Diverse reactivities of picket fence porphyrin atropisomers in

Feb 19, 1992 - media, (2) the mode of catalysis in enzymatic porphyrin meta- lation,11-13 (3) ... alation reactivities in homogeneous solution has bee...
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5106

J . Phys. Chem. 1992,96,5106-51 14

Diverse Reactivities of Picket Fence Porphyrin Atropisomers in Organized Media: A Probe of Molecular Location and Orlentatiodp2 David C. Barber, Tamar E. Woodhouse, and David C. Whitten* Department of Chemistry, University of Rochester, Rochester, New York I4627 (Received: February 19,1992)

The reactivity of picket fence porphyrins (meso-tetrakis(o-(alkylcarbonyl)aminophenyl)porphyrins) toward metalation with Cu2+or ZnZ+shows an extremely wide rate range in aqueous anionic surfactant solutions (sodium dodecyl sulfate (SDS) micelles and Aerosol OT (AOT) reversed micelles, a factor of 35000) compared to homogeneous solution (N,N-dimethylformamide, range = 156). The rates vary systematically as a function of the chain length of the N-alkyl substituent and, more significantly, the atropisomer configuration. A correspondingbroad range of apparent porphyrin basicities (core diprotonation constants) spanning 7.0 pK, units is observed with the different sets of atropisomers in aqueous SDS, again under conditionswhere relatively little variation is observed in homogeneousmedia. The correlation of basicities with metalation rates suggests that anionic interfacial catalysis plays an important role in both processes; the spread in rate and equilibrium constants for different porphyrin atropisomers suggests that topologially controlled orientation of the porphyrin macrocycle with respect to the interface governs reactivity. Specifically, the observed behavior in SDS and AOT is consistent with two separate equilibrating interfacial orientations for the 4,O atropisomer whose relative proportions vary with side chain length, and a more nearly perpendicular orientation for short chain cis-2,2 and trans-2,2 derivatives. These orientations lead to an inverted reactivity order for the short chain compounds in organized media (i.e. 4,O < 3,l < cis-2,2 < trans-2,2) as compared to those in homogeneoussolution. Spectra intermediacy of the porphyrin monoacid (H,PC) during acid titration occurs exclusively for the short chain 4,O derivatives, for which an “inverted” interfacial orientation (hydrophobic core solubilization) is proposed; monoacid “stabilization”is attributed to prerequisite porphyrin migration/reorientation into a more anionic microenvironment prior to diprotonation. Catalytic metalation and elevated interfacial basicities strongly support interfacial solubilization for all of the tetraamidophenyl porphyrins studied such that the porphyrin core is located in close proximity to surfactant head groups while solubilization in sites more remote from the anionic head groups is proposed for hydrophobic porphyrins such as tetraphenylporphyrin (TPP).

Introduction The details of molecular solubilization at interfaces, essential to an understanding of interfacial phenomena, have been the subject of numerous investigation^.^ Questions addressed include the modification of reaction rates and pathways4J compared to the reactivity in homogeneous solution, as well as probes of the interfacial microenvironment and solubilization site: Biological studies have focused on environment-sensitiveprobes of membrane structure and dynamics,’ the control of molecular conformation and binding properties by the presence of hydrophilic and hydrophobic moieties as found in enzymes, interfacial electron transfer,s and factors controlling molecular solubilization in aqueous or nonaqueous environments. However, elucidation of the specific microenvironment experienced by a solute in an orphotoganized assembly has often proven d i f f i ~ u l t . Although ~ physical properties of solute molecules have been employed to probe assembly microenvironments, their precise location within the assembly is usually uncertain. Relatively few studies clearly demonstrate the direct correlation between interfacial reactivity, solubilization site, solute orientation, and molecular topology which are essential to a mechanistic understanding of reactivity in organized assemblies.I0 The behavior of porphyrins in model membrane systems is of interest as it relates to (1) modification of reactivity in organized media, (2) the mode of catalysis in enzymatic porphyrin metalation,’ (3) the role of microenvironment in influencing reactivities for membrane bound p~rphyrins,’~J~ and (4) the activity and fate of porphyrin photosensitizers in biological systems. Studies of porphyrins in organized assemblies have attempted to address a variety of biologically relevant processes including oxygen binding to hemoglobin,I6 mitochondrial electron transport,” and photosynthetic energy transferIs where careful attention is paid to the precise location and orientation of the porphyrin core. Electron-transfer and/or electrooptical studies of vesicle or m o n ~ l a y e r ’incorporated ~-~~ porphyrins have been used to assess the precise location and orientation, respectively, of the porphyrin core within the assembly while studies in micellar solution have probed interfacial and counterion effects on metal ion exchange and ligand binding to metall~porphyrins.~~-~~ However, in the majority of studies of porphyrins incorporated into surfactant 0022-365419212096-5 106$03.00/0

