You will be presented with the full text of a science research paper. Please write an abstract for this paper. Your response should include the abstract and no additional text.

Paper text:
a recent frontier in physics is to investigate organic and living matter , trying to formulate models able to capture their main features .
this investigation has the double mark of being in itself interesting due to the lack of knowledge in the subject and of exploiting wide potentialities in the production of a new generation of electronic devices . as a matter of fact
, some specific tasks of sensing protein can be reproduced in artificial devices to improve their performances .
it is the case of nano - biosensors which aim to mimic _ in vitro _ the peculiar sensing abilities shown _ in vivo _ by proteins and organic materials .
accordingly , much attention has been devoted to transmembrane proteins ( tmps ) , which interface external and internal cell environments .
specifically , tmps are proteins consisting of one or several helices crossing the cell membrane and able to modify their structure when capturing a specific molecule or a small protein ( ligand ) . among available experiments , here we report on those performed on two different seven - helix tmps , bacteriorhodopsin ( br ) and proteorhodopsin ( pr ) , proteins able to transform energy coming by light in energy useful for the cell survival .
therefore , they appear very interesting to work out bio - solar - cells and/or bio - diodes . to this purpose ,
recent measurements on the electrical properties of br and pr have revealed the possibility to use them _ in vitro _ like organic transistors . in particular , they are almost insulators at the lowest values of the applied bias , and their conductance suddenly increases for orders of magnitude , above some threshold value where they enter a sharp super - ohmic regime . furthermore , in the presence of green light , i.e. after a conformational change , the conductance exhibits a further increase above the dark value , thus confirming the hypothesis of a structure - mediated charge transfer .    to provide a microscopic model of the main features described above
, we developed a numerical approach based on the analogy between a single protein and an impedance network .
the charge transport is obtained by introducing a procedure of stochastic changes of the network impedances .
the investigations are performed on the br , whose current - voltage ( i - v ) responses were carried out on a wide range of applied voltages @xcite , then extended to pr @xcite , on which experiments are only at the preliminary stage of low applied bias .
former experiments were carried out on br , probably the best known protein in the seven - helix tmp family .
samples of br in its natural membrane were used in a metal - insulator - metal structure @xcite and their i - v characteristics measured in dark @xcite and also in green light @xcite .
the samples exhibited a small conductivity ( less than 1 ps / cm ) at low applied field .
this conductivity was found to increase as the protein sample is irradiated with green light and/or when the electrical field overcomes a critical value . in particular , green light produces a higher current with respect to dark conditions , as a consequence of the modification of the chromophore - opsin complex @xcite .
the super - linear i - v characteristics point toward a tunneling charge - transfer mechanism @xcite . in particular , a transition between a direct tunneling ( dt ) regime and a fowler - nordheim ( fn ) tunneling regime is revealed ( see )
. first measurements of i - v characteristics in pr were reported in ref .
the experiment was performed on a thin - film device with a contact distance of 50 @xmath0 , just about @xmath1 times longer than the br nanolayers used in the previous experiments @xcite .
the film was obtained by dehydrating pr in a buffer solution .
the protein activity after dehydration was tested by absorbtion measurements that confirmed the preservation of the structure and the function after the treatment .
accordingly , the experiments performed on br and pr evidence the possibility to obtain , _ in vitro _ like so _ in vivo _ , a significant photocurrent , a result of great interest for optoelectronic applications .            following the indication of experiments @xcite ,
we assume that charge transfer through the amino acids of the single protein is the main mechanism of transport .
therefore , a microscopic interpretation of experiments is given in terms of the single protein electrical properties , taken as intrinsic characteristics of the protein morphology .
to this purpose we make use of the impedance network protein analogous ( inpa ) technique already detailed in @xcite . in short , each protein is associated with a topological network , consisting of nodes and links , as pictured in .
each node corresponds to a single amino acid , and its position is taken as that of the related @xmath2 atom @xcite .
each couple of nodes is connected with a link when their distance is less than an assigned interaction radius , @xmath3 ( usually in the range @xmath4  ) , so that the network captures the protein topology .
the graph turns into an impedance network when an elemental impedance is associated with each link . in the present case of a static response
only resistances are considered .
