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J. Phys. Chem. B 1998, 102, 8852-8857
pH Modulation of Porphyrins Self-Assembly onto Polylysine Roberto Purrello,*,† Emanuele Bellacchio,† Sergio Gurrieri,‡ Rosaria Lauceri,† Antonio Raudino,† Luigi Monsu` Scolaro,§ and Anna Maria Santoro† Dipartimento di Scienze Chimiche, UniVersita` di Catania, Viale Andrea Doria 6, 95125, Catania, Italy, Istituto per lo Studio delle Sostanze Naturali di Interesse Alimentare e Chimico-Farmaceutico, C.N.R., Via del Santuario 110, 95028 ValVerde (CT), Italy, and Dipartimento di Chimica Inorganica, Chimica Analitica e Chimica Fisica, UniVersita` di Messina, ITCPN C.N.R. Sezione di Messina, Messina, Italy ReceiVed: July 2, 1998
The tetraanionic meso-tetrakis(4-sulfonatophenyl)porphine (H2TPPS) in the pH range 5-12 exists in a monomeric form, and its fluorescence is not pH-dependent. However, in the presence of polylysine, absorption, circular dichroism, and resonance light scattering data indicate extensive polymer-induced self-aggregation of the porphyrins. In addition, the fluorescence intensity vs pH behavior is deeply modified, showing a sigmoidal profile. In particular, at low pHs (e7), the protonated polylysine promotes porphyrins binding and self-aggregation with consequent strong quenching of their fluorescence, while at pHs higher than 9-10, the porphyrins exist in solution essentially as free monomers which retain their fluorescence. Interestingly, the molecular recognition processes leading to the formation of these aggregates can be modulated by using porphyrins containing different central metal ions with particular coordination geometry.
Introduction A supramolecular photochemical species is a chemical complex in which one or more photoactive component(s) is (are) assembled together by and with an “organizing receptor”.1,2 The properties of such species are “modulated by the arrangement of the bound units as determined by the organizing receptor”;1 i.e., there is a strong interplay between the properties of the photochemical species and the molecular recognition processes which lead to its formation. Therefore, the knowledge of these processes and a reasonable prediction of the photophysical variations of the photoactive component(s) upon assembly should allow the design of complex species characterized by new properties. Recently, it has been reported that the water-soluble tetraanionic mesotetrakis(4- sulfonatophenyl)porphine (H2TPPS, Figure 1) interacts with polylysine, leading, under appropriate conditions, to the formation of aggregates.3 The very first recognition processes are primarily driven by electrostatic interactions: those between the negatively charged peripheral groups of the porphyrins and the protonated side chains of the matrix and those among porphyrins themselves.4 However, most likely, also other processes (such as desolvation processes or matrix conformational changes upon dye binding) contribute to the total free energy change. Once a large enough porphyrin “critical” concentration onto the polymer matrix has been reached, van der Waals and solvophobic forces induce extensive porphyrin aggregation which, in many cases, leads to remarkable fluorescence quenching.5 In this context, we hypothesized that it should be possible to control porphyrin fluorescence by modulating, through pH * Corresponding author: Dipartimento di Scienze Chimiche, Universita` di Catania, Viale Andrea Doria 6, 95125, Catania, Italy. Telefax: Int. code +(95)580138. E-mail:
[email protected]. † Universita ` di Catania. ‡ Istituto per lo Studio delle Sostanze Naturali di Interesse Alimentare e Chimico-Farmaceutico, CNR. § Universita ` di Messiana.
Figure 1. Molecular structure of the meso-tetrakis(4-sulfonatophenyl)porphine (H2TPPS).
