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Spectroscopy of Porphyrins

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Spectroscopy of Porphyrins BORIS F. KIM and JOSEPH BOHANDY SPECTROSCOPY OF PORPHYRINS Porphyrins are an important class of compounds that are of interest in molecular biology because of the important roles they play in vital biochemical systems such as biochemical energy conversion in animals, oxygen transport in blood, and photosynthetic energy conversion in plants. We are studying the physical properties of the energy states of porphyrins using the techniques of ex­ perimental and theoretical spectroscopy with the aim of contributing to a basic understanding of their biochemical behavior. INTRODUCTION Metalloporphin Porphyrins are a class of complex organic chemical compounds found in such diverse places as crude oil, plants, and human beings. They are, in most cases, tailored to carry out vital chemical transformations in intricate biochemical or biophysical systems. They are the key constituents of chlorophyll in plants and of hemoglobin in animals. Without them, life would y be impossible. t Free base porphin These molecules display a wide range of chemical and physical properties that depend on the structural details of the particular porphyrin molecule. All por­ ~x phyrins are vividly colored and absorb light in the visible and ultraviolet regions of the spectrum. Some exhibit luminescence, paramagnetism, photoconduc­ tion, or semiconduction. Spme are photosensitizers Wavelength (nanometers) or catalysts. Scientists from several disciplines have been interested in unraveling the principles that cause Fig. 1-The chemical structures for the two forms of por· this diversity of properties. phin are shown on the left. A carbon atom and a hydrogen The simplest compound of all porphyrins is por­ atom are understood to be at each apex not attached to a nitrogen atom. Metalloporphin has a metal atom (desig· phin. This aromatic molecule exists in two basic con­ nated by M) complexed at the center of the molecule, while figurations (Fig. 1), depending upon the nature of the free base porphin has two hydrogen atoms in the center. material at its center. When a metal ion (such as iron, The general class of porphyrins are obtained from porphin magnesium, zinc, or cobalt) is attached at the center, by replacing the peripheral hydrogen atoms with other the compound is said to be a "metalloporphin." chemical groups. The optical absorption spectra shown characterize these compounds when they are in solution at When two hydrogen atoms are attached, the com­ room temperature. 0 0 in metalloporphin is due to a pure pound is termed "free base porphin." The periphery electronic transition (no molecular vibrations) between the of these structures contains hydrogen atoms that can electronic ground state and the first electronic excited state; 0 is due to the same electronic transition but, in ad· readily be displaced by one or more molecular 1 dition, molecular vibrations are involved. This band is groups, thus forming the general class of porphyrins. called a vibronic band. The subscripts 0 and 1 for free base In some cases, these peripheral groups playa role in porphin carry a similar interpretation. The x and y sub· the particular properties of the porphyrin. In other scripts refer to the orientation (polarization) of the electric cases, they serve to attach the porphyrin molecule to vector of the absorbed light with respect to the axes shown a chemically inert substrate such as a large protein on the chemical structure diagram for free base porphin. molecule. In some highly sophisticated biochemical systems, the substrate places the chemically active phyrins in molecular biology. Figure 2b shows sche­ porphyrin into a highly specific spatial position to matically the structure of hemoglobin, which trans­ permit its interaction with other chemically active ports oxygen in the blood. There are four porphyrin species. groups attached to four protein chains. Other impor­ The metal ion in metalloporphyrins has a great in­ tant heme-containing proteins are myoglobin, which fluence on the properties of the particular porphyrin stores oxygen in muscle, and the cytochromes, which molecule. The iron porphyrin shown in Fig. 2a is perform many cellular functions involving oxygen called "heme" and is one of the most important por- metabolism and the production of stored biochemi- Vo lume 2, N umber 3, 1981 153 Chlorophyll a Vitamin B1 2 (a) (b) Fig. 3-The chemical structures. of two important mol· ecules that contain a metalloporphyrin, chlorophyll a, and vitamin 812. tronic charge in the molecule and by its electronic energy states. The vibrational modes of the molecule and their corresponding vibrational energy states comprise the vibrational structure. Collectively, these structures are referred to as "vibronic structure." The basis for the chemical action of porphyrins is Fig. 2-(a) The chemical structure of heme (iron protopor· phyrin); (b) a representation of the entire hemoglobin founded in their quantum