El articulo está disponible en: http://pubs.acs.org/cgi-bin/article.cgi/bichaw/2007/46/i51/pdf/bi701440z.pdf. La editorial no permite la inclusión del articulo en un repositorio institucional, pero está disponible on-line para quien tenga acceso a la suscripción.A pigment-deficient reaction center of photosystem II (PSII)-with all the core pigments (two molecules of chlorophyll a and one of pheophytin a in each D protein) but with only one molecule each of peripheral chlorophyll a (Chlz) and beta-carotene (Car)-has been investigated by pump-probe spectroscopy. The data imply that Car and Chlz are both bound to D1. The absence of Car and Chlz in D2 allows the unprecedented observation of secondary electron transfer in D1 of PSII reaction centers at room temperature. The absorption band of the Car cation in D1 (Car(D1)(+·)) peaks around 910 nm (as against 990 nm for Car(D2)(+·)), and its positive hole is shared by ChlzD1, whereas Car(D2)(+·) can disappear by capturing an electron from ChlzD2.This work was funded by the Research Council of Norway, the Spanish Ministry of Science and Education (Grant No. BFU2004-04914-C02-02), the Czech Ministry of Education, Youth and Sports (Grant Nos. MSM6007665808 and AV0Z50510513), and the Spanish and Czech bilateral collaboration program (Grant No. CZ2004001).Peer reviewed