HYDROGEN-BONDING OF WATER TO CHLOROPHYLL A AND ITS DERIVATIVES AS DETECTED BY H-1-NMR SPECTROSCOPY

Using H-1-NMR spectroscopy, evidence is presented for the hydrogen bonding of water to the C-13(1) keto carbonyl of chlorophyll a and some of its derivatives. This evidence is based on the observation of a low-field proton signal at approx. 11 ppm from samples prepared by dissolving the water adduct of each phorbin derivative in tetrahydrofuran-d(g). When the temperature of the sample is elevated or lowered, the low-field signal becomes broader or narrower, respectively. After the addition of a small amount of (H2O)-H-2 to the solution, the signal disappears in approx. 10 h at -40 degrees C. Because the pyre derivatives lacking the 13(2)-methoxycarbonyl group and the 'prime derivatives' possessing the 13(2)(S) configuration as well as the Mg-free derivatives of these phorbins, also show a similar low-field proton signal, it is concluded that this signal arises from the 13(1)-C=O...H2O hydrogen bond. The strength of this hydrogen bond is estimated to be approx. 8 kcal mol(-1)(33.5 kJ mol(-l)). The difficulties encountered in the dehydration of chlorophyll preparations are interpreted in terms of this relatively strong hydrogen bond and the central Mg...OH2 interaction of comparable strength. The importance of preserving chlorophyll preparations in their anhydrous and amorphous form is emphasized, because of the higher stability of these forms as compared to the photoactive chlorophyll-water adducts.