A Mathematical Approach to Protein Biophysics by L. Ridgway Scott & Ariel Fernández

Author:L. Ridgway Scott & Ariel Fernández
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

hen egg-white lysozyme

133L

130

34

13

human apomyoglobin

2HBC

146

34

3

monomeric human insulin

6INS

50

30

14

human -microglobulin

1I4F

100

17

9

The experimental techniques reviewed in the previous section suggest that the density of dehydrons correlates with protein stickiness. However, the techniques are based on measuring the aggregate behavior of a large number of proteins. One might ask for more targeted experiments seeking to isolate the force of a dehydron or at least a small group of dehydrons. Such experiments were reported in [147] based on atomic force microscopy (AFM).

We will not give the details of the experimental setup, but just describe the main points. The main concept was to attach hydrophobic groups to the tip of an atomic force microscope. These were then lowered onto a surface capable of forming arrays of dehydrons. This surface was formed by a self-assembling monolayer of the molecules SH–(CH)–OH. The OH “head” groups are capable of making OH–OH hydrogen bonds, but these will be exposed to solvent and not well protected.

The data obtained by lowering a hydrophobic probe on such a monolayer are complex to interpret. However, they become easier when they are compared with a similar monolayer not containing dehydrons. In [147], the molecule SH–(CH)–Cl was chosen.

The force–displacement curve provided by the AFM has similarities for both monolayers [147]. For large displacements, there is no force, and for very small displacements, the force grows substantially as the tip is driven into the monolayer. However, in between, the characteristics are quite different.

For the OH-headed monolayer, as the displacement is decreased to the point where the hydrophobic group on the tip begins to interact with the monolayer, the force on the tip decreases, indicating a force of attraction. Near the same point of displacement, the force on the tip increases for the chlorine-headed monolayer. Thus, we see the action of the dehydronic force in attracting the hydrophobes to the dehydron-rich OH-headed layer. On the other hand, there is a resistance at the similar displacement as the hydrophobic tip begins to dehydrate the chlorine-headed monolayer. Ultimately, the force of resistance reaches a maximum, and then, the force actually decreases to a slightly negative (attractive) value as the monolayer becomes fully dehydrated. It is significant that the displacement for the force minimum is approximately the same for both monolayers, indicating that they both correspond to a fully dehydrated state.

The force–displacement curves when the tip is removed from the surface also provide important data on the dehydronic force. The force is negative for rather large displacements, indicating the delay due to the requirements of rehydration. Breaking the hydrophobic bond formed by the hydrophobic groups on the tip and the monolayer requires enough force to be accumulated to completely rehydrate the monolayer. This effect is similar to the force that is required to remove sticky tape, in which one must reintroduce air between the tape and the surface to which it was attached. For the chlorine-headed monolayer, there is little change in force as the displacement is increased by four Ångstroms from the point where the force is minimal. Once the threshold is reached, then the force returns abruptly to zero, over a distance of about one Ångstrom.

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