variations, the interactions between the anionic porphyrins (the photoactiVe components) and the protonated polylysine (the organizing receptor, apparent pKa ≈ 9.9).6,7 Indeed, under appropriate conditions, this is feasible, and such systems can behave as supramolecular fluorescent “pH-sensors”.8 This behavior allowed us to preliminarly characterize these aggregates and, most importantly, to understand some of the factors which modulates their stability and drive the underlying molecular recognition processes. Experimental Section Polylysine of different lengths (polymerization degrees (dp) of 46 and 633) was obtained from Sigma Chemical Company. Its concentration is expressed as moles of lysine residues per liter and was determined using 205 ) 3300 M-1 cm-1 in doubly distilled water.9 Meso-tetrakis(4-sulfonatophenyl)porphyrin was obtained from Mid-Century as the tetrasodium salt. Porphyrin metalation was performed by literature methods.10 H2TPPS, SnIVTPPS, and ZnTPPS concentrations were determined using 414 ) 5.33 × 105, 418 ) 6.2 × 105, and 421 ) 6.8 × 105 M-1 cm-1, respectively. Spectrofluorimetric measurements were recorded on a Jasco FP-777. Resonance light scattering (RLS)11 measurements were performed on a SPEX F111 spectrofluorimeter fitted with a 150-W xenon lamp, using right angle
10.1021/jp9828686 CCC: $15.00 © 1998 American Chemical Society Published on Web 10/09/1998
pH Modulation of Porphyrins Self-Assembly onto Polylysine
J. Phys. Chem. B, Vol. 102, No. 44, 1998 8853
Figure 2. Absorption spectra of H2TPPS (1 µM) in the absence (continuous line) and in the presence (dotted line) of polylysine (100 µM). The inset shows the second derivative of the spectrum obtained in the presence of polylysine.
Figure 3. RLS spectra of H2TPPS (1 µM) in the presence of different concentrations of polylysine. R is the polylysine/porphyrin ratio.
geometry. The excitation and emission monochromator wavelengths were coupled and adjusted to scan simultaneously. Spectrofluorimetric and RLS measurements were carried out using disposable metacrylate cuvettes. These cuvettes proved to be superior for these studies with respect to those made of quartz. In fact, adsorption of polylysine on the quartz surface was observed, causing spurious results. All spectrofluorimetric titrations were run at least three times. Absorption measurements were carried out on a HP8452A. CD spectra were recorded on a Jasco J-600 spectropolarimeter. The concentration values used to calculate the ∆ have been derived by the spectrofluorimetric curves, having in mind the same considerations and approximations used to calculate the apparent pK. All measurements were performed at [NaCl] ) 0.05 M. In these experimental conditions the ionic strength variation, due to the addition of the titrant, can be neglected. Doubly distilled water was used throughout. Results and Discussion H2TPPS-Polylysine System. We started our investigation by spectroscopically characterizing the behavior of H2TPPS in aqueous solution both in the absence and in the presence of polylysine. Differently from previous reports3a,b we have worked under conditions of controlled ionic strength ([NaCl] ) 0.05 M). In fact, owing to the already mentioned electrostatic nature of these interactions, ionic strength variations might lead to misleading results. In the present experimental conditions ([H2TPPS] ) 1 µM, pH range 5-12) the tetraanionic porphyrin is essentially in the monomeric form. However, the absorption spectra (Figure 2) shows that, upon addition of polylysine, the Soret band of the free-porphyrin (λmax ) 414 nm) experiences a strong hypochromic effect (≈60%) and some broadening. The second derivative of this spectrum (inset of Figure 2) resolves the broad Soret band in two components at 400 and 414 nm, which correspond to the absorption of the bound and free porphyrins, respectively. From the intensity ratio (400 nm/414 nm) of the resolved bands it turns out that, under these experimental conditions, most of the tetraanionic porphyrin (≈95%) is interacting with the protonated polypeptide. The remarkable spectral variations observed upon the addition of polylysine strongly indicate that porphyrins are not simply monodispersed on the matrix surface but, most likely, are self-
Figure 4. CD spectra of H2TPPS (1 µM) in the presence of polylysine (100 µM), at pH 7 (continuos line) and pH 10 (dotted line).