accordingly , the elemental resistance between the @xmath5-th and @xmath6-th nodes , say , @xmath7 is taken as @xcite : @xmath8 , where @xmath9 , is the cross - sectional area between the spheres centered on the @xmath10 nodes , respectively , @xmath11 is the distance between these nodes , @xmath12 is the resistivity . by positioning the input and
output electrical contacts on the first and last node , respectively , for a given applied bias the network is solved within a linear kirchhoff scheme . by construction , this network produces a parameter - dependent static i - v characteristic . to account for the super - linear current at increasing applied voltages , a tunneling mechanism of charge transfer
is included by using a stochastic approach within a monte carlo scheme@xcite . in particular ,
the resistivity value of each link is chosen between a low value @xmath13 , taken to fit the current at the highest voltages , and a high value @xmath14 , which depends on the voltage drop between network nodes as : @xmath15 where @xmath16 is the maximum resistivity value taken to fit the i - v characteristic at the lowest voltages ( ohmic response ) and @xmath17 is the height of the tunneling barrier between nodes .    )
, @xmath18 is the variance @xcite of x. ]    ) , @xmath18 is the variance @xcite of x. ]    the transmission probability of each tunneling process is given by : @xmath19 \   \hspace{0.06 cm }   ( ev_{i , j }   < \phi ) , \hspace{0.4 cm } \mathcal{p}^{fn}_{ij}=\exp \left[-\alpha\ \frac{\phi}{ev_{i , j}}\sqrt{\frac{\phi}{2 } } \right ] \    \hspace{0.06 cm }   ( ev_{i , j } \ge
\phi ) \   \label{eq:1}\ ] ] where @xmath20 is the potential drop between the couple of @xmath10 amino acids , @xmath21 , and @xmath22 is the electron effective mass , here taken the same of the bare value .
the investigation is carried out through a comparative analysis of pr and br properties .
the inpa model is calibrated on the experiments performed on br monolayers @xcite . in particular , starting from the experimental i - v characteristic @xcite the model parameters used to get the best agreement are : @xmath23  , @xmath24 j , @xmath25   , and @xmath26    @xcite . reports the i - v characteristics calculated for a single protein of br and pr , on a wide bias range . in the common range of applied voltages
, calculations agree with experiment when properly rescaled to take into account the sample size .
reports the data on scale axes appropriate to evidence the transition between dt and fn tunneling regimes , the latter being evidenced by a sharp increase ( over ten orders of magnitude ) at high voltages of the dependent variable .
the average chord - conductance , @xmath27 , is calculated over a large ensemble of configurations , and is taken as a measurable global - quantity and order parameter in recent investigations @xcite . as a further step ,
the conductance fluctuations which emerge as a result of the same numerical simulations are estimated .
indeed , these fluctuations are essential to determine the average value of the global quantity @xmath28 since they are particularly high close to the transition between the dt and fn regimes , as generally expected near to a phase transition .
indeed , the probability distribution function ( pdf ) of these fluctuations , normalized to its first and second moment , is found to be well described by the generalized gumbel distribution , @xmath29 , @xmath30 where @xmath31 is a positive number , @xmath32 the variance , @xmath33 with @xmath34 .
furthermore , @xmath35 , with @xmath36 , @xmath37 and @xmath38 indicating the gamma , digamma and trigamma function , respectively . finally , @xmath29 is a normalized pdf with zero mean and unitary variance ; it is a skewed function , recently used to describe finite - size systems in both critical and not critical conditions @xcite .
the numerical value of the shaping parameter @xmath31 is related to the system internal dynamic @xcite . in the present case , the lack of a gaussian behaviour is due to the existence of a low and upper bound on the conductance values @xcite .
the values of @xmath31 which , in a wide range of applied bias , better describe the pdfs for br and pr , are @xmath39 and @xmath40 , respectively ( see ) .
integer values of @xmath31 allow to interpret the @xmath29 as the distribution of the @xmath31-th minimum / maximum of the independent variables , i.e. @xmath39 should be correlated with the existence of a single dominant conductance value . otherwise , the interpretation of on integer @xmath31 value remains an open question .
microscopic investigations of the i - v and conductance characteristics of the two light receptors br and pr reproduce available experiments and predict the pr behaviour on a bias range not yet explored . from experiments , the protein activity after dehydration was tested by absorption measurements that confirmed the preservation of the structure and the function after the treatment . from theory ,
the pdf of the fluctuations of the conductance , @xmath41 , taken as order parameter are found to follow the generalized gumbel distribution @xmath29 , for both proteins .
these results confirm the inpa approach as a powerful numerical method to investigate protein electrical properties on the basis of their tertiary structure .
19 i casuso , l fumagalli , j samitier , _ et al .
_ , h melikyan , b -e khishigbadrakh , a babajanyan , _ et al .
_ , y jin , n friedman , m sheves , _ et al .
e alfinito , j -f millithaler and l reggiani h m berman , j westbrook , z feng , _ et al . _ a noullez , and j -p pinton , ; s c chapman , g rowlands and n w watkins , e alfinito and l reggiani ,
we report a theoretical / computational approach for modeling the current - voltage characteristics of sensing proteins . 
 the modeling is applied to a couple of transmembrane proteins , bacteriorhodopsin and proteorhodopsin , sensitive to visible light and promising biomaterials for the development of a new generation of photo - transducers . 
 the agreement between theory and experiments sheds new light on the microscopic interpretation of charge transfer in proteins and biological materials in general .    
 # 1#2#3#4#1 _ # 2 _ * # 3 * # 4