CHART I: Picket Fence Porphyrin Structure and Atropisomer Representations R

R

O
4,O cis-2,2 > 3,l) is incongruent with their order of metalation reactivities (4,O > 3,l > cis-2,2 > trans-2,2). The nearly opposite basicity trend and the displaced position of the 4,O isomer in the order of basicities is noted only for the C6 side chain length. In contrast to the

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5110 The Journal of Physical Chemistry, Vol. 96, No. 12, 1992

’

0

’

2

1

o.o+ -5.0

‘

I

-4.0

8

’

-3.0

-2.0

’

9

-1.0

I

’

0.0

1.0

log [k(Cu)l Figure 1. Basicity-reactivity correlation. The curve is fit through the data in organized media (El); data for the for TAc isomers in 9:l DMF/water (m) are plotted for comparison.

behavior of 4,O THex at 60 mM SDS (pK3K4”P = 7.9), the apparent basicity of 4,O THex in the premicellar state (at 6 mM SDS, pK3K4”PPC 4.4) where the porphyrin exhibits J-aggregate absorptions3is comparable to that noted for the least basic PFPs in micellar SDS (60 mM; Le., for TPP pK3K4”P = 3.84; for 4,O TPiv pK3K4aPP= 3.22; see Table V).

Discussion The wide range of reactivities for the porphyrins studied in SDS micelles and AOT reversed micelles as compared to homogeneous solution demonstrates both topological control of reactivity at an organized interface and differentiation of reactivities based on molecular hydrophobicities. In general, the greatly elevated metalation reactivities and enhanced basicities noted for all PFPs investigated in anionic surfactant assemblies, as compared to that noted for less hydrophilic derivative such as TPP, meso-tetrapentylporphyrin and the TAm isomers, suggest interfacial solubilization of the PFPs. However a closer inspection of the rates for different atropisomers of a given side chain suggests that the average solubilization sites may be quite different as a function of atropisomer identity. Reactivity and Topological Control. The overall effect of apparent enhanced porphyrin basicity and enhanced metalation at the anionic interfaces investigated is probably attributable to two factors. One of these is an enhanced concentration of cationic reactants, H+ and Cu2+.X (X = solvent), near the interface, while the second is stabilization of products (protonated porphyrin in the case of enhanced basicity) or intermediates/transitionstates involved in the metalation. It is not easy to make a separation of the two effects; however the overall shift in pKas observed with the most affected porphyrins compares with those ohserved in other studies.62 For the rate process, a slightly better estimation of the separation of the two contributing components may be made since we can make a reasonable approximation of the local Cu2+ concentration. For the SDS micelles, we estimate that the [Cu2+]intf can be no more than 100 times that in homogeneous solution; a similar estimate of [Cu2+Iintf can be made for reversed micelles, as described in the Experimental Section. This reduces the kobsvalues to microscopic metalation rate constants (kmiao), as shown in Tables I and 111. These values, in several cases, are several orders of magnitude higher than the corresponding rate constants for reaction in homogeneous solution (e.g., in 9: 1 DMF/water, k t 2 , =~ 7.5 ~ ~X ~lO-’and ke2,2TAc = 1.0 X la-4 M-’ 2.5 X and k t 2 , 2 = ~ ~2.1 ~ X lo-’ M-’ S-’; in DMF k,2,2TAc s - ’ ) . ~ ~Although metalation is a relatively slow process compared with the dynamics of micelle structural rearrangement,the rather striking variation of microscopic reaction rate constants suggests that the “topology” of the different porphyrin atropisomers can

