the invention described hereinafter relates to the obtaining and applicative use of nanoassembled complexes in the form of nanoparticles comprising a nucleus of polyavidin , obtained by the nucleation of several avidin units onto one or more nucleic acid molecules , then stabilized by the presence of surface protecting agents so as to be able to remain as discrete and stable entities in saline aqueous solution and free from further non - specific interactions . with the nanoassembled complexes of the present invention , discrete nanoparticles are obtained which are stabilized against risks of : a ) aggregation in aqueous saline environments and b ) non - specific interactions with other molecules in solution , by virtue of the presence of protective elements on their surface . said protective elements are themselves present on the particle surfaces in controlled and highly defined quantities . moreover , surface protection according to the preparative method developed by the inventors takes place without destroying the nucleic acid - avidin self - assembled complex and without modifying the total capability of assembled avidins for binding to biotin ( i . e . without modifying biotin binding sites ). the size of these nanoparticles can be established from the length of the nucleic acid which is the assembling nucleus of more avidin units , and accordingly , particles characterized by different sizes and different charges on the avidin can be obtained by suitably varying the size of the nucleating nucleic acid ( na ). the characteristics of said particles are precisely defined and their properties can be modulated by the user by varying : c ) the nature and quantity of the protecting agent present on the surface . for the purposes of the present invention the compounds object of the same are nanoassembled complexes comprising a nucleus obtained by nucleation secondary to a high affinity interaction of several avidin units onto one or more nucleic acid molecules , and stabilized by a biotinylated surface protecting agent , represented by the general formula ( i ) nb n av y ( b - x a - pa b ) z ( i ) nb are the single nuclobases of a single or double stranded nucleic acid ; av is an avidin unit ; b - x a - pa b is the biotinylated surface protecting agent in which pa is a polymer unit having at least one or two functionalizable residues of which one binds , by a covalent bond either directly or through a spacer x , to a biotin residue b by means of its carboxyl functional group ; n is a number varying from 16 to 10 , 000 , 000 ; y is an integer equal to or greater than (≧) 1 and being relative to n can vary from ( 0 . 0001 )· n to ( 0 . 0454 )· n . if a value comprised in the range ( 0 . 0001 - 0 . 0454 )· n is less than (& lt ;) 1 , then y is equal to (=) 1 ; z is an integer equal to or greater than 1 and being relative to y can vary from ( 0 . 02 )· y to ( 4 )· y . if a value comprised in the range ( 0 . 02 - 4 )· y is less than (& lt ;) 1 , then z is equal to (=) 1 ; a is a number varying from 0 to 50 and is preferably comprised from 0 to 10 ; b is a number varying from 1 to 128 . if z is less than 4 , and hence the biotin binding sites present on the nucleus nb n av y are not saturated by binding with biotin b of the protecting agent ( b - x a - pa b ), the nanocomplexes of the invention can bind additional biotinylated compounds , different from the protecting agent , onto said binding sites . consequently , nb means a nucleic acid consisting of a number of nucleobases ( nb ) equal to n , with n varying from 16 and 10 , 000 , 000 , referring to the total number of bases , irrespective of whether the nucleic acid is single or double stranded . preferably the nucleic acid consists of a base number varying from 30 to 100 , 000 and more preferably the base number is from 3 , 000 to 50 , 000 . i ) any sequence of a single stranded ( ss ) or double stranded ( ds ) deoxyribonucleic acid ( dna ) polymer ; ii ) any sequence of a ribonucleic acid ( rna ) polymer in single stranded form or hybridized with a rna or a complementary dna chain ; iii ) a sequence , in accordance with the above points , in which a part of or all the bases have been chemically modified . moreover , the usable nucleic acid for the nanoassembled complexes of formula ( i ) can be in linear or circular form , in a relaxed , coiled or supercoiled state . with reference to the term avidin , avidin is defined as being derived from chicken eggs or another similar source ( eggs of birds in general ) or from recombinant technology , either in glycosylated or deglycosylated form . also included are other chemically or genetically modified avidin forms , provided they can assemble onto a single or double stranded nucleic acid as previously established . in view of the relationship between the number n of nb and the number y of avidin units self - assembling onto the nucleic acid , y is preferably comprised from ( 0 . 