Fine tuning of copper(ii)–chlorophyll interactions in organic media. Metalation versus oxidation of the macrocycle

Abstract

The nature of chlorophyll interactions with copper(II) ions varies considerably in organic solvents, depending on the dominant coordinative form. Besides formation of the metallo tetrapyrrolic complex, Cu(II) ions can cause oxidation of the pigment, reversible or irreversible, which can lead to the destruction of the macrocyclic structure. All these reaction types can be distinguished within a quite narrow range of reaction conditions. The ability to form new metallo derivatives in either metalation or transmetalation reactions is obviously limited by the concentration of the potential oxidant, but can be secured below this level via suitable composition of the reaction system. The decisive factor in the selection of a specific reaction pathway is the presence of a potential ligand that can affect the reactivity of Cu(II) for example by shifting its redox potential. Spectroscopic and electrochemical studies were performed in order to determine the predominant species of Cu(II) in methanol, nitromethane and acetonitrile in the presence of chloride and acetate ions, as well as to assign their appropriate oxidizing ability. This allowed us to estimate the boundary conditions for the electron transfer processes in chlorophyll-Cu(II) systems. Chlorophyll and its free base can undergo both types of electron transfer processes, however, they reveal different susceptibilities that make this class of ligands quite versatile markers in tuning the reactivity of metal ions in solutions. IntroductionInteractions of chlorophylls (Chls; including chlorophyll a, Chla, its free base pheophytin a, Pheoa, and other derivatives) with metal ions, although not strictly related to their basic biological functions, are important for proper metabolism of the photosynthetic pigment. They are decisive not only in biosynthesis of Chla (Fig. 1) at the stage of enzyme-driven Mg 2+ incorporation into protoporphyrin IX (3,7,12,17-tetramethyl-8,13-divinyl-2,18-porphinedipropionic acid), [1][2][3][4] but also in the spontaneous substitution of metal ions occurring in plants growing on soils contaminated by heavy metals. 5 The latter effect, observed clearly for Cu 2+ , Zn 2+ , Cd 2+ and Hg 2+ , results in the irreversible loss of photosynthetic abilities. [5][6][7] In recent years, deeper insight into the mechanisms of metal ion -Chl interactions has become important also for the development of procedures for modification and synthesis of new tetrapyrrolic complexes in terms of their potential biomedical applications. [8][9][10] Although the primary function of chlorophylls is related to electron transfer processes, their occurrence in biological systems is not due to pigment interactions with redox-active metal ions. Properties of natural photosystems, View Article OnlineView Journal | View Issue including the composition of endogenous ligands, prevent degradation of the pigment's macrocyclic structure in the presence of Cu 2+ and other potential metallic oxidants.Displacement of the chlorophyllic central ion in vivo is limited to a few me...

Factors controlling the reactivity of divalent metal ions towards pheophytin a

“…The metalations in DMF are generally slower ( k 1 values between 1.3 × 10 −6 and 1.4 × 10 −2  M −1  s −1 ) and Co 2+ and Ni 2+ show no sign of insertion after 24 h. Pb 2+ turns out to insert slightly faster than most of the other M 2+ . The insertion of Cu 2+ (triflate) is the fastest in the whole series but an excess of it leads to Pheo a degradation, due to the high oxidation potential of this cation [52]. The broadest range of metalation rates was observed in ACN, in which hardly any reaction can be observed with Cd 2+ , the formation of Co–Pheo a is slower than in MeOH ( k 1  = 3.9 × 10 −4 vs. 7.2 × 10 −2  s −1  M −1 ) and the insertion of Zn 2+ occurs particularly fast ( k 1  = 2.6 × 10 0  s −1  M −1 ).…”

“…2), which indicates the contribution of at least two processes to the overall kinetics. The minor one is either a slow macrocycle degradation [52] or it is a parallel formation of Cu 2+ –Pheo a , e.g. by some less-populated metal species present in a particular solvent [36, 52].…”

“…The minor one is either a slow macrocycle degradation [52] or it is a parallel formation of Cu 2+ –Pheo a , e.g. by some less-populated metal species present in a particular solvent [36, 52]. The slower components, contributing to , may be disregarded in the subsequent discussion, because .…”

“…All the other systems can be satisfactorily approximated to the simple rate law (Eq. 3), in which the k 2 term can be assigned to a parallel rather than a reverse reaction due to the following: (1) the kinetics profiles are independent of [M 2+ ], (2) M 2+ –Pheo a complexes in non-acidic solutions are stable [29, 36, 57, 58], and (3) the metal ions used have complex speciation in organic solvents [36, 52]. The [M 2+ ]-independent reaction most likely relates to metalation involving minor coordinative forms of M 2+ .…”

Cu and Na contents regulate N uptake of Leymus chinensis growing in soda saline-alkali soil

“…The correlation between Cu and N is more significant in saline-alkali grassland sites with 70%-40% cover. Cu is a metal with unique catalytic properties [42] that can form complexes with pigments and stabilize chlorophyll function [43] and can also help activate Kdependent stress resistance mechanisms that confer tolerance of saline-alkali stress and promote leaf growth in L. chinensis. The direct path coefficient of K and N was 0.43 in sites with 70%-40% cover and 0.6 in sites with 40%-10% cover.…”