BioPHYSICAL chemistry

the optical nerve. In addition to questions concerning the response

mechanism, the reason why why vertebrates respond to blue light

remains an open question.

17.20 The proton pathway is critical to function and loss of Asp-85 breaks

the proton pathway, which must have a series of ≈2-Å steps.

17.21The retinal binds at a site very similar to that of bacteriorhodopsin.

Nine of the surrounding residues are conserved and water is found,

as was true for bacteriorhodopsin. A single chloride ion was found

near the retinal at a location corresponding to that of Asp-85. Ion

translocation is proposed to occur through a mechanism related

to that of bacteriorhodopsin. The ion is proposed to be driven by

ion–dipole interactions involving the NH group of the retinal before

it is released toward the cytoplasm and a new chloride ion enters

the transport site. Only the replacement of the negatively charged

Asp-85 as a proton acceptor in bacteriorhodopsin by chloride in

halorhodopsin changes the kinetic preference and therefore the ion

specificity. Thus in both cases the retinal serves as a switch for the

movement of the proton or ion.

CHAPTER 18

18.1 The lipid bilayer is basically impermeable to ions and polar

molecules. Water, in contrast, readily traverses membranes. The

permeability of small molecules ranges over several orders of

magnitude and is correlated with the solubility in nonpolar

solvents relative to water.

18.2

18.3

18.4

=(−2.7 +14.5) kcal mol−^1 =+11.8 kcal mol−^1

18.5

18.6 ΔV

c

c

out

in

=−58 6.log=−58 6.log=

60

3

meV meV −−76 2 .meV

ΔV

c

c

out

in

=−58 6.log=−58 6.log=

20

1

meV meV −−76 2 .meV

ΔGRT=+ 23 .log ()(. −)(.+

20

200

1 965kJ mol^1010 ))

ΔGRT==× 23 .log. (.) .RT+=+

100

20

23 07 095kkcal mol−^1

ΔGRT= .log.

×

(^23) + =− −

2

210

2 27 kcal mol^1

ANSWERS TO PROBLEMS 481

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