aggregated.12 The formation of H2TPPS assemblies was confirmed by the appearance of RLS bands in the Soret region (Figure 3, see further for a more detailed discussion). Indeed, this tetraanionic porphyrin has a well-known tendency to form aggregates.13 Moreover, the blue-shift of the Soret band, in the presence of the cationic matrix, suggests the reciprocal disposition of the aggregated porphyrins to be face-to-face (H-type aggregates).14,15 The circular dichroism (CD) spectrum of the same solution reveals the presence of two induced bisignate bands (Figure 4, continuous line), indicating the presence of chiral porphyrin aggregates.16,17 It is worth recalling, at this point, that the porphyrins used in this study are not CD active. For such porphyrins, a dichroic absorption in the Soret region has been observed only if induced by interaction of the porphyrins with chiral molecules,3,17,18 or the porphyrins being part of chiral superstructures.5,19 However, under the experimental conditions in which the CD spectrum of Figure 4 was recorded (pH 7), polylysine is extensively protonated and, as a consequence of
8854 J. Phys. Chem. B, Vol. 102, No. 44, 1998
Figure 5. Absorption pH-titrations (pH 7.2 ÷ 10.3) of H2TPPS (1 µM) in the presence of polylysine (100 µM).
the electrostatic repulsion between its positive charges, should not be in R-helical conformation.6,7 Therefore, in order to explain the appearance of induced CD signals we hypothesize an interaction between the sulfonated groups of the anionic porphyrins and the protonated -amino groups of polylysine, which partially shields the above-mentioned electrostatic repulsion, inducing localized ordered regions along the polymeric matrix. However, we cannot exclude that other mechanisms could be responsible for the presence of the induced CD features.20,21 Increasing the pH, the intensity of both CD signals decreases and goes to zero, and, eventually, the spectrum of only a single species is obtained (Figure 4, dotted line).22 The observed flat CD can be caused either by the absence of interactions between the dye and the chiral template or by the contemporary presence of species having opposite dichroic signals, such as, for example, conformational isomers.20,23 The absorption data strongly support the latter hypothesis. In fact, at the pH at which no CD signal is observed (pH ≈ 9.5), the intensity of the Soret band shows that about 60% of porphyrins is still interacting with polylysine (Figure 5). In addition, the presence of a “quasi-isosbestic” point in the absorption pHtitrations strongly suggests that the above-mentioned conformers, which are quite different from a dichroic point of view, have, on the contrary, very similar absorption spectra (Figure 5). Spectrofluorimetric pH-titrations show that, in the absence of polylysine, the fluorescence intensity of H2TPPS does not change in the investigated pH range (5 ÷ 11.5) (Figure 6). On the other hand, in the presence of polylysine, the profile of the fluorescence intensity vs pH shows a sigmoidal shape (Figure 6). Back-titrations with HCl show that these aggregation processes are fully reversible. Therefore, it turns out that such sigmoidal shape must be due to the equilibrium between aggregated and free anionic porphyrins. Here, fluorescence variations report only the porphyrins aggregation state, and the pH dependence of the equilibrium indicates that the formation of the supramolecular species is triggered by the presence of positively charged amino groups of polylysine. Thus, at low pHs, where polylysine is extensively protonated, the equilibrium is shifted toward binding and concomitant aggregation (strong fluorescence quenching in Figure 6). Here, only the bound porphyrins in aggregated form (dimers, trimers, ...) contribute to the emission quenching. At higher pHs the porphyrins tend to dissociate from the polylysine matrix and eventually exist as monomers in solution (no fluorescence quenching in Figure 6).
Purrello et al.
Figure 6. pH dependence of the fluorescence emission (λex ) 405 nm, λem ) 640 nm) of H2TPPS (1 µM) in the presence (continuous lines) and in the absence (dotted line) of polylysine (polymerization degree ) 46). Redrawn in a slightly modified way from ref 3c with kind permission from Kluwer Academic Publishers.
