Steric interaction between the peripheral substituents of 10(S)‐chlorophyll derivatives and its conformational consequences: A proton magnetic resonance study

Abstract

The improved methods developed recently for the preparation of lo(@-chlorophyll derivatives have permitted a detailed comparison of their conformations with those of the corresponding 10(R) derivatives, utilizing ' H NMR spectroscopy. Starting from the highly purified lo(@ derivative and following its conversion to the corresponding 10(R) form by repeated ' H NMR spectral measurements in acetone-&, reliable values for the differences in the chemical shifts (As) for each epimeric pair of compounds have been obtained. The relatively large A8 values (0.1-0.2ppm) observed for the P-l-CH,, P-H-2, P-3a-CH3 and 10b-CH3 signals were interpreted as indicating the proximity of the C-10 methoxycarbonyl group and the olefinic region of the phytyl group in the lo(@ derivatives. The proximity between these side-chains was considered to be derived principally from the three closely spaced carbonyls in the lo(@ derivatives. To miniize repulsion among the carbonyls, the C-7 and C-10 side-chains presumably assume conformations allowing maximal mutual distances for the carbonyls. As a compromise of various energetic factors, the phytyl group takes up a conformation where it bends to the right hand side of the macrocycle, or above rings V and III. This results in steric crowding between the olefinic region of the phytyl group and the C-10 methoxycarbonyl group. In addition, the other chemical shift increments show that the steric crowding between the bulky side-chains of the lo(@ derivatives increases the steric strain at the periphery of the macrocycle, relieved by pronounced conformational changes in rings IV and V and, to a lesser extent, also in the whole phorbin ring. The conformational differences were found to be larger for the magnesium-free epimers than for the chlorophylls. In the former case, they were also manifested by relatively large A6 values (0.1-0.5 ppm) for the pyrrole NH protons.

A 3-D Structural Model of Solid Self-Assembled Chlorophyll a/H2O from Multispin Labeling and MAS NMR 2-D Dipolar Correlation Spectroscopy in High Magnetic Field

“…Note. Compiled information from this work and published sources (23,30,31 ). The numbering is according to Fig.…”

“…A low concentration ensured that the Chl a exists in the monomeric form, in line with previous work (30,31). By using newly obtained and published data, a complete set of 1 H and 13 C shifts σ H liq , σ C liq for a monomer in solution was produced ( Table 2).…”

“…By using newly obtained and published data, a complete set of 1 H and 13 C shifts σ H liq , σ C liq for a monomer in solution was produced ( Table 2). The shifts in solution compare well with literature data reported previously for chlorophylls in various solvents (23,30,31 ), and were used for the calculation of aggregation shifts σ = σ − σ liq ( Table 1). The aggregation effects on the chemical shift and the resulting σ are analyzed in terms of classical ring-current calculations which are sufficiently accurate for describing the observed shift differences (18).…”

Chlorophyll synthetase cannot synthesize chlorophyll a′

“…Peak 2a is probably chlorophyll a esterified with cis-phytol (see Fig. 5 (Hynninen and Lotjonen 1985) considering the low-field shift of the 13'-H signal introduced by pyridine (Abraham et al, 1989). To see whether chlorophyllide a'can be esterified at all, we incubated a mixture of chlorophyllide &a' (containing 3 0 2 3 % a?…”

Supramolecular Structures of the Chlorophyll a‘ Aggregate and the Origin of the Diastereoselective Separation of Chlorophyll a and a‘

“…Chl a′ is thermodynamically less stable than Chl a due to the steric hindrance between these bulky substituents (the equilibrium Chl a′ mole fraction ) 0.24, see ref 17). Though visible absorption 18,19 and fluorescence spectra (Oba et al, unpublished results) and redox properties (Mazaki et al and Iijima et al, unpublished results) of Chl a and a′ are almost the same due to a common π-conjugated system, the epimers are distinguished from each other by NMR 20,21 and CD spectra, 18,19 rate of pheophytinization (demetalation), [22][23][24] reducibility of the C13 1 -keto carbonyl moiety, 24 and elution behaviors in high-performance liquid chromatography (HPLC). 25 Hynninen and co-workers reported that Chl a′ was more soluble 23 and had a somewhat weaker intermolecular aggregation tendency in nonpolar organic solvents than Chl a.…”