0001 )· n to ( 0 . 0357 )· n and more preferably comprised from ( 0 . 01 )· n to ( 0 . 0357 )· n . for example , if n = 10 , 000 , y can vary from 10 to 357 , preferably being from 100 to 357 . if instead n = 100 , 000 , y is comprised from 10 to 3 , 570 and is preferably from 1 , 000 to 3 , 570 . in addition , with reference to the biotinylated surface protecting agent b - x a - pa b : b means biotin ; pa means preferably a linear unit of a hydrophilic polymer of any molecular weight capable of binding to biotin by a covalent bond , either directly or through a spacer x , by means of the biotin carboxyl group . if pa has two functionizable residues , the second of said residues is free or protected by protecting groups known to an expert of the art , for example a methoxyl group . if b is greater than 1 , and hence pa represents a hydrophilic polymer consisting of several polymer units , these latter are joined together by a further ligand having a number of functionalites equal to or greater than 3 3 ) of which one binds to the spacer x or to biotin b and the remaining other functional groups bind to the polymer units pa ; x is a spacer consisting of a bifunctional molecule of general formula ( ii ) y , y ′ being the same or different from each other are coo —; — nh —; — o —; so 2 —; — s —; — so —; co —; cos —; — nh — co —; — nh — co —; hn — so — nh —; r can be an alkyl , an alkenyl , an alkinyl , a cycloalkyl , or an aryl with a carbon atom number comprised from 1 to 20 and preferably from 5 to 20 , also optionally substituted . therefore , the bond between the spacer x and biotin b and that between the spacer x and the hydrophilic polymer pa can be indiscriminately an amide bond , an amino bond , a carbamide bond , an ester bond , a ketone bond , an ether bond , a thioester bond , a thioether bond , an urea bond , a thiourea , sulphonic or sulphoxide bond . in view of the relationship between the number y of avidin units and the number z of biotinylated surface protecting agent b - x a - pa b units , z is comprised from ( 0 . 02 )· y to ( 4 )· y , and preferably is comprised from ( 0 . 4 )· y to ( 4 )· y . for example : in a particle with n = 10 , 000 and y = 357 ( 0 . 0357 · n ), z varies from 7 to 1 , 429 , and preferably from 143 to 1 , 429 ; in the case of a particle with n = 10 , 000 and y = 100 , z varies from 2 to 400 and , more preferably , from 40 to 400 ; in the case of a particle with n = 50 , 000 and y = 1 , 786 (= 0 . 0357n ), z varies from 36 to 7 , 143 and , more preferably , from 714 to 7 , 143 ; in the case of a particle with n = 50 , 000 and y = 500 ( y = 0 . 01n ), z varies from 10 to 2 , 000 , and more preferably from 200 to 2 , 000 . in the nanoassembled complexes of formula ( i ) of the present invention , the polymer units pa are biocompatible and preferably hydrophilic polymers and are known polymers ( owens d e and peppas na 2006 ref . cit .) in which the polymer unit pa has a molecular weight preferably comprised from 400 to 40 , 000 and more preferably from 1 , 000 to 20 , 000 . said polymer units are preferably selected from the group consisting of polyethylene oxide or polyethylene glycol ( peo or peg ) also optionally substituted , a copolymer of polyoxyethylene and polyoxypropylene ( peo - ppo ), polyvinylpyrrolidone ( pvp ), polyacryloylmorpholine ( pacm ), a polyoxamine , a polylactide ( pla ), a polyglycolide ( plg ), a copolymer of lactic acid and glycolic acid ( plga ). more preferably the polymer pa is a substituted polyoxyethylene ( peo ) and is therefore characterized by the following formula ( iii ): r 1 , r 2 , r 3 and r 4 can be independently equal to hydrogen , alkyl , cycloalkyl , aryl , alkenyl , alkinyl , alcoxyl , thioalkoxy , aryloxy and thioaryloxy if the polymer consists of several polymer units , and