Exploring the coupling between electron transfer and protein dynamics in photosynthetic reaction centers embedded into dehydrated amorphous matrices

The interplay between protein dynamics and electron transfer (ET) has been extensively investigated in the bacterial photosynthetic reaction center (RC) from Rhodobacter sphaeroides by hampering RC internal motions at low temperatures [1]. Alternatively, the RC dynamics can be inhibited at room temperature by incorporating the RC into dehydrated trehalose matrices [2]. In the glasses the recombination kinetics of the charge separated state P+QA are accelerated and distributed in rate as compared to solution, mimicking at room temperature the effects observed at 10 K in water-glycerol. This is taken to indicate inhibition of the RC relaxation from the dark- to the light-adapted conformation, as well as of the RC thermal fluctuations [1,2]. We proposed that the inhibition is mediated by residual water molecules of the RC hydration shell which bridge protein surface groups with trehalose molecules of the matrix by forming a network of multiple hydrogen bonds [3]. Consistently, similar effects have been observed also in RC films dehydrated in the absence of sugar [4]. However, striking differences are found between RC-trehalose glasses and RC-films: (a) The thermal stability of the RC is tremendously enhanced in the trehalose matrix. (b) In RC-trehalose matrices, the P+QA recombination after a few seconds of continuous photoexcitation is only partially decelerated as compared to the one recorded after a laser flash, whereas in dried RC films a comparable period of continuous illumination leads to a total recovery of the kinetics observed in the hydrated system. These data indicate that in films the protein dynamics can be easily regained, in contrast to trehalose glasses, which reveal much stronger structural constraints. We are extending these studies to a series of RC mutants characterized by a widely altered P+/P midpoint potential relative to wild-type [5]. In these mutants, the acceleration of P+QA recombination induced by cooling to 10 K in the dark decreased with increasing midpoint potential [6]. Interestingly, incorporation into dehydrated trehalose matrices causes instead acceleration of the kinetics by the same factor for all P+/P midpoint potential values.