I would like to programatically modify the property of a field of
SPList item Requiered = false.

I am not sure what I am missing?

SPGlobalAdmin globaladmin = new
SPGlobalAdmin();
Context.Items
[SPGlobalAdmin.RequestFromAdminPort] = true;
Page.RegisterHiddenField
("_REQUESTDIGEST",globaladmin.AdminFormDigest);

globaladmin.AllowUnsafeUpdates = true;

//SPWeb site_rating =
SPControl.GetContextWeb (Context);
SPSite my_site_add_user = new SPSite
("http://members.immunetolerance.org/sites/Tolerance
Proposal/default.aspx") ;

SPWeb my_web = my_site_add_user.AllWebs[""];

my_web.AllowUnsafeUpdates = true;

SPList my_list_Vote = my_web.Lists["Vote and
Review"];

// get the list item and change the property
of the list item
my_list_Vote.Fields["Please rate this
Proposal"].Required = false;


my_list_Vote.Update();
my_web.Update();

has anybody achieved in modifying the properties of a list
programatically?