Temperature dependence of absorption spectra of chlorophyll a in hydrocarbons over the temperature range 300-77K. Enthalpy and entropy changes for the formation of dimeric chlorophyll a.

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The absorption spectra of chlorophyll a in isopentane, 3-methylpentane, and methylcyclohexane solutions were studied over the temperature range 300–77 K. In those hydrocarbon solutions which were prepared using methanol treatment, chlorophyll a forms hydrated dimer at low temperatures. The dimer has an absorption peak at 695 nm at 160 K, which gradually shifts to 706 nm with decreasing temperature from 160 K to 77 K. The enthalpy change (–ΔH) for the formation of the dimer was obtained to be 41.0±0.8 kJ mol−1, independent of the solvent used. The entropy changes (–ΔS) were 113±8 J K−1 mol−1 for isopentane, 118±8 J K−1 mol−1 for 3-methylpentane and 128±8 J K−1 mol−1 for methylcyclohexane solutions. Comparative studies carried out of the solutions prepared without the methanol treatment suggest the existence of two kinds of hydrated monomeric chlorophyll a.
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