1,3-dimethylisoguanine, a new purine from the marine sponge amphymedon viridis - Chehade, C C; Dias, R. L. A.; Berlinck, Roberto Gomes de Souza; Ferreira, A G; Costa, L V; Rangel, M; Malpezzi, E L A; Freitas, J. C.; Hajdu, E (Journal of Natural Products, 1997)