these are bound together by a polyfunctional ligand with functionality equal to or greater than 3 (≧ 3 ), said ligand can be lysine , glutamic acid , aspartic acid , cysteine , a dendrimer . the term “ dendrimer ” means a symmetrical macromolecular compound consisting of branches repeated around a central core consisting of a smaller molecule or a polymeric nucleus . the functional groups present outside the dendrimer , whose number depends on its number of branches , are themselves functionalizable with other molecules including , for example , pa polymers . furthermore , if the polymer unit pa is bifunctional , it can further covalently bind , through a second free functional group , to a compound suitable for the uses pursued with the nanoassembled complex , and in particular compounds selected from ligands , sugars , chromophores or fluorophores , drugs , chelating agents for radionuclides , peptides , antibodies , proteins , enzymes and the like . the preparation of the nanoassembled complexes of the invention comprises three successive steps in aqueous solutions : in the first step nanoparticles consisting of only avidin and nucleic acid are obtained , constituting the central nucleus of the complexes of the invention . the two subsequent steps comprise optionally preparing the biotinylated surface protecting agent b - x a - pa b but mainly adding said surface protecting agent to the nucleic acid - avidin nanoparticles obtained in the first step . therefore , the method for preparing the nanoassembled complexes of general formula nb n av y ( b - x a - pa b ) z ( i ) comprises at least the steps of : a ) preparing the self - assembled primary nucleus nb n av y by mixing avidin av with nucleic acid in predefined stoichiometric molar ratios of avidin to nucleobases ; b ) mixing the biotinylated surface protecting agent b - x , — pa b with the previously obtained primary nucleus . optionally , preparation of the nanoassemblies of the invention can also comprise preparation of the biotinylated surface protecting agent b - x 2 - pa b . the first step is undertaken by mixing , under stirring , the solutions of avidin and nucleic acid , preferably both in salt - free water . in this first step the molar ratios of avidin to nucleobases nb is within the range from 0 . 44 to 0 . 0001 and preferably from 0 . 133 to 0 . 0044 , and more preferably 0 . 044 . the reagents are mixed under continuous stirring at a temperature from 0 to 50 ° c . for a time between 1 and 600 seconds . the biotinylated surface protecting agent b - x a - pa b is prepared by synthesis or , if commercially available , is purchased . preparation of b - x a - pa b by synthesis involves conjugating the biotin molecule to the polymer pa b by chemical means , using classical bioconjugation techniques known to any expert of the art . subsequently , the previously prepared or purchased biotinylated surface protecting agent b - x a - pa b is added in a stoichiometrically controlled quantity relative to the concentration of biotin binding sites present in the solution , which are themselves relative to the avidin concentration . the molar ratios of avidin : b - x a - pa b are hence comprised between 4 and 0 . 02 . addition of the biotinylated surface protecting agent b - x a - pa b is also carried out under stirring in aqueous solutions at a controlled temperature from 0 to 50 ° c . for a time between 1 and 120 minutes . moreover , the nanoassembled complexes of the invention can be prepared by a method in which steps a ) and b ) are substantially inverted , hence the preparation method can comprise : a ) adding the biotinylated surface protecting agent b - x a - pa b to the avidin in pre - defined stoichiometric molar ratios of biotin to avidin ; b ) adding nucleic acid to the conjugate av y ( b - x a - pa b ) z obtained in the preceding step in pre - defined stoichiometric molar ratios of avidin to nucleobases . the preparation conditions are the same as those previously described for the first method . if necessary , as well as the aforementioned steps , whether the nanoassembled complexes are prepared by the first or second process , the preparation method can further comprise the purification of the particles from any monomeric avidin eventually present in the solution as a residue of the first step . purification can be undertaken after either step