structure. Although a de­ molecule, consisting of four protein chains denoted by CX1, tailed and valid theory that relates structure to activi­ CX 2 , {31, and {32 . The four planar structures represent heme ty does not yet exist, in a few cases the relationship of molecules to which oxygen binds in the lungs and which structure to the chemical role of porphyrins is under­ release oxygen in the capillaries. stood, as, for example, in recent reports of photode­ struction of malignant tissue by a hematoporphyrin cal energy from foodstuffs. The heme group is usual­ derivative. I Its role is understood in terms of its ly attached to a large protein molecule by a bond be­ quantum structure, which permits absorption of light tween the iron atom and a nitrogen atom of a histi­ in the red region of the spectrum of the molecule and dine amino acid residue of the protein. The chemical subsequent transfer of the excitation energy to a activity is due to the heme group. Two other impor­ triplet oxygen molecule. This, in turn, forms excited tant porphyrin derivatives, as seen in Fig. 3, are mag­ singlet oxygen molecules that attack and destroy the nesium-containing chlorophyll and cobalt-containing malignant tissue. In another example, two hydrogen vitamin B 12 • atoms on the periphery of the porphyrin structure in We are interested in the quantum structure of por­ chlorophyll change the electronic structure of the phyrins. Atoms and molecules are comprised of molecule sufficiently so that it absorbs more light in bound particles, which can be described by standing the redl green region of the spectrum, causing the wave functions using the theoretical methodology molecule to perform photosynthesis more efficiently. called quantum mechanics. The conditions to which The overall objective of the porphyrin spectros­ the standing wave modes correspond are called copy project at APL is to advance the state of knowl­ "states." The quantum structure of molecules in edge of the electronic and vibrational energy struc­ solids or liquids may be considered to consist of their ture of porphyrins and to study the effects of chemi­ electronic and vibrational structures. The electronic cal perturbations on this structure. Implicit in this structure is determined by the distribution ofelec- objective is a desire to contribute to the understand- 154 Johns H opkins A PL Technical Digest ing of the relationship between structure and chemi­ cal and biological function. -----ej +En Optical and microwave spectroscopies are the ex­ ~:~ited state -----el + En perimental methods that are used to probe the quan­ r-------....:...-~---- En tum structure of porphyrins. In optical spectroscopy, changes in quantum states are detected when the molecule absorbs energy to achieve a higher excited energy state or when it loses energy as it decays into a Second I-----e i' + E2 lower energy state. The energy is related to the fre­ excited state --'!-;?-----el + E2 quency of the radiation by the Einstein relation -+t--------'---=------ E2 E = hv, where h is Planck's constant and v is the frequency. Figure 4 is a diagram of typical energy levels. For porphyrins, the light absorption at Qo (see Fig. 1) corresponds to the El energy level in Fig. 4. In ab­ sorption spectra, one observes transitions from the ground state (Eo) to the various excited vibronic states of the molecule (El + e1 , El + e2 , etc.). Thus, the absorption spectra reveal excited electronic energy levels (El' E 2 , ••• ) and associated vibrational energy levels (e I , e2 , ••• ). The molecule will eventually Absorpti on Em ission (lumi nescence) release the excitation energy and return to its elec­ tronic ground state (Eo). When the molecules are in a solid or liquid phase, which is the case in these studies, an excited molecule (in E 2 , for example) will ej + Eo decay to the first (or lowest) excited state (E1 ), with Ground the energy loss being released in the form of heat. state I el + Eo The subsequent decay from this state to the ground electronic state is often accompanied by the emission of electromagnetic radiation, which is referred to as Fig. 4-Absorption and emission processes. The E's are "luminescence." The transition from the lowest ex­ electronic energy levels, typically greater than 16,000 cm- 1 cited state, E 1 , to the electronic ground state, Eo, for porphyrins. The e's are vibrational energy levels may terminate on a ground-state vibrational level as superimposed on the E's. For porphins (porphyrins with no shown in Fig. 4. Thus, it follows that the side groups on their periphery) the e's are less than 3600 cm -1. For metalloporphyrins, transitions to E1 correspond luminescence spectra can yield vibrational energy to 00 while transitions to E2 correspond to the Soret band levels in the ground electronic state. in Fig.1. Electron spin resonance is a microwave spectro­ scopic technique that is used when the molecular species in its ground state contains a magnetic mo­ phyrins have only two. The structure of the spectra in ment.
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