Highly ordered surface structure of large-scale porphyrin aggregates assembled from protonated TPP and water

https://doi.org/10.1016/j.molstruc.2014.02.055Get rights and content

Highlights

  • A doublet finding near 1000 cm−1 establishes proton sharing in water–porphyrin complex.

  • The 1608 and 1483 cm−1 frequencies characterize behavior of protonated dimeric complex.

  • Vibrational properties of water confined in aggregates depend on solid or liquid support.

  • Highly ordered structures of the protonated TPP aggregates are found by SEM.

  • Structural features on the surface indicate an intimate relation to liquid water structure.

Abstract

Large-scale aggregates assembled from protonated meso-tetraphenylporphine (TPP) dimers and water have been investigated by IR and resonance Raman spectroscopy and also by scanning electron microscopy (SEM). It was found that the properties of water confined in the aggregates depend on the physical state of the support. When the aggregates were deposited on a solid CaF2 plate, they showed properties consistent with a quasi-crystalline structure. But when the aggregates were dispersed in oil, their IR characteristics were different; the vibration bands of the confined water were like those of water in liquid state. A doublet at about 1000 cm−1, components of which have been attributed to specific vibrations of H3O+ and H2O bound in the structure of water-porphyrin dimeric complex, was found in IR and resonance Raman spectra (λex = 441.6 nm) of protonated TPP aggregates. This doublet indicates the hydrogen ion involving in the vibrational system of water-porphyrin dimeric complex with hydrogen bonding by similar way as in so-called Zundel cation. The resonance Raman spectrum shows evidence for proton sharing between protonated water dimer and single bondNdouble bond groups of the pyrrole rings. SEM results indicate that the large-scale aggregates of the protonated porphyrin possess highly ordered structure, are only observed when using extremely pure water.

Introduction

The idea of the cooperative nature of interaction between water molecules in the liquid state was suggested by Frank and Wen [1]. Their “flickering cluster” model of the structure of liquid water in support of cooperativity of the water structure was based on chemical and theoretical arguments. The concept of “hydrophobic bond” [2] was proposed to explain cooperativity between proteins and water molecules. According to this concept, the stability of a unique spatial protein structure can be explained by the presence of hydrophobic bonds. Later the concept was revised and the term “hydrophobic interactions” was adopted instead. Despite detailed investigations of liquid water structure [3], [4], [5], [6], [7], the problem of protein stability and the nature of protein cooperative interaction with water have not been fully clarified. Although it is already clear that the properties of water clusters [8] are responsible for such interactions. Many similar phenomena as water clusters containing hydrogen ions are continued to be of interest [9]. The wave-cluster model was proposed to explain the stability of unique protein structures, the behavior of small clay particles, and for the formation of array of water clusters [10], [11], [12], [13]. It has been suggested that the model can explain such phenomenon as self-organization of TPP dimeric complexes into large-scale aggregates of microscopic proportions.

The formation of such large-scale aggregates [14] assembled from water molecules and protonated TPP dimers (see the Scheme 1) was observed by scanning electron microscopy. The neighboring porphyrin molecules in the dimer are connected to each other via water molecules so that mono-protonated and di-protonated TPP dimers are usually generated in solution. The results obtained in this study indicate that although there are many different models for liquid water, only the wave-cluster model can explain self-organization of an object of microscopic proportions consisting of porphyrin and water. However, further studies are needed to clarify the properties of large-scale aggregates assembled from different protonated TPP dimers that have been described. These water clusters containing aggregates with the size of living cells can be used to mimic interactions in living cells to understand the underlying principles of the structural organization of biomacromolecules. Furthermore, these aggregates can be used as model systems for the study of self-organization of supramolecular structures and for realization of practical applications such as investigation of photosynthetic processes, water-clay mixtures and the development of new materials.

In the present work large-scale aggregates assembled from protonated TPP dimers and water were investigated by IR and resonance Raman spectroscopy and also by scanning electron microscopy.

Section snippets

Materials and methods

The synthesis of meso-tetraphenylporphine was carried out according to procedures described elsewhere [15]. Tetrahydrofuran (THF) and other chemicals and organic solvents were of high-grade purity. Highly purified de-ionized water with a resistance of more than 18 MΩ was used for protonated TPP aggregates formation in aqueous THF solutions for microscopic purposes. Distilled or twice distilled water was used in a similar procedure for the preparation of aggregates for infrared spectra

Results

The electronic absorption spectra of protonated TPP aggregates were recorded to characterize the nature of the aggregates in aqueous-organic solution. In general four main bands are observed in aqueous THF solution (Fig. 1). The broad band in the UV region with a maximum at ca. 360 nm indicates the protonated state of porphyrin. Two Soret bands with the maxima at 403 nm and 465 nm (±2 nm) belong to mono-protonated TPP dimers with different configurations of the porphyrins in the dimers (see the

Conclusions

The results presented above show that protonated TPP aggregates consisting of only water and protonated porphyrin form a solid structure when immobilized on CaF2 plates that contains a quasi-crystal structure of water confined in the aggregates. However, when the aggregates were placed in water-free oil, i.e. in a liquid support, some characteristics, in particular, shape of the band of stretching vibrations were changed. Changes are found for other bands of the confined water too; shape and

Acknowledgement

The European Union supported this work under the INTAS programme Grant No. 01-2101.

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