a ) or step b ). purification of the nanoassembled complexes from any monomeric avidin present in solution can be carried out by known methods , for example ultrafiltration or size exclusion chromatography . in the case of ultrafiltration , suitable systems are used , characterized by a cut - off equal to or greater than (≧) 100 kda . in the case of size exclusion chromatography , chromatographic media are used which are suitable for retaining protein molecules of sizes up to (& lt ;) 200 kda . if the biotin binding sites present on the avidin of the nucleus are not saturated by the biotinylated surface protecting agent b - x a - pa b the preparation method can also comprise a further optional step of adding additional biotinylated compounds equal or different each other . with the previously described preparation methods , nanoassembled complexes having the features of nanoparticles of any size can be obtained . in particular , said nanoassembled complexes are in form of nanoparticles of at least 10 nm in size and preferably from 50 to 1 , 000 nm in size . the use of nanoassembled complexes in nanoparticulate form herein described extends to all currently known applications of the avidin - biotin system , for which they act as “ amplification ” systems . examples of these applications include their use as : a ) detection means in in vitro diagnostics ; b ) amplifiers in the localization and patterning of molecules on surfaces ( for example microarrays , protein chips and dna ); c ) instruments for in vivo diagnostics ; d ) systems for active or passive is targeting of drugs . the use will depend on the nature of the biotinylated compounds which can further be introduced onto nanoassembly surfaces through biotin binding sites present on the avidin and not saturated by binding with biotin of the protecting agent b - x a - pa b . some examples of the preparation of the nanoassembly compounds of the invention and their characterization wilt be given hereinafter by way of non - limiting illustration . in particular , the nanoassembly compounds obtained by the previously described preparation were characterized by : c ) the number of biotin binding sites available for introducing additional biotinylated functions . this assessment was carried out using the haba assay , as described in the literature ( green n m biochem . j . 1965 , 94 : 23c - 24c ); d ) the speed of aggregation in a buffered medium , using light scattering techniques ; e ) their stability to freezing and thawing , and to lyophilization , using light scattering techniques . preparation and characterization of nanoassemblies obtained with plasmid dna and avidin in different molar ratios without addition of a surface protector the complexes were prepared by mixing aqueous solutions of avidin ( av , belovo , belgium ) and nucleic acid ( pnm , plasmid p - egfp cl ( clonetech # 6084 - 1 ) ( 4 . 7 kb )) in varying molar ratios as given in table 1 below . the solutions were left to equilibrate for an hour at 0 ° c . in an ice bath , and after centrifugation ( 15 , 000 rpm for 5 minutes ), the sizes of the nanoassemblies in solution were analyzed by light scattering using an instrument system consisting of a spectra physics stabilite 2017 laser , a pacific scientific “ nicomp 370 computing autocorrelator ” and a system for temperature controlling the samples . the size distribution measured on the first sample is given in fig1 . from the figure said assembly can be seen to be characterized by a moderate polydispersivity . the data in table 1 also show that the sizes and polydispersivity of the nanoassembly increase as the yin ratio decreases , indicating that as this value decreases , the degree of condensation of the nucleic acid molecule in the assembly is less . the size variation as y / n varies is however limited to within the values of about 70 to 200 nm . preparation and characterization of nanoassemblies obtained with plasmid dna , avidin and surface protecting agents different quantities of the various surface protecting agents ( b - x a - pa b ) were added to sample 1 av - pegfp 3 , prepared as described in example 1 , using b - x a - pa b : avidin molar ratios varying between 0 and 2 . 