Barber et al. result in drastically different “average solubilization sites” with respect to the charged interface which are most likely responsible for both the spread in basicity and metalation reactivity. Basicity-Reactivity Correlation. A strong correlation of basicities and reactivities in organized media form a Bronsted plot (see Figure 1). The data for homogeneous solution fall off-line compared with the SDS micelle data; the deviation for the data in homogeneous solution may be attributed to a reduced activity of hydrogen ions (basicity deviation) and the fact that the metalating reagent differs (metalation deviation) in 9:l DMF/water relative to SDS micellar solution. Limited studies of Cu(I1) incorporation in 2-propanol/water indicate that the initial portion of the reaction in this solvent, containing a weakly binding solvent ligand (Le., 2-propanol vs DMF), is -5 times more rapid than that noted in 9:l DMF/water; these data may more closely a p proximate the metalating complex in the anionic assemblies ( C U ( H ~ O )vs ~ ~Cu(DMF),(H20),). + Second-order Cu(I1) incorporation rate constants in organized media are up to 540 000-fold higher than those noted in homogeneous solution. After correcting for the concentrating effect of the micelle or reversed micelle assembly, we note that the microscopic metalation rate constants greatly exceed those observed in homogeneous solution, in most cases, by factors of up to 12000 (for trans-2,2 TAc in reversed micelles vs 9:l DMF/water). Although there is a relatively poorer basicity-reactivity correlation if one includes the homogeneous solution data, we still see very large factors of acceleration in organized media over homogeneous solution, suggesting catalysis at the anionic interfaces, even considering the maximum interfacial [Cu(II)] .63 From our orientational hypotheses (see discussion below), we suggest that these interfacial accelerations correlate with the proximity of the porphyrin core to the anionic surfactant head groups quite independently of facial steric effects; this results in the trans-2,2 and cis-2,2 isomer of TAc being much more reactive than the 4,O atropisomer in organized media opposite the trend noted in homogeneous solution.5* Porphyrin SohMizatiamSites m Anionic Surfactant Assemblies. Incorporation of the PFPs into the micelle in aqueous SDS is suggested since the porphyrins are insoluble in water and have a strong tendency to self-aggregate in simple aqueous solution. In aqueous SDS (60 mM), they exhibit sharp Soret absorption similar to that noted in homogeneous organic solution, indicative of monomeric micellar solubilization. UV-visible spectral studies of the THex atropisomers in aqueous SDS indicate that the porphyrins are strongly aggregated below the critical micelle concentration (cmc) but exhibit monomeric solubilization at and above the C ~ C Since . ~ ~the micellar solutions are composed of an aqueous phase and micellar pseudophase in which there may be a gradient of solubilization sites, porphyrins containing exclusively nonpolar functionalities may be solubilized in less polar regions. Thus TPP, which is 50 times more basic toward core diprotonation than the TAc isomers in homogeneous solution,s8 is signifi*mtly less basic than the PFPs in aqueous SDS (seeTable V). Accordingly, the least reactive PFP toward Cu(I1) incorporation in SDS micelles, 4,O TPiv, is of comparable basicity to TPP in micellar solution. It is reasonable to conclude that the PFPs are significantly more hydrophilic than TPP or related compounds due to the polar amide functionalities resulting in more polar “interfacial” solubilization and elevated metalation and diprotonation reactivities. Thus in the SDS micelle, porphyrins may equilibrate between relatively hydrophobic and hydrophilic solubilization sites; the metalation reaction must occur at the surfactant/water interface since Cu(I1) is soluble in the aqueous phase while the porphyrins are restricted to the micelle. In the case of reversed micelles, the porphyrins may equilibrate between the surfactant interface and the bulk heptane phase as indicated in (1) and (2); since the PFPs are insoluble in water, they should be excluded from the water pool. By varying the Pbulk + Intf e Plntf PIntf+ M2+

+

MP

+ 2H+

(1)

(2)

reversed micelle concentration at a constant AOT/Cu(II)/water

Picket Fence Porphyrin Atropisomers ratio, the interfacial concentration may be varied to determine if the porphyrin resides entirely at the interface (same Cu(I1) incorporation rate observed) or resides partially in the bulk heptane phase (Cu(I1) incorporation rate increases with increasing concentration of interface). Strong interfacial binding was noted for the hydrophilic 4,O and trans-2,2 isomers of TAc, since their metalation rates were identical over a 3-fold range of reversed micelle concentrations. In contrast to the behavior of the TAc isomers, the solubility of the THex isomers in heptane solution is - 5 and 80 times greater for cis-2,2 and trans-2,2 THex, respectively, as compared to 4,O and 3,l THex, demonstrating the relative hydrophobicities of the cis-2,2 and trans-2,2 isomers. Accordingly, the reactivities of these two isomers toward Cu( 11) is much reduced compared to that of 4,O and 3,l THex (see Table I), suggesting that these more hydrophobic porphyrins may partition into the heptane phase. The extremely sluggish reactivities for TPP and meso-tetrapentylporphyrinsuggest that they partition strongly away from the interface. It is unclear whether the unreactivity of 4,O and trans-2,2 TPiv can be attributed to strong partitioning into the bulk phase or “steric exclusion” of these compounds from the interface due to their bulky pivaloyl side chains. An analogous situation is noted for the 4,O isomer of cI&6 P w s in microemulsions, where they are 26 times more reactive than TPP,43although 50-250-fold less reactive than TPP in homogeneous ~ o l u t i o n . ~This ~ - ~suggests interfacial solubilization for the PFPs and solubilization of TPP in the hydrocarbon phase. Interfacial Orientations (Porphyrin CoreHead Group h o x imities). The reactivity trends of PFPs toward metal ion incorporation both in organized assemblies and toward H+ (apparent basicities) in SDS micelles are consistent with different interfacial orientations for the porphyrin core as a function of solute topology. The normal order of metalation reactivities in homogeneous solution (4,O > 3,l > cis-2,2 > trans-2,2), which occupy a fairly narrow reactivity range (156-fold variation for Cu2+incorporation in DMF)51is reversed for short chain PFPs (the TAc and TPro isomers) in both micelles and reversed micelles (see Tables I and 111). This inverted order appears to be independent of the identity of the metal ion studied, since the same inversion is noted for Znz+ incorporation into the tetraacetyl isomers in SDS micelles (see Table IV) and suggests that the variations in reactivity order are related to solute topology and/or the interactions of the polar side chains with the hydrophilic/hydrophobic interface. While trans-2,2 and cis-2,2 TAc and TPro are much more reactive than all other PFPs and exhibit accelerated metalation in anionic surfactant assemblies, reactivities of the short chain PFPs, 4,O TAc and TPro, are among the slowest of the PFPs studied in organized media (see Tables I and 111). For the 4,O isomer, reactivities increase systematically with increasing side chain length, leveling off at longer chain lengths. The behavior of the 4,O isomer suggests a chain length dependent equilibrium, as shown in Figure 2 depicting limiting orientations for short (orientation A) and long chain (orientation B) PFPs. Thus 4,O TAc and 4,O TPro may adopt an orientation in which the hydrophilic acetamide or propionamide side chains may bind to the hydrophilic interface with the porphyrin core at a somewhat more remote site from the interface; in this orientation, approach of the metal ion is strongly hindered by the four side chains. Metalation rates from the substituted face of 4,O PFPs in DMF are greatly reduced, presumably due to direct steric effects?8 and would be expected to persist at these anionic interfaces. The faceexposed orientation, favored when side chain hydrophobicity increases in the 4,O isomer (Le., orientation B), permits easy access of metal ion to the porphyrin core and provides accessibility of surfactant bead groups and a more polar microenvironment which may promote metalation. While the cis-2,2 and trans-2,2 isomers are the least reactive in homogeneous solution due to steric hindrance from side chains on both sides of the porphyrin core,58 cis-2.2 and trans-2,2 TAc and TPro are the most reactive toward metalation in organized assemblies; they are much more reactive than any 4,O PFP (see Tables I and 111) and exhibit accelerated reactivity relative to 9:l DMF/water of up to 540000 in AOT

