A charge transfer process in the visual pigments |
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Authors: | Ignacio G Galindo |
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Institution: | (1) The University of Chicago, Committee on Mathematical Biology, Chicago, Illinois;(2) Present address: Seccion de Radiacion Solar, Instituto de Geofisica, UNAM, Ciudad Universitaria, Mexico, D.F., Mexico |
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Abstract: | A physical model that incorporates all the experimental information on the formation of the visual pigment rhodopsin is presented.
The visual pigments consist of a chromophore bound to an appropriate protein. Thus rhodopsin (λm 505 mμ) is formed by a Schiff’s base linkage C19H27CH=NH+-opsin (λm 440 mμ) between 11-cis retinal (λm 380 mμ) and the protein opsin (λm 280 mμ). It is found that there exists a red shift in the spectrum of rhodopsin from the Schiff’s base. The model brings
an explanation for this red shift. It is shown that such a shift may be due to a charge transfer process (R. S. Mulliken,J. Am. Chem. Soc.,74, 811–824, 1952) between an electron at the double bond of carbons C11−C12 and an atomic orbital of the sulphur present in cysteine. This provides an explanation of the presence of SH-groups in the
protein after the absorption of light. A one-electron approximation is used and the dipole momentμ
NV
; hence, the oscillator strengthf of the transitionNV is estimated and compared with the experimentally determined extinction coefficient ∈m by mixing 3.5×10−3 M of 11-cis retinal with 8.3×10−5 M of cysteine at pH ranges 6 through 8. Reasonable agreement is found. Solvent, concentration and temperature dependence
are shown also. |
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