4 as shown in table 2 . five different b - x a - pa b agents were used ( i , iia , lib , iii and iv ), whose chemical formulas are given as follows : said protecting agents b - x a - pa b were synthesized and characterized as described below . b - x a - pa b i : was obtained by condensing the 6 - amino - n - hexylamide of biotin with the n - hydroxysuccinimidyl carbonate of monomethoxy polyethylene glycol 2 , 000 ( monfardini c , schiavon o et al . bioconjugate chemistry 1995 , 6 : 62 - 69 ). b - x a - pa b iia : was obtained by condensing α - amino , ω - methoxy - polyethylene glycol 5 , 000 ( fluka cat # 06679 ) with the n - hydroxysuccinimidyl carbonate of biotinyl - n - hexanolamide ( morpurgo m , bayer e a et al . j . biochem . biophys . meth . 1999 , 38 : 17 - 28 ). b - x a - pa b iib : was obtained in a similar manner to b - x a - pa b i using monomethoxy polyethylene glycol 5 , 000 instead of 2 , 000 . b - x a - pa b iii : was obtained by condensing the n - hydroxysuccinimidyl carbonate of monomethoxy polyethylene glycol 2 , 000 with the amino groups of the amide of 2 , 6 diaminohexanoic acid and with biotinyl - n - hexyldiamine ( 2 , 6 - diamino - hexanoic acid ( 6 - biotinylamidohexyl )- amide ). b - x a - pa b iv : was obtained in a similar manner to b - x a - pa b iii using monomethoxy polyethylene glycol 5 , 000 instead of 2 , 000 . the dimensions of the final nanoassembled complexes in the assembling solutions were measured by light scattering , as described in example 1 . the size results are summarized in table 2 and fig1 . all the samples , initially prepared in salt - and ion - free water , were then diluted in pbs buffer and their aggregation rate was measured by light scattering . the aggregation kinetics are shown in fig2 . it can be seen from the figure that when the b - x a - pa b s are introduced onto the surfaces of the nanoassemblies they slow aggregation of the latter , in a salt - containing environment , until they inhibit it completely . the protective efficacy of each b - x a - pa b increases with increasing surface concentration of b - x a - pa b . the protective efficacy also depends on the type of b - x a - pa b , with b - x a - pa b iv and b - x 2 - pa b iib being the most effective of all those tested . preparation and characterization of nanoassemblies obtained with plasmid dna and avidin , and purification by ultrafiltration alexa - fluor546 - biocytin ( molecular probes # a12923 ) was added to sample 1 av - pegfp 3 , prepared as described in example 1 , in a quantity equal to that needed to saturate 2 % of total biotin binding sites . the prepared product was subjected to various ultrafiltration steps using vivaspin 100k pes membranes ( sartorius , 100 , 000 da cut - off ) so as to enable monomeric but not nanoassembled avidin to pass through . the avidin concentration in the supernatant and in the filtrate was determined by fluorescence , based on the signal of the alexa - fluor546 fluorophore . the supernatant obtained after four ultrafiltration steps was analyzed by light scattering . the avidin : nb ratio in the nanoparticulate system was calculated from the avidin concentration present therein with the assumption that the dna present was the same as that present prior to ultrafiltration . from the results given in the table it is apparent that ultrafiltration treatment is able to remove excess monomeric avidin introduced in the preparative stage . particle sizes are found to be slightly larger than those recorded before purification . this difference ( not statistically relevant ) is probably ascribable to the lower level of dna packing recorded as the y / n ratio in solution decreases , as already described in example 1 . preparation and characterization of nanoassemblies obtained with genomic dna and avidin in different molar ratios , with and without addition of surface protector the nanocomplexes were prepared by mixing aqueous solutions of avidin ( av , belovo , belgium ) and fragmented bacterial genomic nucleic acid ( gen pnb , sigma cat # d1760 ) ( average size about 16 - 24 kb ) in a variable molar ratio ( see table 4 ). the solutions were left for an hour at 0 ° c . in an ice bath and after centrifugation ( 15 , 000 rpm for 5 minutes ) the dimensions of the nanoassemblies in solution ( table 4 ) were analyzed by light scattering as described in example 1 . a biotinylated antibody ( anti - hpsma ) was immobilized by spotting ( 1 μl ) onto nitrocellulose membranes . the membranes were blocked by immersing into pbs containing 2 % w / v of bsa ( pbs / bsa ) then treated with solutions containing avidin ( 1 . 