All language subtitles for Non-covalent bonds 7

Section 4.

Non-covalent bonds.

Hydrogen bonds.

Not hydrogen bombs, but hydrogen bonds.

Much more peaceful things.

Hydrogen bonds.

We've agreed that oxygen is greedy compared to a hydrogen.

And so if I have a water molecule,

I have some negative charge over there, a little bit

of positive charge over there.

Suppose I have another water molecule over here.

I have a little bit of negative charge over there,

and I've got a little bit a positive charge over there.

Not that much, but I got some.

This is like a little magnet, got a positive end

and a negative end.

This thing's here a little bit like a magnet, positive end

and a negative end.

What happens when I have two magnets?

They attract, or repel.

And so what's going to happen is these guys

are going to attract.

Now that doesn't sound very impressive,

just these two little atoms-- these two little molecules.

But I got more of that over here.

I got more of this over here.

And when I have a big cup of water, all of those molecules

are doing that to each other.

They're all weakly interacting by those little magnets,

hydrogen bonds.

So in fact, what I end up with are big cages like that.

The water's all coupled to each other by these weak forces.

And these aren't like huge forces.

These bonds as compared to my 80 kilocalories per mole,

these are five kilocalories per mole, just five

kilocalories per mole.

And it turns out that, nonetheless,

because I have so many of them, water

is highly, highly structured.

They're all coupled to each other,

moving around some, structure.

This is so strong bugs can walk on water.

Bugs are walking on hydrogen bonds.

The net the bugs are walking on, that surface

is provided by the strength of all those hydrogen bonds.

No hydrogen bonds, the bugs sink.

You can tell that.

You put in something disrupts the hydrogen bond, soap.

They sink.

So you may not be impressed by hydrogen bonds, any one

hydrogen bond, but aggregate hydrogen bonds are pretty cool.

Next time you think of water, think of water as a highly,

highly structured business.

All right.

Strength in numbers.

Next we have ionic bonds.

Sometimes, sometimes we have the situation

where a hydrogen may come off here, and move over

here to create a net positive charge here

and a net negative charge there.

So here this is no longer the sharing

of electrons, but really the exit of an electron--

the exit of a positive charge to create a negative charge in one

place and a positive charge in the other place.

This is an ionic bond between charged ions

here, between ions.

So these depend, of course, on their distances

as any of these forces depend, ionic, of these magnetic forces

depend with distance.

They're also, on the whole, relatively weakish.

But you add them up together, and these all have effects.

Hydrogen bonds have effects.

Ionic bonds have effects.

Another kind of bond, even weirder.

Van Der Waals forces.

Van Der Waals bonds.

I told you that carbon carbon, nonpolar.

Carbon hydrogen, nonpolar.

Equal sharing.

That's all very nonpolar.

Well, I'm slightly lying.

It turns out that even though there's

an equal sharing on average, at any given instant

the negative charge might be a little on one side

or a little on the other side.

It sort of fluctuates back and forth.

But if these guys are right near each other,

and it so happens that this instant, this is

a little more negative and this is a little more positive,

just temporarily, what is that negative going to do over here

to the negative charges?

Push them away.

It's going to repel those negative charges.

And they'll go over here a little bit.

And this will become a little more positive.

But of course that's only instantaneous,

because it's a fluctuating back and forth.

When it's more positive on that side,

it's going to be pulling negative charges over.

So both bonds are fluctuating, but they can fluctuate

in concert with each other.

They entrain each other.

And if you have those two bonds that are fluctuating,

and they can manage to entrain each other,

then it's as if you have little magnets temporarily going back

and forth and back and forth.

And that works, too.

Now this is even less impressive than hydrogen bonds.

This gets you an absolutely ridiculously puny one

kilocalorie per mole.

Not very impressive, is it?

But if I have some big molecule, like a protein,

the sum of all these forces-- the sum

of hydrogen bonds and ionic bonds

and these Van Der Waals interactions--

add up to big, big numbers.

Big numbers.

And in fact, biological molecules

are held together, yes, by their covalent bonds.

They matter a lot.

But the shape they take up depends much less

on their covalent bonds than all of

these non-covalent interactions that are taking place.

The hydrogen bonds, the ionic ponds, the Van Der Waals

interactions, to determine whether some long molecule

folds up this way or folds up that way or whatever.

Now just to make you appreciate how powerful those forces are,

I bring you the gecko.

The gecko can climb up glass.

It can climb this way.

Why is the gecko not falling down?

Well people thought maybe the gecko has glue on its pads.

It does not.

What holds the gecko up?

The gecko is held up by Van Der Waals forces.

It manages to have its palm have these little flappy pads, that

can make so much contact that it's

making lots of these ridiculously tiny Van Der Waals

interactions with the surface.

And that is enough to hold up a gecko.

Do not try this yourself, as it turns out

that you lack the right kind of pads and, I'm sorry to say,

are too heavy to be held up.

But for something the size of a gecko,

it can be held up by Van Der Waals interactions.

Strength in numbers.

We have to understand that to understand biology.

You're almost done.

But before we go on we've got a couple questions for you

about different kinds of bonds.