect every juncture. But when you have kind of like in you can't you know head tail head tail. Think of it like they can snuck together. Yeah.

[00:13:20]Okay. Back to this uh example here. So let me flesh it out more. So iso electric point no net charge here for this case is a pH 6.8.

[00:13:31]When you add more bases hydroxide, you will take more negative charge there. So you have more anionic character. Right?

[00:13:39]On the other hand, starting from here, when you add H+, you attain more H+ surrounding it. You will have more cation character.

[00:13:51]Okay, this is just a graph to plot on the solubility of the beta leto globalin. Here you notice that when you have a pi there, the solubility becomes almost zero. It's a low side because when you dissolves in water, you want something that's charged, right? Your iron double moment. Yeah. In this case, right, where lowest charge, no net charge, it doesn't dissolve that well.

[00:14:20]When your overall more charge, it kind of have a trend of having higher solubility.

[00:14:30]So I've kind of finished today's class.

[00:14:33]In conclusion, we have covered what are buffers, right? These are molecules that resist the change in pH when a small amount of acid base is added to it. where your blood buffers why it's important to know them and how pH affects the charges in the amino acid and the protein and what is pi iso electric point.

[00:15:03]So with that I'll see you in the lab here. This is a moment for you to learn by doing chemicals. Stay safe. I'll see you soon. Cheers.