D. Dolphin

Porphyrins XVII. Vapor absorption spectra and redox reactions: Tetraphenylporphins and porphin

Abstract Vapor absorption in the range 800-200 mμ are given for porphin, free-base tetraphenylporphin (H 2 TPP), and the Mg, Cr(III), Mn(III), Fe(III), Co, Ni, Cu, Zn, Cd, Sn(IV), and PbTPP complexes. Electronic bands Q , B , and M characteristic of the ring are found in the regions 540 mμ (18 500 cm −1 ), 405 mμ (24 700 cm −1 ), and 205 mμ (48 800 cm −1 ). The region between the B and the M shows rather unstructured absorption, an order of magnitude less intense than the B region. Based on spectroscopic measurements, the Δ H of vaporization and absolute vapor pressures are calculated. The pressures average to ∼0.05 Torr at 390°C. Vacuum uv spectra of H 2 TPP, NiTPP, CuTPP, and ZnTPP down to 145 mμ are presented which show absorption across the entire range with more intense bands at 185 mμ (54 000 cm −1 ), 164 mμ (61 000 cm −1 ), and 150 mμ (67 000 cm −1 ), and no evidence of Rydberg structure. The vapors of Cr, Mn, Fe, and Sn complexes show unusually different spectra from their solutions. Spectral changes on electrolytic reduction establish that in the Mn, Fe, and Sn cases the vapors are those of the divalent metal. Solution spectra of H 2 TPP, CuTPP, and FeClTPP were studied up to 300°C. Broadening comparable to the vapor spectra is observed. Also the visible bands shift to the red while the Soret band shows no change.

Porphyrins. XXVII. * Spin-orbit coupling and luminescence of Group IV complexes

Luminescence studies are reported on compounds M(IV)X2P: M is Si, Ge, Sn, Pb; X is F, Cl, Br, I, OH, benzoate; P is etioporphyrin or octaethylporphin. (One tetraphenylporphin is reported for comparison.) We find fluorescence yields 2 × 10−1 ≥slant φf ≥slant 2 × 10−4; phosphorescence yields 7 × 10−2 ≥slant φp ≥slant 3 × 10−3; and phosphorescence lifetimes 100u2009msec ≥slant τp ≥slant 1u2009msec. The contrasting vibronic envelopes of phosphorescence for octaethylporphin and tetraphenylporphin derivatives is explained by attributing the former to transitions 3Θ± 1 → 1ΘGND and the latter to 3Θ± 9→ 1ΘGND, where ± 1 and ± 9 are pseudoangular momentum quantum numbers. The spin‐orbit interaction is calculated by the extended Huckel model, and it is found that the ligands have far more effect than the metal, in agreement with the data. However a simple relation between decay rates and spin‐orbit coupling fails quantitatively, and the extended Huckel model appears to exaggerate the contribution of the ligand to the spin‐o...

THE CHEMISTRY OF PORPHYRIN π‐CATIONS*

The redox properties of metalloporphyrins have been studied extensively, and particular attention has been paid to the changes in oxidation states of the metal.’-” In general, the most stable metalloporphyrins are those in which the metal is in the + 2 oxidation state. However, most of the transition metalloporphyrins show a variety of oxidation states. Thus Co(L1) porphyrins can, like the corresponding cobalt-containing vitamin B I Z , be oxidized to the CO(LII),~ or be reduced to the Co(1) complex; and like the Co(1) containing vitamin BI2(BL2 s), the Co(1) porphyrin is nucleophilic and undergoes oxidative addition with alkyl halides to give the corresponding alkyl cobalt porphyrins. Manganese porphyrins show + 2, + 3, and +4 oxidation states,6 and it has recently been shown’ that Pb(I1) porphyrins can be oxidized to Pb(1V) while the more stable Sn(Lv) can be reduced to Sn(LL) ~ y s t e r n s . ~ ~ ~ A particularly striking example of the stability of the divalent complexes is that of the Ag(1L) porphyrins.*O Few examples of divalent silver complexes are known, but the planar tetradentate porphyrin ligand stabilized this unusual oxidation state which can, however, be oxidized to the trivalent Although the redox properties of metalloporphyrins are fascinating and important in their own right, the initial focus and interest in the oxidation states of these systems stems from the redox properties of the cytochromes (which are enzymes containing iron porphyrins, and function catalytically via theFe(I1) Fe(LLL)couple) and the function of hemoglobin (an Fe(I1) porphyrin which, unlike simple Fe(I1) porphyrins, is not oxidized by oxygen to Fe(LI1) but reversibly binds oxygen at the Fe(LL) oxidation level) as well as from the incompletely determined role of valence changes of iron in the peroxidase/catalase class of heme enzymes. The central role of the iron in these naturally occurring systems and the considerable efforts that have been expended on elucidating the roles of metals in these and other metalloporphyrins resulted in the widely held assumption that the macrocyclic porphyrin ligand serves merely to modify the redox potentials of the metals, and to act as a convenient bridge between the metal and the protein-an assumption which is far from true.