The Journal of Physical Chemistry, Vol. 96, No. 12, 1992 5111

A Organic Phase

YO Aqueous Phase

Orientation

YO

Orientation B

A KB/A

B

Organic Phase

Yo

Aqueous Phase

YO

Orientation C Figure 2. Interfacial orientations of the PFPs: (A) orientations for the 4,O atropisomer; (B) orientation for short chain cis- and trans-2,2 PF’Ps. reversed micelles and 41 000 in SDS micelles for trans-2,2 TAc. Their elevated reactivities, considering the greater steric hindrance to approach of a metal ion to the porphyrin core, is consistent with a perpendicular interfacial orientation (orientation C; see Figure 1) from which metal ions can be efficiently delivered from a surfactanthead group on either side of the porphyrin core. Anionic catalysis may also contribute to the large observed accelerations, as discussed later in this paper. Calculation of tbe Interfacial Equilibrium Constant (KBA) for 4,O Isomers. Since the metalation rate constants from the two faces of the 4,O isomer differ such that metalation from the hindered face is negligible in homogeneous solution (Le., the calculated metalation rate constant for 4,O TAc from its hindered face in DMF is 650 times lower than that from the unhindered face and 3500 times lower for 4,O THA) and noting that core reactivities are identical for 4,O TAc and THex in DMF and 9:1 D M F / ~ a t e r , ~one * can estimate the interfacial equilibrium constant KBIAfor the 4,O isomers (see Figure 2).65 Calculated values for KBA indicate that orientation B is populated to a leaser extent for 4,d TAc in SDS micelles (0.1%) than in the AOT reversed micelle at w = 10 (1.3%). The same is true for 4,O TFYo and 4,O TBu (0.2 and 0.3% in SDS and 5.5 and 15%in AOT, respectively). For longer chain lengths in AOT reversed micelles, it appears that at intermediate chain lengths both orientations are equally pop ulated and that orientation B is nearly entirely populated for the longest chain lengths (4,O THex (50% B), 4,O THept (54% B), 4,O TOct (73% B), 4,O TDec and 4,O THA (-100% B)). Interestingly, for PFPs with branched side chains, this calculation suggests that orientation A is strongly favored in both media (in SDS, 4,O TlBu (0.42% B), 4,O TPiv (0.04% B); in AOT, 4,O TPiv (0.2% B)). Monoacid stabilization for compounds proposed to adopt orientation A in SDS micelles gives striking evidence in support of this orientational equilibrium for the 4,O atropisomer, as discussed below. Topologicrl Orientation and Interfacial Basicities in Surfactant Assentbks. “Apparent porphyrin basicities”, pKSdnPPvalues, are reported for the PFPs in aqueous SDS since the interfacial pH is not known. A great increase in basicity is observed upon incorporation of the PFPs into the which may be attributed to two factors: (1) a high interfacial pH as has been noted from studies of other acid-base indicators in organized media6“’ and (2) electrostatic stabilization of the porphyrin diacid by a strong anionic field in the vicinity of the porphyrin head groups. Hence, not only should interfacial solubilization play an important role in the enhancement of porphyrin basicities but so also should

