y between the various groups R and R???, m and p represent, independently of each other, an integer which may vary between 1 and 6, and n represents an integer which may vary between 0 and 6, wherein, when n is equal to 0, then at least one of R1 and R2 is other than hydrogen, and when n is greater than 1, m is able to take different values and R3 is able to take different meanings within the gene