It appears that you are initializing a new variable M_ at the start of your code and by default it is just an empty double-class array. Doubles do not use or allow dot notation. Also, is there a reason why you are trying to initialize your variable as a global variable? If not, then I would recommend steering away from a global as this can create problems in subsquently run scripts if you are not careful.
If you are intending for M_ to utilize the dot notation in MATLAB, then you will need to generate a table or structure class. See help documents for struct and table classes.