Absrraci-Electrochemical properties of biological molecules and their interaction with sulfhydryl radioprotectors as WR-1065 or glutathione were investigated (in absence of any applied electrical field) b y an analogic noise spectrograph built in our laboratory. Now we present a digital noise spectrograph permitting to precisely determine the statistical properties of the electrical signal due to ionic brownian motion. This new device, devoted to numerical signal analysis, is able to show off the permanent moment of biological molecules or the modification of molecule behaviour when a drug is added.In recent years we have investigated the principal properties of sulfhydryl compounds and their interaction with DNA or lipids taken as membrane models [I]. We have particuiarly studied the competition between the strongest radioprotector WR-1065 and glutathione [2]. We have evaluated their intrinsic properties in absence of any applied electrical field, using an analogic noise spectrograph built in our laboratory [31,[4]. Now we present a digitai noise spectrograph permitting to precisely determine the statistical properties of the signal.Brownian motion of charge Carriers produces an elecmcal noise. The noise generated by the studied solutions is analysed by a noise spectrograph built in our labomtoy. The Set up is shown schematically on figure 1. B Fig. 1. Blockdiagram of the noise spectrographThe random voltage n(t) produced by the solution or by standard resistors is treated immediately by two identical parallel measurement devices.In both channels A and B, n(t) is amplified and fiitered at a frequency U with a bandwidth : 2Au = 0.10 D . If nA(t) and ng(t), the two amplifiers proper noises are taken into account. the voltages at the multiplier inputs are : NA(t) = G[n(t) + n~(t)l and NB(t) = G[n(t) + ng(t)lwhere G is the total amplification gain in each channel. The NA(t) and Ng (t) C r o s s -c o r r e~~ function iS : R N~N~ (2) = &A(t + T)NB(t)> Because nA(t) and ng(t) are nofi-correIated and independant of n(0. R N~N B (~) = G~R~w where R&) is the auto-comiation function. By this technique it is possible to eliminate the device's parasitic proper noises. The multiplier-integrator computes the value of the NA(t) and Ng(t) cross-correlation function R N * N~( T ) for z = 0 : so we obtain the autocorrelation function of the noise produced by the studied solution in the band [U k AV] : Rn(0) = in the band [U +-AV]. DIGITAL SPECIROGRAPH Now we present a digital noise spectrograph allowing to determine the second order moment of the signal with precision. This spectrograph includes a microprocessor AT 386 DX 25 MHz with a 106 MO disk and a Data Acquisition Processor (DAP) Microstar 240014 allowing A/D conversion with a maximum sampling rate of 156,000 sampledsecond and a 12 bits resolution. A lot of standard commands are available from any high level language [SI : the mai...