Porphyrins: XVI. Vapor absorption spectra and redox reactions: Octalkylporphins

Abstract Vapor absorption spectra in the range 800-200 mμ are given for free base octaethylporphin (H 2 OEP), Mg etioporphyrin-I, VOOEP, CoOEP, NiOEP, ZnOEP, PdOEP, MnClOEP, MnAcOEP, and FeClOEP. Electronic bands Q , B , N , and M characteristic of the ring are found in the regions 570 mμ (17 500 cm −1 ), 390 mμ (25 500 cm −1 ), 325 mμ (30 800 cm −1 ), and 213 mμ (47 000 cm −1 ), respectively. All compounds show at least one “extra” band between N and M of variable intensity and energy. Based on spectroscopic measurements, we found all compounds show p ∼ 0.1 Torr at 330°C and heat of vaporization, Δ H ∼ −23 kcal/more. Vacuum UV spectra of H 2 OEP and CuOEP down to 145 mμ are presented which show absorption across the entire range with more intense bands at 194 mμ (51 500 cm −1 ), 162 mμ (62 000 cm −1 ), and 150 mμ (75 000 cm −1 ) and no evidence of Rydberg structure. The vapors of the Mn and Fe complexes show unusually different spectra from their solutions. Similar spectra to the vapor are obtained when solutions of MnAcOEP and FeClOEP are subject to electrolytic reduction, suggesting that the vapor species are the reduced forms. Spectra of H 2 OEP, CuOEP, and MnClOEP in silicone oil at T ∼ 300°C are given. Broadening comparable to the vapor spectra is observed. Also the visible bands shift to the red while the Soret band shows no change.

Energy Transfer between Covalently Linked Metal Porphyrins

Abstract A zinc porphyrin is linked covalently to either a copper or a cobalt porphyrin by a peptide linkage containing either a p-phenylene or an ethylene group. These double porphyrins were prepared by reacting the acid chloride of 2-carboxyl-12,17-diethyl-3,7,8,13,18-pentamethyl-porphyrin with either p-phenylenediamine or ethylenediamine and subsequent reaction with a second acid chloride porphyrin. In rigid solutions of 2-methyltetrahydrofuran at 77° K intramolecular energy transfer from the zinc triplet state to that of the copper tripdoublet is manifested in a shortening of the zinc porphyrin triplet state lifetime from 46 ± 1 to 34 ± 1 msec in the ethylene bridged dimers. In the cobalt-zinc ethylene bridged dimers triplet energy transfer from the zinc chromophore is more efficient as manifested in the complete quenching of the zinc phosphorescence. Calculations suggest that in the copper-zinc case energy transfer proceeds by a triplet-triplet exchange mechanism whereas in the cobalt-zinc case, quenching of the zinc triplet proceeds by the formation of a charge transfer complex. There is no evidence for triplet-triplet energy transfer in the phenylene bridged double porphyrins. Singlet-singlet energy transfer is not observed in any of the above systems.

Low temperature chiral nematic liquid crystals derived from β‐methylbutylaniline

The synthesis and some basic physical properties of three new liquid crystals are reported. Each of these compounds has been synthesized in both pure chiral and racemic versions. It is demonstrated that a mixture of the chiral and racemic versions of a compound produces a chiral nematic liquid crystal which behaves as a one component system in which the torsion of the helical structure varies linearly with composition, while other thermodynamic properties are independent of composition.

ESR STUDIES OF PORPHYRIN π-CATIONS: THE 2A1u and 2A2u STATES*

Recent work has established that metalloporphyrins can be reversibly oxidized and that electrons can be abstracted from theorganicmoiety, fromthechelatedmetal, or from both.-7 ESR spectra of the radicals of zinc-meso-tetraphenylporphyrin (ZnTPP + .) and of magnesium octaethylporphyrin (MgOEP+ .) provide clear evidence of electron removal from the porphyrin r system but show marked differences in spin density distrib~tion.~ Comparison6 of the optical spectra of several r cations indicates that they fit into two classes represented by ZnTPP+ and MgOEP+ . The latter shows a major absorption band near 700 nm with a high energy shoulder, whereas the former exhibits several overlapping peaks in the region of 5W700 nm. We propose to review here the evidence for oxidation of the porphyrin n system and to discuss the correlation between the experimental data and molecular orbital calculations which have led us to suggest the existence of two distinct ground states. These are typified by the radicals MgTPP+ or ZnTPP+ (AzU) and MgOEP+ or ZnOEP+ (2AI,), which show remarkably different properties.