3 μg / ml in pbs / bsa ), with previously added biotin - alexa - fluor ® in a quantity so as to saturate 25 % of total biotin binding sites . the avidin in said solutions was used in the monomeric or nanoassembled form ( table 5 ). after 2 hours of incubation at ambient temperature , the membranes were washed with pbs and visualized with a fluorescence microscope ( fig3 ). it can be seen from the figure that nanoassembled avidin is more effective at detecting the immobilized sample on the membrane . second comparison of efficiency of avidin in monomeric form and in nanocomplexed form with nucleic acid , in dot blot fluorescent detection varying quantities of biotinylated bsa ( 100 , 50 , 20 , 10 , 5 , 2 ng of protein corresponding respectively to 10 , 5 , 2 , 1 , 0 . 5 and 0 . 2 pmoles of biotin / spot ) were immobilized by spotting ( 0 . 1 μl ) onto nitrocellulose membranes . the membranes were blocked by immersing into pbs containing 2 % w / v of bsa ( pbs / bsa ) then treated with solutions containing avidin ( 5 μg / ml in pbs / bsa ), with previously added biotin - alexa - fluor ® in a quantity so as to saturate 40 % of total biotin binding sites . the avidin in said solutions was used in the monomeric or nanoassembly form ( table 6 ). after 2 hours of incubation at ambient temperature , the membranes were washed with pbs then visualized with a fluorescence microscope ( fig4 ). it can be seen from the figure that the detection limit using monomeric avidin is equal to 1 pmole of biotin , whereas when avidin is used in the nanoparticulate form , biotin is visible even in quantities equal to or less than 0 . 2 pmoles . the detection limit with the nanoassembly system was not achieved in this experiment . stability to freezing / thawing of the nanoassemblies in the absence and presence of b - x a - pa b agents the nanoassembly samples obtained with genomic dna as given in example 4 were subjected to a freeze - thaw cycle . the size measurements of the particles present in solution after thawing were compared to those of the same preparations before treatment . the results are shown in table 7 . it can be deduced from the results that the particles devoid of protection agent are not resistant to the freeze - thaw process , subsequent to which they aggregate irreversibly . when instead the protecting agent b - x a - pa h is present on the surface , aggregation is inhibited . comparison of efficiency of avidin in monomeric form and in nanocomplexed form with nucleic acid , in an enzyme - linked detection system varying quantities of biotinylated - igg ( igg - b ) ( 0 . 054 , 0 . 18 , 0 . 6 , 2 . 0 , 6 . 7 and 22 . 3 ng of protein were immobilized by spotting ( 0 . 5 μl ) onto nitrocellulose membranes . the membranes were blocked by immersing into pbs containing 2 % w / v of bsa ( pbs / bsa ) then treated with solutions containing avidin ( 5 μg / ml in pbs / bsa ). the avidin in said solutions was used in the monomeric or nanoassembly form ( table 7 ). after 1 hour of incubation at ambient temperature , the membranes were washed with pbs and incubated ( 1h ) with biotin - horseradish peroxidase ( sigma - aldrich , 4 μg / ml in pbs / bsa ). membrane development was carried out with diaminobenzidine ( dab ). spot density was analyzed through the imagej software and translated into the graph of fig5 . it can be seen from the figure that the detection limit using monomeric avidin is equal to 0 . 6 ng of igg - b , whereas when avidin is used in the nanoparticulate form , igg is visible even in quantities equal to or less than 0 . 054 ng . the detection limit with the nanoassembly system was not achieved in this experiment .
the present invention discloses new nanoassembled complexes consisting of a central nucleus formed by a high - affinity interaction from nucleic acids and avidin , wherein said nucleus is stabilized in aqueous solutions , even saline , and protected from further unspecific unwanted interactions by means of suitable polymeric agents capable to mask totally or partially the nucleus itself . the nanocomplexes obtained have been shown to be stable in aqueous solutions and to have nanoparticle features . in addition , the nano - complexes have shown characteristics useful for use in biotechnological field and in nanomedicine .