5112 The Journal of Physical Chemistry, Vol. 96, No. 12, 1992

porphyrin orientation and the precise proximity attainable between the porphyrin core and surfactant head groups at the interface which is so important to the metalation process. While it is clear that the neutral free base PFPs (H2Ps, see Scheme I) are solubilized in the micelle since they are insoluble in water, UV-visible absorption spectra of the PFP diacids (Hp2') measured in water are shifted relative to those in aqueous SDS, indicating that they remain associated with the micelle in aqueous SDS following diprotonation. Moreover, the fact that the TAc isomers are 1.5-8.5 pK, units more basic in SDS micelles than in homogeneous solution (pK3K4= 1.7 for the four isomers in 9: 1 DMF/water)58 indicates that they must reside at the micelle interface. Other studies indicate that three or more permanent charges are required to solubilize other tetraarylporphyrins in aqueous s o l ~ t i o n . ~ ~ ~ ~ ~ ~ ~ ~ Apparent basicities for the PFPs span a range of 7.0 pK, units in SDS micelles; this wide range can be attributed to topological discrimination at the hydrophilic/hydrophobic interface. Presuming that the trans-2,2 and cis-2,2 isomers of TAc and TPro adopt a perpendicular interfacial orientation (i.e., orientation C, Figure 2), their corresponding diacids should be strongly stabilized by ion pairing with anionic surfactant head groups at the surfactant/water interface and should consequently exhibit greatly enhanced basicities. Similarly, for the 4,O isomer, compounds adopting orientation B should exhibit higher basicities than those in orientation A due to physical isolation of the porphyrin core from the surfactant head groups and location of the porphyrin core in a site remote from the anionic charge gradient at the micelle surface. The data presented in Table V clearly reveal the role of the surfactant head groups in stabilizing the porphyrin diacid as a function of molecular topology. The short chain transand cis-2,2 PFPs, which exhibit highly accelerated reactivities in organized assemblies, exhibit the greatest enhancements in core basicities over those noted in homogeneous solution (Le., pK3K4"*P = 8.8 in aqueous SDS and 1.7 in 9:l DMF/water), consistent with a perpendicular interfacial orientation (orientation C). This basicity enhancement is in contrast to Phillips' pK3 measurements for a number of deuteroporphyrin dimethyl esters which are found to be only about 1 pK, unit more basic in SDS micelles than in water.O ' We suggest that these natural porphyrin esters, for which a stabilized monoacid can be isolated in aqueous SDS, reside in hydrophobic micellar solubilization sites where they are relatively protected from hydrogen ions and the effects of a negatively charged interface. In contrast to the protective effect of zwitterionic ~ e s i c l e s ~and l - ~poly(vinylpyrr01idone)~~ ~ toward Cu(I1) incorporation and core protonation of porphyrins incorporated into these systems, all porphyrins studied in anionic micelles exhibit enhanced basicity. Compared to the short chain cis- and trans-2,2 PFPs, short chain 4,O PFPs and relatively hydrophobic porphyrins (e.g., TPP, (o-aminopheny1)porphyrin (To-AmPP), and natural porphyrin esters) merely exhibit smaller basicity enhancements and no net protection in SDS micelles. An intermediate value for 4,O THex (pK3K4'PP = 7.85), which is proposed to adopt orientation B, and much lower values for the short chain 4,O PFPs (pK3K4"p values for 4,O TBu (5.99), 4,O TPro (5.64), 4,O TiBu and 4,O TAc (5.11), and 4,O TPiv (3.22)), proposed to prefer orientation A, further support the existence of these topologicaly controlled interfacial orientations. The interplay of side chain steric effects and interfacial orientation is evident in the pattern of basicities and reactivities for the THex atropisomers in the series of compounds studied. The high apparent basicity for trans-2,2 THex in aqueous SDS as compared to cis-2,2 TAc (8.2 vs 8.8) is not accounted for by a correspondingly fast kcuz+(krel= 970 and 22 500, respectively) which presumably evidences larger steric effects to metal ion approach to the core of trans-2,2 THex. More striking is the coincidence of basicities for cis-2,2 and 4,O THex and the discrepancy in metalation rate constants in the micelle (kQn = 2600 and 6000, respectively). The displaced position of the 4,O atropisomer in the order of THex basicities (pK3KdapPvalues follow cis-2,2 > 3,l) as compared to the normal trans-2,2 > 4,O reactivity order toward Cu(I1) observed in homogeneous solution

-

Barber et al.

-.

350

360

la0

A00

AZO

A40

Ai0

Ai0

440

A60

A80

Wavelength (nm)

1.0

8

f

.6

A

2

'350

360

380

A00

Wave-

420

(nm)

Flgwe 3. Spectral changes for PFP titration in aqueous SDS: (A) cis-2,2 THex;(B) 4,O TPro.

