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The First Steps in Vision in the Classroom Revista Brasileira de Ensino de F¶³sica, v. 32, n. 2, 2303 (2010) www.sb¯sica.org.br The ¯rst steps in vision in the classroom (Etapas iniciais para a vis~aona sala de aula) A.C.F. Santos1 Instituto de F¶³sica, Universidade Federal do Rio de Janeiro, Rio de Janeiro, RJ, Brazil Recebido em 15/9/2009; Aceito em 31/2/2010; Publicado em 17/1/2011 We examine, using the particle-in-the-box model, the transient absorption measurements of the cis-trans isomerization by the visual pigment rhodopsin present in the retina [R.W. Schoenlein et al., Science 254, 412 (1991)], which contains the retinene group, a conjugated system, which is formed by single and double carbon- carbon atoms, and works as a light antenna in the process of photon absorption. The present approach is directly applicable to the classroom in undergraduate chemistry and physics major courses. Keywords: vision, quantum mechanics, undergraduate. Analisamos, utilizando o modelo de po»code potencial in¯nito, o processo de absor»c~aode um f¶otonque leva `aisomeriza»c~aocis-trans do pigmento visual rodopsina, presente na retina [R.W. Schoenlein e cols., Science 254, 412 (1991)], que consiste de um composto formado por liga»c~oessimples e duplas entre ¶atomosde carbono, e funciona como uma antena no processo de absor»c~aode um f¶oton.A presente proposta ¶ediretamente aplic¶avel nos cursos de gradua»c~aoem f¶³sicae qu¶³mica,nas disciplinas de f¶³sicamoderna e f¶³sico-qu¶³mica,respectivamente. Palavras-chave: vis~ao,mec^anicaqu^antica, gradua»c~ao. 1. Introduction (violet), which are low compared to normal electronic excitations, and at the same time very large compared The selective absorption of light by molecules is respon- to the molecular vibrational excitations. There exists a sible for the color of most of everyday objects, color of large number of organic pigment molecules, which have the eyes, the wings of butterflies, birds, vegetables, etc ressonances in the visible, like those present in blood. [1]. As opposed to most atoms and molecules, which The chlorophylls constitute another group of natural have resonances in the ultra-violet and infrared part of pigments where part of of the long chain closes in a the electromagnetic spectrum, the chromatic pigments ring. They have at least two functions: they serve to as well as the photoreceptors, i.e, vision cells, contain absorb radiation energy as light antennae, and they are molecules which have resonances in the visible part of responsible for the primary process of photosynthesis the spectrum. The vision process, for pedagogical pur- in the reaction centre. In common, the organic pig- poses, can be divided into a sequence of steps. After ment molecules, whether natural, whether synthetic, entering the eye, the light is focused on the retina. The have long molecular chains constituted by single and photon is absorbed by the rhodopsin, and converted double carbon-carbon bonds, which are known as con- into a physiological message. In the case of living or- jugated systems [2]. One example is the rhodopsin, ganisms, the optically active molecules contained in the Greek for rose-vision, formerly known as visual purple, photoreceptors consist of a dye combined with a pro- which is a big protein that contains the retinene group tein, discovered by George Wald in the 1930's [2]. The and plays a fundamental role in the absorption of light. molecules absorb light and transfer the absorbed en- They constitute states of large wavelength and low en- ergy to speci¯c sites in order to give rise to the chem- ergy. The energy required to excite the weakly bond pi ical processes. The e®ect of photons on rods contain- electrons, in which the magnitude of the component of ing molecules of rhodopsin is to convert them rapidly the orbital angular momentum of each electron along to molecules of retinal. From the excited state, a ra- the molecular axis is h, is relatively low, lying in the diationless transition to a di®erent con¯guration (iso- visible range. The molecules are viewed as a light an- merization). This isomerization reaction is employed tennae, or oscillators with resonance in the visible. For as the light detection mechanism in the eye. The visi- details about the mechanisms of seeing, the reader is ble photons have energies from 1.6 eV (red) to 3.2 eV referred to Refs. [3-8]. 1E-mail: [email protected]. Copyright by the Sociedade Brasileira de F¶³sica.Printed in Brazil. 