The multiple equilibria can be schematically represented as follows
where n is the number of protonated lysine residues per strand and m represents the number of aggregated porphyrins per polylysine strand. Obviously, different values of n correspond to different charge densities on the polymer which, therefore, will affect the number of bound porphyrins (m). It follows that for any values of n and m it pertains the formation of different species and, therefore, a different equilibrium constant for the formation of the supramolecular complexes. In order to understand the effect of the matrix concentration on the formation of these supramolecular species we performed further experiments keeping constant the concentration of H2TPPS and increasing the amount of polylysine. As reported in Figure 6, the fluorescence intensity at pH < 8 decreases drastically increasing the polylysine/H2TPPS ratio (R) from 25 to 50 and then levels off. Also, RLS spectra (Figure 3) obtained at R ) 25 are more intense than those measured at R ) 50, while at R ) 100 no signal is detectable, owing, most likely, to the presence of very small aggregates.24 These observations indicate that the concentration of the total bound porphyrin in the aggregated form increases with polylysine concentration, but H2TPPS aggregates become smaller and smaller. These two effects can be easily explained by a larger availability of binding sites. Also, going from R ) 25 to R ) 100 the inflection point of the various curves is shifted toward higher pHs (Figure 6). No further shift is observed for R values higher than 400. Clearly, the considerations about the multiple coexisting equilibria I hold for each of the curves of Figure 6. However, as shown by RLS data, at a given polylysine concentration pertains an initial value of m (the number of aggregated porphyrins per strand) which differs from those of the other curves and leads to a series of subsequent equilibria I which are function of R.25 Furthermore, other effects than the polylysine/porphyrin ratio may modulate the equilibria I. For instance, using longer polylysine strands (Figure 7) we found at R ) 25 a stronger fluorescence quenching with respect to
pH Modulation of Porphyrins Self-Assembly onto Polylysine
J. Phys. Chem. B, Vol. 102, No. 44, 1998 8855 TABLE 1: Apparent pK for the Supramolecular Complexes Formed by H2TPPS and ZnTPPS with Polylysine (Polymerization Degree 46 and 633)a
R
pK H2TPPS/polylysine (DP ) 46)
pK H2TPPS/polylysine (DP ) 633)
pK ZnTPPS/polylysine (DP ) 46)
25 50 100 400
9.0 (3) 9.5 (2) 9.9 (1) 10.4 (1)
9.8 (2) 10.6 (2)
10.0 (2)
a
10.6 (2)
Standard deviations are reported in parentheses.
Figure 7. pH dependence of the fluorescence emission (λex ) 405 nm, λem ) 640 nm) of H2TPPS (1 µM) in the presence of polylysine (polymerization degree ) 633).
the analogous curve obtained using shorter polylysine strands (Figure 6). This result is probably related to end effects in charged polymer chains (such as stronger electrostatic fields in longer chains). From these considerations a quantitative characterization of each equilibrium step turns out to be quite complex, and it is beyond the purpose of this study. Despite this complexity, the fully reversible behavior of the single fluorescence titration curves suggests that we can treat each curve as a whole system of subsequent equilibria which describes the dissolution of a given initial supramolecular assembly, where each system (i.e., each curve) “melts” at a different pH, which depends on R. Then, a reasonable approach to simplify the multiple equilibria I consists in considering a single comprehensive equilibrium for porphyrin binding, where polylysine does not appear explicitly:
The pH dependence of the equilibrium II prompted us to associate to it an apparent pK which indicates the pH at which the concentrations of aggregated and free porphyrins are equal, independently of the m values. The equation we used to estimate the apparent pK concerning the equilibrium II is
[
pK ) pH + log
]
(Imax - I) (I - Imin)
which involves the following considerations and/or approximations: (i) Imax is proportional to the total concentration of porphyrin, (ii) Imin is the limiting intensity at low pH of the aggregated porphyrins, and (iii) I contains contributions both from free and aggregated porphyrins. Here, the key approximation made is that aggregation leads to 100% fluorescence quenching. This is reasonable considering that high polylysine concentrations lead to a H2TPPS fluorescence quenching close to 100% (Figure 6). Moreover, spectrofluorimetric experiments with the nonaggregating SnIVTPPS (see next paragraph) shows that other routes different from self-aggregation do not cause relevant quenching. The calculated pK values are reported in Table 1. As already discussed, the shift of the inflection points observed by increasing R reflects a different “stability” to pH changes of various supramolecular porphyrin aggregates the size of which strongly depends on R (see RLS data). In particular,
Figure 8. Absorption spectra of SnIVTPPS (1 µM) in the absence (continuous line) and in the presence (dotted line) of polylysine (100 µM). The inset shows the pH dependence of the fluorescence emission (λex ) 420 nm, λem ) 600 nm) in the presence (continuous lines) and in the absence (dashed line) of polylysine.
