Electronic Absorption Bands associated with the Hydrogen Bondings of Thymine and Uracil

Abstract

IN the course of investigations of the electronic absorption spectra of crystals of DNA bases, we have found that an unusual band appears in addition to the usual π → π^* transition when the crystal has the hydrogen bonding. This absorption band is observed in thymine monohydrate, thymine, uracil and 5-ethyl 6-methyl uracil crystals; however, it is not found in calcium thymidylate and barium uridylate¹. In the former class of crystals, hydrogen bondings of the type are present, while in the latter such bondings of the typeare present, while in the latter such bonding are not present, because the bases are separated from each other. Fig. 1a shows a typical absorption spectrum for a crystal of thymine monohydrate; for comparison, Fig. 2 shows the absorption spectrum of calcium thymidylate. Both compounds show similar absorption spectra in solution at about 2640 Å, and corresponding bands occur at about the 2600∼2750 Å region in all such crystals. The extra band at 2850 Å in thymine monohydrate crystals is considered to be a new type, because it is observed only in the particular crystalline state. It was possible to determine the direction of the transition moment of the 2650 Å band (π→π^* transition) from the polarization measurement of all these crystals. It was found to be inclined at about 20° from the N₁ - - - C₄ line towards N₃ atom (compare with Fig. 3). This result is nearly in agreement with the earlier results of Stewart and Davidson² on 1-methyl thymine, who regarded this angle to be either 14° or −19°.