(Le., 4,O > 3,l > cis-2,2 > trans-2,2), is consistent with a change in interfacial orientation for 4,O THex relative to 4,O TAc and 4,O TPro. The anomalous position of 4,O THex in the order of basicities can be accounted for by a face-exposed orientation (orientation B) which places the unhindered face of the macrocycle in close proximity to the anionic head groups. Stabilization of the porphyrin monoacid (H3P+,see Scheme I) in SDS micelles is observed for the least basic porphyrins. In homogeneous solution, the porphyrin monoacid is commonly not observed to a sufficient extent to be spectrally observed during acid-base titration^;^^ a representative spectrum for a direct equilibration between free base and diacid in SDS micelles is shown in Figure 3A. However, during titration with aqueous hydrochloric acid in SDS micelles, PFPs proposed to favor orientation A at the interface (i.e., short chain 4,O PFPs) produce spectral changes in the visible region characteristic of the porphyrin monoacid (i.e., increased absorbance at -590 nm);76shifting "isosbestic" points in the Soret region accompany the formation and conversion of H3P+to H4P2+(see Figure 2). The proportion of H3P+formed increases with decreasing pK3K4nwfor 4,O PFPs. The formation of significant quantities of H3P+ is interpreted in terms of an energy of reorientation from orientation A to an orientation in which the porphyrin macrocycle is more exposed at the interface, such as orientations B or C (seeFigure 3B). That is, for short chain 4,O PFPs, a monoprotonated PFP core residing in orientation A, a relatively less polar solubilization site, is preferred relative to orientation B while core diprotonation requires PFP reorientation at the interface such that it is exposed to a site of high anionic interfacial potential. It is clear, from the basicity data, that the particular PFP topology governs interfacial orientation; interfacial solubilization, as opposed to solubilization in sites of lower anionic potential, is favored by the presence of polar amide functionalities as opposed to the solubilization of TPP and natural porphyrin esters in SDS micelles. The hydrophilic/lipophilic balance about both faces of the PFps appears to dictate their interfacial orientations in such a way that amide exposure at the interface is maximized. Stabilization of the porphyrin monoacid can be understood by comparing previous work by Phillips to that presented here.

The Journal of Physical Chemistry, Vol. 96, No. 12, 1992 5113

Picket Fence Porphyrin Atropisomers

TABLE VI: Acceleration a d Catalytic Factors for PFP Metalation in AOT Reversed Micelles and SDS Micelles Relative to 9 1 DMF/Water porphyrin 4.0 TAc 3,l TAc cis-2,2 TAc trans-2,2 TAc 4.0 THex

reversed micelles acceleration catalysis 290 21000 240000

13 95 10900

530000

24000 560

12500

SDS micelles acceleration catalysis 2.30 700 30800 4 1000

2300

0.023 7.0 308 410 23

Phillips observed an even greater percentage of monoacid formation for porphyrins whose basicities are increased by a smaller amount upon incorporation into SDS micelles (Le. ApK, = 1 unit for natural porphyrin esters vs 1.5-8.5 units for the PFPs). The simplest explanation for isolation of the porphyrin monoacid in Phillips’ study is that monoprotonation of the porphyrin core is possible at sites relatively remote from the interface, where a free energy of relocation and/or reorientation (ACT) placing the porphyrin closer to the surfactant head groups is required for diprotonation in both cases. We conclude that it is not so much the “hydrophilic” nature of interfacial solubilization that promotes porphyrin protonation but rather the “anionic” nature which promotes cations to accumulate (a high interfacial electrostatic field) and an elevated interfacial pH. Comparison of Metalation Rate Constants for the PFPs in Various Media. The ranges of reactivities for the PFPs are the greatest in micelles and reversed micelles (seeTable 11), suggesting that specitic accelerating (i.e., orientation C, Figure 2) and relative “decelerating” factors (Le., orientation A) are operative. The greater acceleration in AOT reversed micelles, in terms of k,,, relative to the SDS micelle is consistent with a more well-defined reversed micelle interface. Yet the greater discrimination of reactivities in SDS micelles (i.e., range for linear PFPs are 2800 in aqueous SDS and 191 in AOT reversed micelles, see Table 11) is consistent with a more flexible interface in the SDS micelle which may more readily accommodatediffering topologies; thus, interfacial structure and solute orientation is more strongly governed by molecular topology and the hydrophobic/hydrophilic balance of the porphyrin in the SDS micelle than by forces controlling surfactant aggregation. As compared to reactivity in homogeneous solution, generally elevated reactivities for the PFPs and diminshed reactivities for TPP or tetrakis(0-aminopheny1)porphyrin evidence PFP solubilization in close proximity to the surfactant head groups. Reactivities in DMF,nonionic microand cationic micelle^^*^^ and at the anionic interfaces studied in this paper reveal the general effect of interfacial charge. The reactivity of 4,O THA in AOT reversed micelles is -980 times faster than in nonionic micro emulsion^.^^ Monolayers, Micelles, and Reversed Micelles. A more general understanding of PFP behavior at hydrophilic/hydrophobic interfaces is provided by comparing their properties in different organized assemblies. Spread monolayers and deposited multilayers of particularly the 4,O atropisomer have been studied in terms of metalation ratesM and pressure-area isotherms.66 In deposited multilayers, 4,O THA is found to be highly reactive compared to 4,O TAc while natural porphyrins, which should adopt a perpendicular orientation near or slightly removed from the interface, are unreacti~e.~-~’ These trends are consistent with what one would expect on the basis of surface area-pressure isotherms for the four atropisomers of TAc, THex, and THA.66 For 4,O TAc, a lift-off area of 180 Az/molecule (an area at which the surface pressure begins to rise) and high surface pressures (-20 mN/m) at 140 A2/molecule both occur above the limiting area estimated for a planar porphyrin ring,” giving rise to a large plateau region for 4,O TAc. This situation is consistent with strong binding to the air/water interface such that the porphyrin core is oriented parallel to the interface. The other TAc isomers do not support high surface pressures, and only the 3,l isomer shows any evidence for a plateau region, consistent with a perpendicular interfacial orientation. The behavior of 4,O THA at an air/water interface is consistent with