it looks like the smaller the aggregate the higher is the resistance of the system to pH variations. SnTPPS-Polylysine System. A different behavior was observed for the hexacoordinated26 SnIVTPPS. This metalloporphyrin was selected because the presence of two axial ligands prevents self-aggregation. The absorption spectra (Figure 8) show that upon the addition of polylysine (R ) 100) the Soret band of this metallo-porphyrin is red-shifted by only 2 nm, and its intensity decreases by about 40%. These spectral variations are much less pronounced than those observed for the analogous system with the free-porphyrin. Accordingly, no induced CD features and RLS bands have been observed. Furthermore, SnIVTPPS fluorescence is only slightly quenched (less than 10%) in the presence of the polypeptide (at pH < 8 and R ) 100, see inset of Figure 8), confirming that the quenching observed for the H2TPPS-polylysine system must be mainly due to shortrange porphyrin-porphyrin interactions, as those occurring in self-assembled structures. ZnTPPS-Polylysine System. The trend of the pK variation with increasing polylysine concentration, observed for the H2TPPS-polylysine system, was confirmed by the data obtained for an analogous system involving the pentacoordinated ZnTPPS, which presents one water molecule axially coordinated to the central metal ion.26 To the best of our knowledge, no evidence of self-aggregation in water solution has been presented to date. However, absorption, CD, and fluorescence data show that ZnTPPS does aggregate in the presence of polylysine, confirming that porphyrin aggregation onto opposite charged
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Figure 11. Schematic representation of a possible ZnTPPS dimeric form. Sulfonic groups have been omitted for clarity.
Figure 9. Absorption spectra of ZnTPPS (1 µM) in the absence (continuous line) and in the presence (dotted line) of polylysine (25 µM). The inset shows the second derivative of the spectrum obtained in the presence of polylysine.
Figure 12. pH dependence of the fluorescence emission of ZnTPPS (λex ) 429 nm, λem ) 604 nm) in the presence (continuous lines) and in the absence (dotted line) of polylysine (polymerization degree ) 46).
Figure 10. CD spectrum of ZnTPPS (2 µM) in the presence of polylysine.
matrices involves specific molecular recognition processes.27 In fact, the spectroscopic features are very similar to those previously described for the parent free-porphyrin. In particular, following the addition of polylysine (R ) 25) the absorption spectra show a strong hypochromicity (about 60%) and broadening of the Soret band (Figure 9). In addition, the second derivative of the absorption spectrum in the presence of polylysine is resolved in three bands at 414, 426, and 450 nm. Also, the CD spectrum shows an intense bisignate induced CD band in the Soret region (Figure 10).28 However, because of the presence of one axial water molecule, we hypothesize that ZnTPPS self-aggregation is limited to the formation of dimers. Following the indications of the absorption spectra (the blueshift of the bound form Soret band), we also hypothesize that most of the dimeric form of ZnTPPS is in a face-to-face arrangement (Figure 11), and we assign to this species the
absorption at 414 nm. The small absorption at 450 nm might be assigned to a very small percentage of head-to-head dimers. Finally, the shoulder at 426 nm is most likely due to monomeric ZnTPPS bound to polylysine. The spectrofluorimetric titrations of the ZnTPPS-polylysine system (Figure 12) are similar to those already described for H2TPPS. However, at R ) 25, the ZnTPPS supramolecular complex is about as stable as that formed by H2TPPS at R ) 100, the pK values being 10.0 (Table 1). These observations suggest again that to small aggregates pertain a series of subsequent equilibria which leads to high pK values. Finally, between pH 7 and pH 8 we observed an additional fluorescence inflection in the ZnTPPS-polylysine titrations which we tentatively assign to the deprotonation of the axially bound water to the metal site.2e This inflection is not present in the absence of polylysine (Figure 12) and, therefore, must be related to aggregation processes. Concluding Remarks In conclusion, we have shown that the interactions of anionic porphyrins with a polymeric cationic matrix is modulated, both in terms of type and extent, by the charge density of the polymer chain and the steric features of the porphyrins. The absence of severe steric constrains leads to assemblies which can behave as fluorescent pH-sensors. The stability toward pH of these supramolecular aggregates seems to be related to their size, which can be tuned by changing (i) the polylysine to porphyrin ratio or, again, (ii) the pH and (iii) the steric features of porphyrins (e.g. the central metal ion), which mainly affect the lateral distribution of the porphyrins along the polymer matrix and hence the quenching.