2303-2 Santos The absorption of visible radiation by pi electrons is analog to the metalic grid which absorbs in the mi- crowave (Fig. 1). The component of the electric ¯eld paralell to the wires accelerates the free electrons in the metal and they may move all the way up and down along the wires. In the case of the metallic grid, the en- ergy absorbed by the electrons is converted into heat by the Joule e®ect, while in the case of retinal, the energy is converted into a nerve impulse in the optic nerve by the functional units surrounding the rhodopsin molecules. Humans beings cannot produce the retinal molecule in their cells [3-8]. One obtains it from the vitamin A, and the absence of this vitamin, provoke the phenomenon night blind. Figure 2 - Schematic ground-state singlet (So) and ¯rst excited- singlet state (S1) potential energy surfaces for isomerization in rodopsin (adapted from Refs. [4] and [5]). The arrow represents the isomerization process. Modeling medium and large molecules is a di±cult task. For practical purposes, the molecular orbitals can be separated and treated distinctly in two ways, namely, the strongly bond and localised σ orbitals, and the weakly bond and delocalised ¼ orbitals. The dif- Figure 1 - The left side (A) shows the microwave absorption by a metalic grid, where the free electrons in the wires are accelerated ferent symmetry characters of both orbitals, and the by the paralell component of the electric ¯eld. This system is higher susceptibility of the pi electrons to external per- the analog of the visible photon absorption by the weakly bond turbations, such as a photon absorption, is the main non-localised pi-electrons in the retinal molecule (shown in part B), which acts as a light antennae. justi¯cation for this approach. Many of the molecules of interest in organic chemistry have conjugated single In this paper, we apply a very simple model, namely and double carbon-carbon bonds. The alternate char- the particle in a box , also known as free-electron acter of the bonds provides a kind of loosing or delocal- molecular-orbital (FE-MO) method [9], in order to esti- ization to the electrons. Each carbon atom contributes mate the longest absorption wavelength of the retinene with one non-localised electron. molecule. This description is behind the physical basis In the FE-MO method, developed in the 50's by of the very complex mechanism of seeing. The present Kuhn, Bayliss, Platt, and Simpson, the (see for in- informal and rough approach is suitable for either intro- stance Ref. [10] and references therein) pi electrons are ductory quantum mechanics or advanced optics courses. treated within the independent particle model (IPM), i:e:, the electron-electron Coulomb correlations 1/rij are neglected, and the e®ect of the nuclei and the core 1.1. The model electrons is represented by a particle-in-a-box potential, and the sigma orbitals will be disregarded. In general, the theoretical outcomes of this simple model is sur- It is well known that a retinal rod can detect a sin- prisingly in good agreeing with the experimental val- gle photon [3-6]. Vision commences when a photon is ues [10]. absorbed by the visual pigment and converted into elec- trical signals which are processed within the retina and The box, is a potential well of e®ective width sent to the brain. Figure 2 shows schematically the L = Nd, where N is the number of carbon atoms (or pi So !S1 electronic vertical transition (11-cis!11-trans electrons) in the active chain and d is the average dis- isomerization) in rhodopsin [4]. The energy di®erence tance of the C-C bond (»1,4 Aº for conjugated systems) between these two states (60 kcal/mole) corresponds to as shown in Fig. 3. The highest ¯lled and and low- a green photon, which corroborates with the absorption est unoccupied energy levels are pi orbitals, and play a curve of the main pigment in retina, the visual purple fundamental role in the spectroscopy of the conjugated (rhodopsin), which has a maximum around 5000 Aº [6]. systems. The ¯rst steps in vision in the classroom 2303-3 Figure 4 - The energy acquired by a conjugated system after an absorption of a photon, corresponding to the longest-wavelength electronic transition (Eq. (3)), ¢E in eV, as a function of the e®ective number of carbon atoms in the chain. For display pur- poses, ¢E is plotted as a continnum function. The horizontal lines represent the visible part of the electromagnetic spectrum (1.6-3.2 eV). The corresponding photon wavelengths can be cal- culated using ¸(A)º = 12398/E(eV). For the case of the retinal molecule, illustrated in Figs. 3 and 4, if one assumes that the e®ective length L, of the optically active part of the molecule is 12,6 Aº Figure 3 - The in¯nite potential well of width L proportional to or 9 carbon atoms, the transition energy is » 2.5 eV . the number of carbon atoms, where the lowest N/2 energy levels This is in agreement with the S !S electronic tran- are ¯lled by the N pi-electrons.
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