30 A. Meybeck Table IV. E.S.R. characteristics of melanoidins obtained by reaction of DHA with aminoacids for 4 h at 100øC Linewidth Spins/g Aminoacid O ( x 10 -•*) Gly 7.5 3.3 Gly+ trieth. 7 4-5 Gly+ Na OH 6.5 2-2 Ala 7 3.2 ¾al 7 3.2 Leu 7 5.9 Ser 7 3'5 Asp 6 2.1 Glu 6-5 1.5 Glu+ Na OH 6 1.8 Arg 6.5 0'4 Lys 9 3'5 Phe+ Na OH 7 5.6 Tyr + Na OH 6 2.7 3-aminopropionic 8.5 6'0 4-aminobutyric 11 10.0 5-aminopentanoic 7 4.0 6-aminohexanoic 11.5 15.0 8-aminooctanoic 9 2.6 Figure 3. E.S.K. spectra of synthetic melanin and of melanoidins prepared by reacting DHA with glycine, alaninc, valine, leucine, serine, aspattic acid, glutamic acid, lysine and 6-aminohexanoic acid at 100oC (37•C for glycine and lysine). All the samples have been purified by dialysis and have been examined as powders. 6-9 G range. Recording the spectrum of DHA and glycine melanoidin sample under vacuum, did not change the signal significantly and it is assumed therefore that these data, which were obtained in air, are representative of the actual linewidths without any line broadening by oxygen. As far as free spin contents are concerned, they fall in the 10 •s

Reaction products of dihydroxyacetone 31 spins/g range except for arginine which gave a lower value. Two of the melanoidins obtained with non-alpha aminoacids have a slightly higher linewidth and a stronger signal. II. REACTIONS CARRIED OUT AT 37øC In order to better investigate the possible reactions occurring actually in the Stratum Corncure, a few experiments have also been made at 37øC with the same starting con- centrations of DHA and aminoacids in water: 0.1 mole of each reactant (0-05 for Lys) in 25 mi. Only glycine and lysine did produce significant amounts of pigments after 24 h (Table V). Arginine reacted very poorly, as could be expected in view of the low yield obtained at 100øC. But what is more surprising is that serine and alaninc which react readily with DHA at higher temperature, are almost inert at 37øC. It should be noted, however, that these two aminoacids were not entirely soluble at this temperature in the rather concentrated media used. Table V. Melanoidins formed by reaction of DHA with aminoacids at 37øC Duration U.V. absorption Spins/g Aminoacid (h) •o Yield of 0'01 • water sol. (x 10 -•8) GIy 10 1.2 1'15 at 320 nm 2-5 Gly 24 1-7 1.40 at 310 nm 1-8 Gly 48 4-8 1-50 at 320 nm 3.1 Gly 72 4.7 1.46 at 310 nm 3-2 Lys 24 26.2 0.9 at 330 nm 7-2 Ala 24 0 Set 24 0 Arg 24 0 Underlined wavelengths correspond to a maximum. The yield in melanoidin obtained for lysine as well as its free spin content, show by comparison with the results obtained at 100øC, that the reaction is already in an advanced stage after 24 h at 37øC. For glycine however, 48 h are necessary to obtain the same num- ber of free spins/g, and after 72 h the pigment yield is still much lower than at higher temperature. But in spite of these quantitative differences, the DHA melanoidins prepared at 37 ¸ or 100øC are very similar as far as their U.V., I.R. and E.S.R. spectra are concerned. Discussion Comparison with melanins One of the most important characteristics of melanins, the natural pigments of hair, skin, squid ink... is to give an E.S.R. signal of 4.8 (6) to 8.8 G (7) linewidth depending upon the origin of the sample and perhaps also upon the conditions used in recording the spectra. It corresponds to free spin contents ranging from 5 x 10XS/g (6, 8) to 20 x 10XS/g (9) for Sepia melanin, and from 0.4 x 10•8/g (8) to 2 x 10X*/g (9) for synthetic melanin prepared by oxidation of DOPA. These data are strikingly similar to those of Table IV. Moreover, U.V. visible absorption spectra of melanins which have no special features