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We are currently investigating the temperature dependence of the pK in order to get information on the ∆H° and ∆S° values. These data, analyzed in the framework of a model for the thermodynamics of self-assembly processes,29 should help us in understanding the role played by weak noncovalent interactions and desolvation processes in stabilizing these complexes.
(13) (a) Pasternack, R. F.; Schaefer, K. F.; Hambright, P. Inorg. Chem. 1994, 33, 2062. (b) Ribo, J. M.; Crusats, J.; Farrera, J.-A.; Valero, M. L. J. Chem. Soc., Chem. Commun. 1994, 681 and references therein. (14) (a) Kasha, M.; Rawls, H. R.; El-Bayoumi, M. A. Pure Appl. Chem. 1965, 11, 371. (b) Osuka, A.; Maruyama, K. J. Am. Chem. Soc. 1988, 110, 4454 and references therein. (15) Fuhrhop, J.-F.; Demoulin, C.; Boettcher, C.; Konig, J.; Siggel, U. J. Am. Chem. Soc. 1992, 114, 4159. (16) The shape of the signal makes one think that both signals are bisignate (conservative or quasi-conservative). Such a kind of induced CD bands, when the total ∆ is close or higher than |100 M-1 cm-1|, are normally associated to porphyrin aggregation.17 (17) Gibbs, E. J.; Tinoco, I.; Maestre, M.; Ellinas, P.; Pasternack, R. F. Biochem. Biophys. Res. Comm. 1988, 157, 350. (18) Pasternack, R. F.; Gibbs, E. J.; Villafranca, J. J. Biochemistry 1983, 22, 2406. (19) It has been recently observed that porphyrin assemblies of the protonated form of H2TPPS (pKa ) 4.8) are chiral by themselves at pHs close to 1. (Ohno, O.; Kaizu, Y.; Kobayashi, H. J. Chem. Phys. 1993, 99, 4128). Also, some authors reported that the interaction with DNA may induce a shift of some cationic porphyrins pKa (Mukundan, N. E.; Petho, G.; Dixon, D. W.; Kim, M. S.; Marzilli, L. G. Inorg. Chem. 1994, 33, 4676. Schneider, H.-J.; Wang, M. J. Org. Chem. 1994, 59, 7473.) However, we do not have spectroscopic evidences that, in our experimental conditions (pH g 5), polylysine induces protonation of H2TPPS (λmax H4TPPS ) 434 nm). (20) Pancoska, P.; Urbanova, M.; Bednarova, L.; Vacek, K.; Paschenko, V. Z.; Vasiliev, S.; Malon, P.; Kral, M. Chem. Phys. 1990, 147, 401. (21) Brittain, H. G. In Analytical Applications of Circular Dichroism; Purdie, N.; Brittain, H. G., Eds.; Elsevier: Amsterdam, 1994; pp 1-13, and references therein. (22) The observed CD features and their pH dependence are very similar for all the polylysine to porphyrin ratio investigated, the only difference being the pH at which the spectral variations start to happen. This is reasonable because, as anticipated, the behavior of these complex species vs the pH depends on R. (23) Stryer, L.; Blout, E. R. J. Am. Chem. Soc. 1961, 83, 1411. (24) This trend cannot be ascribed to a higher extent of porphyrins aggregation because the spectrofluorimetric data show the opposite one, i.e., the concentration of aggregated porphyrins increases on increasing R values. (25) The shift of the inflection point, observed by varying R, cannot be simply related to a shift toward the right side of the equilibria I for concentration effects but rather reflects the variation in population distribution of aggregated porphyrins (dimers, trimers, ...) on varying the number of available binding sites. (26) Hoard, J. L. Porphyrins and Metalloporphyrins; Elsevier: Amsterdam, 1975. (27) (a) Pasternack, R. F., Gurrieri, S., Lauceri, R., Purrello, R. Inorg. Chim. Acta 1996, 246, 7. (b) Lauceri, R.; Campagna, T.; Contino, A.; Purrello, R. Angew. Chem., Int. Ed Engl. 1996, 35, 215. (28) Indeed, in analogy with the H2TPPS-polylysine system, the CD spectrum suggests the presence of two chiral species. (29) Raudino, A.; Gurrieri, S.; Lauceri, R.; Monsu` Scolaro, L.; Purrello, R. Manuscript in preparation.