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a parallel interfacial orientation but weaker interfacial binding (lower surface pressures). These results suggest that the interfacial orientations for the PFPs in monolayers, micelles, and reversed micelles are similar and are determined by molecular topology and side chain length. The particularly strong binding noted in monolayers of 4,O TAc is consistent with hydrophilic exposure of the acetamides in orientation A. The reactivity of the PFPs in organized media is unique. While many studies have focused on the properties of the 4,O isomer, we find equally interesting behavior for all four atropisomers in organized a ~ s e m b l i e sand ~ ~ -homogeneous ~ solution.58 Whereas other porphyrins generally show less dramatic accelerations and may more readily adopt a perpendicular orientation in Langmuir-Blodgett monolayers due to their planar core structure, the PFPs appear to reside in parallel, perpendicular, or inverted interfacial orientations, depending on atropisomer topology. The generality of the PFPs’ behavior at both anionic interfaces studied appears to distinguish these o-amido TPP atropisomers from other porphyrins.

Conclusion The results obtained in this study suggest that metalation-a very general and not too selective process in solution--can become an extremely environment-sensitiveand selective process in organized media and at interfaces. It appears reasonable that topological orientation can control access of the reagent (Cuz+) and affect the stability of (1) intermediates during metalation, (2) the monoacid intermediate (H3P+)for core protonation, and (3) the diacid product. In most cases, specific characteristics of interfacial solubilization for various porphyrins generally enhance both metalation rates and porphyrin basicities concurrently and imply a similarity between the two processes. The basicity-reactivity correlation is reasonable since (1) very similar structural changes are associated with macrocycle distortion in both cases and (2) changes in net charge and the location of charge on porphyrin species are the same. Acknowledgment. We are grateful to the National Science Foundation (Grant CHE-8616361) and the National Institutes of Health (Grant 5 R01 CA48961) for funding this work. We thank Professor Jonathan S. Lindsey for the gift of meso-tetrapentylporphyrin.

References and Notes (1) Taken in part from the doctoral dissertation of D.C.B., University of Rochester, 1989. (2) A preliminary account of a portion of this work has been published: Barber, D. C.; Whitten, D. G. J. Am. Chem. SOC.1987,109, 6842. (3) Kalyanasundaram, K. Photochemistry in Microheterogeneous Systems; Academic Press, Inc.: Orlando, FL, 1987. (4) Anionic Surfactants: Physical Chemistry of Surfactant Action; Lucassen-Reynders, E. H., Ed.; Surfactant Science Series, Vol. l l ; Marcel Dekker, Inc.: New York, 1981; pp 89-91. (5) Takagi, K.; Suddaby, B. R.; Vadas, S. L.; Backer, C. A.; Whitten, D. G. J. Am. Chem. Soc. 1986,108, 7865. (6) Shin, D.-M.; Schanze, K. S.; Whitten, D. G. J . Am. Chem. Soc. 1989, 111, 8494. (7) Barrow, D. A.; Lentz, B. R. J. Biophys. SOC.1985, 48, 221. (8) Tsuchida, E.; Kaneko, M.; Nishide, H.; Hoshino, M. J . Phys. Chem. 1986, 90, 2283. (9) Fendler, J. H. Acc. Chem. Res. 1980, 13, 7. (10) Bunton, C. A.; Moffatt, J. R. J. Phys. Chem. 1988,92, 2896. (11) Jones, M. S.; Jones, 0. T. G. Biochem. J. 1970, 119, 453. (12) Nishida, G.; Labbe, R. F. Eiochim. Biophys. Acta 1959, 31, 519. (13) Labbe, R. F. Biochim. Biophys. Acta 1959, 31, 589. (14) Antonini, F.; Brunori, M. Hemoglobin and Myoglobin and Their Reactions with Ligands; North-Holland Publishing Co.: Amsterdam, 1971. (15) van den Bergh, H. Chem. Er. 1986, 22, 430. (16) Collman, J. P. Acc. Chem. Res. 1977, 10, 265. (17) Brochette, P.; Petit, C.; Pileni, M. P. J. Phys. Chem. 1988, 92, 3505. (18) Nango, M.; Dannhauser, T.; Huang, D.; Spears, K.; Morrison, L.; Loach, P. A. Macromolecules 1984. 17. 1898. (19) Tsuchida, E.; Kaneko, M.; Nishide, H.; Hoshino, M. J. Phys. Chem. 1986, 90, 2283. (20) Nishide, H.; Yuasa, M.; Hashimoto, Y . ;Tsuchida, E. Macromolecules 1987, 20, 459. (21) Phillips, J. N. Reu. Pure Appl. Chem. 1960, 10, 35. (22) Simplicio, J. Biochemistry 1972, 11, 2525. (23) Simplicio, J. Biochemistry 1972, 1 I, 2529.