OXIDATION OF FERRIC PORPHYRINS

Characterization of heme oxidation products falls naturally into two categories : species resulting from electron transfer, and in vivo and in vitro heme degradation products formed by oxidative addition or cleavage. In the former, we propose to include the intermediates appearing during the catalytic cycles of catalase (Cat), horseradish peroxidase (HRP), and cytochrome c peroxidase; the latter class consists of the bile pigments formed by catabolism of heme proteins as well as chemical intermediates such as oxophlorin and verdoheme. Salient features of the catalytic cycles of the enzymes have been discussed previously; briefly, these are the appearance of a green species (compound I) when the enzyme is treated with HzOz or an organic hydroperoxide. The enzymatically active compound I is a two-electron oxidation product of the ferriheme protein and upon reduction forms first a one-electron oxidation product (Compound 11) which may be reduced further to the parent ferriheme. Compound I of cytochrome c peroxidase differs from these general observations inasmuch as its optical spectrum is quite similar to Compounds I1 of HRP and Cat and it displays an EPR2 signal at g 2.0. Although no EPR signal has been observed for HRP I or HRP 11, recent studies on the methyl hydroperoxide complex I1 of catalase show the existence of a complicated EPR spectrum. Other physical properties of interest are magnetic susceptibility measurements4 on Cat I and HRP I, which show that these species have an effective magnetic moment consistent with three unpaired electrons. The Mossbauer spectras- of Compounds I and I1 of HRP and Cat have similar isomer shifts which, in turn, differ from those of the parent enzymes. A number of suggestions have been advanced concerning the chemical constitution, source of oxidizing equivalents, and electronic formulation of Compounds I

The Role of Distortion in the Lysozyme Mechanism

One of the most fascinating features of the mechanism proposed by Phillips and co-workers (Blake et aI., 1967a, b; Phillips, 1966) for the cleavage of oligosaccharides by lysozyme is the distortion of the saccharide bound in subsite D into a half-chair conformation. This mechanism is based upon X-ray crystallographic studies of the non-productive complex of lysozyme with chitotriose, GlcNAc-,B(l -+ 4)-GlcNAc-,B(1 -+ 4)GlcNAc, (bound in subsites A, B, C) and model building. The involvement of substrate distortion in the proposed mechanism is supported by several studies (Chipman et aI., 1967; Chipman and Sharon, 1969; Chipman, 1971; Johnson et aI. , 1968; Rupley and Gates, 1967) which estimate that the contribution to the free energy of binding of oligosaccharides from the saccharide bound in subsite D is unfavorable by 3-6 kcal mole-I. This is exemplified by a comparison of the binding constants for GlcNAc-,B(1 -+ 4)-MurNAc-,B(1 -+ 4)GlcNAc (KA = 2.8 X 105 M-I) and GlcNAc-,B(1 -+ 4)MurNAc-,B(I-+ 4)-GlcNAc-,B(I-+ 4)-MurNAc (KA = 2.1 X 103 ~I) (Chipman and Sharon, 1967). With an estimate of the favorable interactions at subsite D, the energy available for distortion is 6-12 kcal mole-I (Johnson et aI., 1968) , which is approximately that required for the distortion of cyclohexanes from the chair to the half-chair conformation (Eliel et aI., 1965). Since the hydrolysis of natural oligosaccharides by lysozyme is complicated by non-productive binding and tmns-glycosylation, the study of any one feature of the mechanism is difficult without the use of synthetic substrates (Rand-Meir et aI., 1969). In agreement with the mechanism proposed by Phillips and co-workers, Dahlquist et al. (1968, 1969) have concluded that considerable carbonium ion character is involved in the hydrolysis of the

THE DETECTION OF SUBSTRATE DISTORTION BY LYSOZYME: AN APPLICATION OF NMR TO THE STUDY OF ENZYME SUBSTRATE REACTIONS *

Fourier transform (FT) techniques have reduced the time required for the acquisition of an nmr spectrum to roughly the inverse of the resolution to be achieved in the final spectrum.l While this fact has focused attention on the possibility of using nmr to follow reaction kinetics, most applications of FT nmr have capitalized on this time saving to achieve a substantial improvement in sensitivity over continuous wave (CW) methods given the same amount of time. Nevertheless, the combination of the structural information available from nmr experiments with the time scale of rapid mixing experiments for the study of reacting systems has a great potential, especially in the observation of intermediates in chemical and biochemical reactions.? The great advantage of nmr over other detection methods (e.g., optical) commonly used in transient kinetic experiments is the type of detailed information available about each species whose appearance and/or disappearance is being followed during the reaction. The FT nmr techniques required for the study of reacting systems are described in this paper together with the application of these techniques to the study of the lysozyme-catalyzed cleavage of the tetrasaccharide (NAGNAM) ?