Acknowledgment. This work was supported in part by the Consiglio Nazionale delle Ricerche (CNR) and the Ministero dell’Universita’ e della Ricerca Scientifica e Tecnologica (MURST). References and Notes (1) Lehn, J.-M. In Frontiers in Supramolecular Organic Chemistry and Photochemistry; VCH: Weinheim, 1969. (2) (a) Bissel, R. A.; de Silva, A. P.; Gunaratne, H. Q. N.; Lynch, P. M. L.; Maguire, G. E. M.; Sandanayake, K. R. A. S. Chem. Soc. ReV. 1992, 187. (b) Fabbrizzi, L.; Licchelli, M.; Pallavicini, P.; Perotti, A.; Taglietti, A.; Sacchi, D. Chem. Eur. J. 1996, 2, 75, and references therein. (c) Grigg, R.; Norbert, W. D. J. A. J. Chem. Soc., Chem. Commun. 1992, 1298. (d) Grigg, R.; Norbert, W. D. A. J. Chem. Soc., Chem. Commun. 1992, 1300. (3) (a) Ikeda, S.; Nezu, T.; Ebert, G. Biopolymers 1991, 31, 1257. (b) Nezu, T.; Ikeda, S. Bull. Chem. Soc. Jpn. 1993, 66, 25. (c) Purrello, R.; Gurrieri, S.; Bellacchio, E.; Lauceri, R.; Monsu` Scolaro, L. In Spectroscopy of Biological Molecules: Modern Trends; Kluwer: Dordrecht, Carmona, P., Navarro, R., Hernanz, A., Eds.; 1997; pp 91-92. (4) (a) Hunter, C. A.; Sanders, J. K. M. J. Am. Chem. Soc. 1990, 112, 5525. (b) Hunter, C. A. Angew. Chem., Int. Ed. Engl. 1993, 32, 1584. (c) Schneider, H.-J.; Wang, M. J. Org. Chem. 1994, 59, 7464. (d) Newcomb, L. F.; Gellman, S. H. J. Am. Chem. Soc. 1994, 116, 4993. (5) (a) Ojadi, E.; Selzer, R.; Linschitz, H. J. Am. Chem. Soc. 1985, 107, 7783. (b) Hofstra, U.; Koehorst, R. B. M.; Schaafsma, T. J. Chem. Phys. Lett. 1986, 130, 555. (6) The ionization state of the side chain amino groups drives pHdependent conformational changes, the polypeptide being an R-helix above pH 9.7 (7) (a) Cantor, C. R.; Schimmel, P. R. Biophysical Chemistry; Freeman: New York, 1980. (b) Snell, C. R.; Fasman, G. D. Biopolymers 1972, 11, 1723. (8) The term sensor in the following context is not used to indicate a device but rather a chemical system able to recognize a species and to report its recognition (e.g. by spectroscopic variations). (9) Rosenheck, K.; Doty, P. Proc. Natl. Acad Sci. U.S.A. 1961, 47, 1775. (10) Hermann, O.; Mehdi, S. H.; Corsini, A. Can. J. Chem. 1978, 56, 1084. (11) Pasternack, R. F.; Collings, P. J. Science 1995, 269, 935 and references therein. (12) For example, the external binding of tetracationic porphyrins onto natural or synthetic DNA leads to small variations of the absorption spectra. On the other hand, porphyrin intercalation (not possible for our system) or aggregation induces more substantial spectral variations (see, for example: Pasternack, R. F.; Gibbs, E. J. ACS Symp. Ser. 1989, 402, 59).