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Barber et al. (57)Collman, J. P.; Gagne, R. R.; Reed, C. A.; Halbert, R. R.; Lang, G.; Robinson, W. T. J. Am. Chem. Soc. 1975, 97, 1427. (58) Barber, D. C.; Lawrence, D. S.;Whitten, D. G. J. Phys. Chem., in press. (59)Barber, D. C.; Whitten, D. G. J. Am. Chem. Soc. 1987,109,6842. (60)Rao, V. H.; Krishnan, V. Inorg. Chem. 1985, 24, 3538. (61)Soret maxima for the PFPs are blue shifted in AOT reversed micelles (--3 nm; A- = 417.2-421.4 nm) and SDS micelles (-4nm; A- = 417.8-421.0nm) relative to those noted in DMF solution (A- = 419.6426.0 nm) such that the values in micellar solution are comparable to those noted in ethanol or methanoLJ3 Although micellar solubilization in a hydrocarbon core has been suggested, the preponderance of data suggest that moderately polar molecules such as porphyrins or other aromatics are solubilized in relatively wet and hence interfacial environments. Absorption maxima of the porphyrin diacids in SDS micellar solution are shifted in the Soret and/or visible hands relative to those in aqueous hydrochloric acid in all cases consistent with micellar association. (62) Fernandez, M. S; Fromherz, P. J . Phys. Chem. 1977, 81, 1755. (63)Estimating the maximum interfacial Cu(I1) concentration for a 4:l composition of CuSO, and TPP core using the densities of CuSO, and an organic hydrocarbon and considering the loading ratio of Cu(I1) to sulfate head groups in the SDS micelle, one calculates a maximum local interfacial [Cu(II)] of 0.32M which is 16% lower than that estimated considering that the reactants are confined to the volume of the micelle used to calculate ‘microscopic metalation rate constants”; see Experimental Section. (64)Turay, J.; Hambright, P. Inorg. Chim. Acta 1981, 53, L147. (65)The ratio of k w a / k D M Fgives corrected rates (k,) for an individual compound which accounts for differences in inherent core reactivities (see ref 58). k,/k,-, where ,k is the maximum metalation rate constant observed for the 4,O isomer in a particular medium, gives the percentage in orientation B assuming that the most reactive 4,OPFP studied in this assembly entirely adopts orientation B. (66) Collins-Gold, L. C.; Barber, D. C.; Hagan, W. J.; Gibson, S.L.; Hilf, R.; Whitten, D. G. Photochem. Photobiol. 1988, 48, 165. (67) Hartley, G. S.Trans. Faraday Soc. 1934, 30.444. (68)Shamin, A.; Hambright, P.; Williams, R. F. X . Inorg. Nucl. Chem. Lett. 1979, 15,243. (69)Kassner, R. J.; Wang, J. H. J. Am. Chem. Soc. 1966, 88. 5170. (70)F a 4 J. N. Porphyrins and Meralloporphyrins; Elsevier: Amsterdam, 1964; Vol. 2,p 27. (71) Furhop, J.-H.; Hosseinpour, D. Liebigs Ann. Chem. 1985, 689. (72) Furhop, J.-H.; Lehmann, T. Liebigs Ann. Chem. 1984, 1057. (73) Furhop, J.-H.; Wanja, U.;Bunzel, M. Liebigs Ann. Chem. 1984,426. (74)El Torki, F.M.; Casano, P. J.; Reed, W. F.; Schmehl, R. H. J. Phys. Chem. 1987, 91, 3686. (75)HIP+ was not evident in titrations of the TAc isomers or for TPP in 9:l DMF/water; see ref 58. (76) Dempsey, B.; Lowe, M. B.; Phillips, J. N. Hacmatin Enzymes; International Union of Biochemistry Symposium Series, Vol. 19; Pergamon Press, Inc.: Oxford, U.K., p 29. (77)Demel, R. A.; Guerts Van Kessel, W. S.M.; Zwaal, R. F. A.; Roelofson, B.; Van Deenen, L. L. M. Biochim. Biophys. Acta 1975, 406,97.